Thanks for all the comments so far. Some assumptions/approximations for the dynamic model: 1. The 'tibia' or shin length of the link is assumed to be the hypotenuse between the actual tibia length and ankle to cleat measurement. This is an approximation to make it work in the four bar analysis. Ankle flexion assumed constant. 2. Crank angular velocity assumed constant. This is imperfect as the crank vel varies with load in 1 rotation.
Eduardo Bueno What’s interesting is if you repeated that mathematical process with 10 different riders (or even the left hand side of the same rider) you’d more than likely get a different result. Again, it’s pretty easy with the right tools to measure the angles your study has extracted. Studies have shown that more elite riders have a tendency to drop the heel less in the first phase of the stroke. The pedal stroke dynamics of individual riders is learnt (by huge amounts of repetition) and is very individual in itself. The real world challenge for bike fitters is to allow for individual movement patterns and more often than not, asymmetries in those patterns.
@@JerrysScenicCycling I would imagine this to be the case indeed. Cleats position affects muscle recruitment. Far too forward requires more calf to stabilize the foot for pedaling. Another issue is the shoe. Sometimes, larger-size shoes are purchased to deal with wide feet. unfortunately further moving the cleat screw-in position forward (as opposed to buying actual wide-style shoes).
Lyan Villacorta Such an important point. I see so many bike fit clients who turn up with big shoes because they have wide feet. There are some great wide fit shoes on the market now, so if you have wide feet it’s really worth taking the time to get the right shoe. Different brands have different hole positioning relative to their last too. The first Fizik shoes that were released 10 years or so ago had holes REALLY far forward. Bont have them pretty far back. Do your homework before buying.
Being a shorter rider (173cm) I changed from 170 cranks to 165 a few years ago, it changed my life on the bike. It isn't just about the 4 bar. Shorter cranks allow the saddle to be moved up and back a few millimeters, for the same closed hip angle as the longer cranks, which unloads the weight on the hands, in my case it stopped numbness in my hands.
Worst thing about this video was that it ended! Really informative and thoughtful analysis of something I’m currently trying to get my head around. Thanks 👍
2 key problems I have with this analysis are failure to adjust seat height with crank length and the acceleration analysis assuming consistent velocity. When crank length is reduced by any measure, if saddle height is adjusted such that the bottom of the pedal stroke remains consistent, the effective change at the top of the pedal stroke is doubled. Also, there are several studies that show that reducing crank length results in increased preferred cadence. This would mitigate some if not all of the acceleration difference we see in your graphs. I also suspect that most cyclist's have a preferred/threshold of force per pedal stroke, and this is why we see higher preferred cadence for higher wattage.
Been riding for 30+ years. Always had 170s, am 5' 7", and noticed that I always had trouble accelerating from a dead stop, or ever, as compared to my taller compadres. Switched to 165s about 10 years ago and, man, what difference! Changed everything. And now I know why. Thanks so much for this.
Fascinating study you’ve done here. I’m very interested in following your progress. My academic background was in Human Factors Engineering and Ergonomics many years ago before I started cycling. I have been searching for an analysis just like yours. I like that you also mentioned leg mass as well. I am about 185 cm tall with large feet (size 47), and big legs. I also have fairly long tibias. I do not use a power meter but I do get the feeling that I cycle better with shorter cranks. The joint angle I am concerned with is the knee. There is a very powerful fulcrum there as well and I want to continue to preserve my 65 year old knees. I haven’t measure power but is seems to me I am able to generate more power at the pedal with the more open knee angle a shorter crank provides. Knee angle and hip angle are related but I haven’t paid attention to hip angle. I look forward to seeing your next video on the subject. Thanks!
FFT!!! Could you add a video for all us fast-tempo long distance for-fun-pleasure riders. Less so than this video for explosive-power-racing types !!!! I'd love to think I'm fast and unlike Alehandro I was still fast at 40 and now 43 I've lost power and now just enjoy zone 4 no zone 5's
I touched on dynamics at high school, remember fluid dynamics in physics class. I remember we had to calculate the torque and kilowatts required for an oil pump to deliver X amount of litres per hour at a given RPM. The fluid viscosity was set and the pump volume per cycle was also set. Pretty simple with given data much harder to engineer.
The time spent reading or watching how to improve my performance normally yields a negative return on investment vs spending the time actually training. This video is an exception. It inspires informed experimentation which may indeed uncover flaws in my current setup. Being able to present significant ME theory to the point of almost sounding sexy is quite a testament to your overall skill set. 😊
I rotated my handlebars to have the drops closer to horizontal like you suggested in another video and I am now more confident and comfortable in the drops and use them far more often. Thanks, dude!
I moved fom 172.5 to 165mm to alleviate anterior knee pain. I also have te seat forward & raised. So much more comfy in the drops or TT position. Great vid!
Good analysis. I ride long cranks (old school 185 and 180mm). Recently I have a bike with shorter (175mm) cranks. The feeling is very different. My second ride with the bike resulted in "exploded" legs after 75km. You really need to adjust to it timing wise. My cadence did not change (94rpm average). In several studies they changed crank length and tested in a short period of time. That would never give a good result.
Excellent video. Love hearing the workings of someone who knows his stuff. Had a quick look through the title of previous videos to see if there's interesting stuff there. Saw the name Hambini so instantly subscribed.
Tri dork here. Loved this video! I found your data on hip angle using saddle position very interesting. Thanks for making this. I bet we could make a small smart phone app doing something similar here. Or maybe there already is something on the market that's easy. 🙄
Noice! Have you considered or already done the 5 linkage version, adding in the ankle-to-cleat link? The whole "cleat under ball of foot" rule of thumb has also been subjected to actually doing the science and thinking about it from a biomechanical POV, with cleats being closer to the midfoot showing some real benefits, such as increased foot stability and reduced calf loads/fatigue.
Great video, very informative and I love the use of engineering to debunk cycling myths. Did you ever make the follow up video about force? I couldn't see it
Great video ! With the reduction of crank length, should your model not also increase the seat height since the maximum extension of the leg is correspondingly shortened when the shorter crank is a the bottom of the rotation? That would open up the hips even more.
It does but you must consider that as one raises the saddle the setback incrementally increases too if the axis point is fixed so if you bring the saddle forward your hip angle is the same.
Great video! I would like to add a few comments and thoughts though. With the shorter crank installed, the saddle should be raised at least the equivalent amount of difference. This would open up the hip and knee angle further. Additionally, the angles can be opened up with a more rearward cleat position. The other factor to consider, on a road bike, is depending how far the saddle is moved forward, this can result in pressure and discomfort in the hands, wrists, shoulders and neck and effect breathing. There of course many other changes to be made, as a result of what I have listed above - "knock on effects" Enjoying the videos, keep them coming!
I would love to see how an oval chainring plays into this equation and also the other interest would be how cadence speed would it should change to create the same output. Great clip.
it's neat to see the benefits of shorter crank lengths and I'd be interested to see how this could be adapted for MTB use where shorter cranks have inherent value anyways- fewer pedal strikes/lower BB for cornering aggressively. I wouldn't be entirely surprised if this is the next portion of geometry that gets pushed to the practical limit (like head angle, bar length, top tube length and stem length already have).
I'm 186cm but prefer 170mm cranks to 175mm due to bad knee (yeah, most people say I should use 172.5 - 175mm). Never thought about angles, but a friend recommended that I try shorter cranks to see if it helps with knee pain. I was sceptical at first, because in my simple mind, shorter crank means more torque and worse pain... But it worked! Your video explains a lot why it is so. I might experiment with saddle position as well. Thanks!
Id like to see you do a force vs. Femur/Tibia angle at the pedals. That coupled with the crank length should give you the best idea of optimal crank length to run.
Watched this video and would like to comment on your CAD model/dynamic simulation. In that model the angle between your ankle and lower leg is held constant, which you do mention at approximately 12:25. For your CAD model what you should do is constrain the angle of the foot to the ground, not to the lower leg. If you watch riders from a side view you will see that through a full pedal rotation, the foot is held at 20-30 degrees to the road surface. This varies throughout the stroke, and if you could model this correctly in your CAD skeleton, you would see different results in hip angle especially at the top of the stroke (330°-60°). See if you can find videos of experienced cyclists on trainers from a side view and you will see what I mean.
I commissioned a custom-built frame with 80° STA and paired it with a 152mm crank. It took a little getting used to, but found no downside other than it forced my seat a bit higher, which made the seat-to-grip height difference a bit worse. I also had to switch from using RPM to using "foot velocity" instead. Thought cornering would be worse, but instead it actually felt like I got more consistent at them. In my head, I thought my legs were doing more of a rapid "piston-like" movement, like stair climbing, and speculated that this kind of position would benefit my fitness on foot (running), which made me wonder if triathletes were going after the same thing. Made me question the whole idea of going for different positions to rest overused muscles was even wise, compared to just giving the most used muscles even more training.
You could model the next video on a selection of frames in one series. Then mark out the heatmap of where the effective seatpost could even be, given the extension of the fore-aft, and similar for reach given available stems, and the same for crank length. What does that look like with a different colour for each frame, if all frames are stationary at the BB? Not only do some bike models have the same geo across their range, they also sport the same seat, stem, cranks, and gears (and hopefully BB height for the comparison). That seems wanting.
Really interesting. I'm trying to square it all with the fact that I just perform better on longer cranks (180mm). Anecdotal - I've got a better jump, I don't need to get out of the saddle coming out of tight corners, under effort I think more about muscle recruitment and ankle flexion, I'm thinking round the pedal stroke, ....and I really like the wider range of motion under light load for recovery. I'm going to try the CAD modelling, there might be something else going on if you remove the foot/tibia approximation and make it a 5 bar linkage. I think the ankle acts as a fluid coupling and damps that peak acceleration. Like you say, hip ROM is a red herring, there's lots of other stuff you can do to solve that.
This was a very useful exercise - it would be great if you did a follow up or two I'm particularly interested to see a comparison with Greg Lemond's method of setting saddle height from the BB
When I was racing off road in the 90s, everyone was using 175+mm cranks, but being under 5.5 ft tall with short legs, I found 175 cranks just too long. I switched to 170s and never looked back.
As I aged, I noticed that my warm-up for rides to the point where I could accelerate with the group was taking longer and longer. I am 6'4" tall and have used 175 cranks forever. I decided to try 172.5's, and that warm-up period more or less disappeared. Also, my pedal stroke up very steep hills smoothed out. Since I am about to turn 79 (tomorrow) and over the last two years, my power has almost disappeared, so every little bit helps. Over the past 12 months I've ridden 5,000 miles and climbed 80,000 feet which is about one third my normal climbing (last winter destroyed many of the hill roads) I have been unable to easily find the climbs best suited to me. Plus, with my degradation of power, I am too slow to ride with the group and so have to ride solo much of the time. But the improvement in power with the shorter cranks. I am able to continue even on the worst sections, though I am crawling over the steep sections.
Great analysis. I think the main problem is that road bikes are still way too conservative on their geometry. Crank length is one point - but the much larger problem is that road bikes seem archaic when it comes to findings on what is faster on mountainbikes, think Mondraker Forward Geometry or Pole bikes - mow more or less implemented in all bikes. Ideally we want much steeper seat tubes - maybe 76-77°, maybe even 78° as on Pole bikes (though then the more you use your bike for climbing - the steeper you like your seat angle), longer reach but shorter stems, and much slacker head angles (and yeah wheel base will rocket from the paltry 99cm to 110-115cm or so - notice enduro mountainbikes have arrived at 130cm already for a size large). The main problem is the handlebar width - going 10-20cm wider would likely be really beneficial fore most going uphill, but really inefficient in the wind. This would resolve a lot of too much weight on hands/shoulders problems. Someone really needs to think forward, forget UCI norms, and look at things that made mountainbikes fast. Then get people to train on a new geometry for some months and compare speed and comfort. Some things from the mountainbike side may not work on roadbikes (for sure no 75-80cm wide handlebars) but a lot will. To counter the slacker head angle and weight distribution - the chainstay length will need to grow too - however road bikes likely need much less weight on the front wheel vs mountainbikes - so shorter CS = less weight on your arms = more comfortable. And yeah if crankarms reduce in size, the long needed evolution of chainring sizes going smaller needs to continue too. Average Joes tend to run way too big rings up front instead of spinning faster and trying to choose a small ring that allows them never to drop below 80RPM on their steepest common climbs. It's crazy how conservate and stupid a lot of the bikefitting is on roadbikes - trying to fit people onto frame geometries that are simply wrong from the start. And yeah it needs a radical departure from the status quo. Most of bike fitting is simply an reaction to wrong bike geometries.
I suspect: What the acceleration peak allows is for you to drop the toe a bit through ankle flex. The toe drop absorbs the acceleration which works perfectly to begin the pull up cycle. If so, that could be an advantage.
This is a very interesting analysis thats is very well done, I understand the constant angle flection assumption as adding more DOF can drastically change the complexity of the analysis. One thing I was thinking whist watching is that muscles have different force production profiles depending on the amount of flection at the joint (amount of stretch on the muscle), and although such impediments as hip impingement is a first priority, to maximise performance with a force analysis, consideration to each muscles force production profile is needed for an accurate depiction. I have an inkling that the ratio of force production between the glutes and quads at their respective angles would contribute towards a total picture. Again great work, and thanks for the video.
Hi, thanks for the comment. You're completely correct I think. It just highlights how complicated an optimisation problem it is, there are sooo many variables.
I switched from 172.5 to 165 cranks a few years back for the reasons you so ably have shown in your presentation. Your presentation is the best I have encountered so far on this topic and validates my decision to switch to shorter cranks. Good to know that moving seat forward also helps to open hip angle. Maybe you could have a look at oval chainrings too, another controversial topic. Cheers
Thanks Norman. There are lots of assumptions and its a simplified model. Mixing engineering and biological/chemcia/neurologicall functions will never truly align.
Great video and discussion thx. When you say you raised the saddle with the shorter cranks in Case 1, how much did you raise it and why? Are your normalizing for 'feel,' hip angle at the bottom of the stroke, or leg extension? If you were doing a similar kinematic analysis on a mechanical system, you wouldn't "raise the saddle." It's only when I started fitting using digital motion capture tools a decade ago that I realized the futility of this. I had spent the previous 20 years of my bike fitting career fixated (as many fitters are) on 'optimizing' leg extension for the pedal at the bottom of the stroke. There's not much power being generated there. Takes a bit of work but we can now test for different FTPs at different saddle heights, and most of the research finds that within a ~2cm band, power output doesn't change much (just as it doesn't change much for crank length). Raising the saddle also erases the significant aerodynamic advantage you'd get from the shorter crank (and resultant ability to lower the upper body while maintaining hip angle constant).
Thank you for the video, as you shorten the crank length you have shown that there is a realized benefit of reduced peak torque, but you can shorten the crank length only so much. When does the crank length become too short such that it becomes a "negative" effect or change on the riders effectiveness?
Earlier this year, after watching this video, I went and changed the crackset on the cheap on one of my bikes from 172.5 to 165 mm (for about $40). I tried this out for a few weeks and it felt less fatiguing overall and enabled pedaling at about 10 rpm faster. It also alleviated saddle discomfort for me, especially at higher cadences.
Great piece of work, thinking and visually attractive presentation. OK it's a model. There are assumptions. But it's interesting to see what can be derived from that, knowing that it's not a perfect repesentation of reality (yet... :-) ). Curious to see what your real world experiences are with the 165mm cranks. Look forward to a follow-up!
Interesting about the power curves around 16 mins in - my return to road bikes came via a TT rig after 25 years on MTB’s or various types - it came with oval rings - when i bought a “proper” road bike so i could group ride (did’nt happen in the end) i ended up after a painful “proper” bike fit - Retuel scrapping most of it and sort of replicating my TT setup going from a layback post to straight and a short nose saddle - $$ is only reason I’ve not swopped out the TT/road cranks from 172.5’s to 160’s but it’s on the cards for this year Back to to the ovals - my own experience has been they smooth out the pedal action - with my mtb running a 32t up I tried a oval and immediately removed the chop chop chop on a high cadence on a flat ride - no science here just my own thoughts and experience - new subscriber too
Cheers! What do you do if you have a massive difference in femur and tibia length? I am 172 cm tall with 45.72 cm femurs and 38 cm tibiae. I use 170 cranks. On all my bikes I have to jam the seat back with set back seat posts to be comfortable or powerful. Any thoughts?
+1 for the constraint based sketches! +1 for the consideration of acceleration! I’m cringing while thinking about my femur crashing into my tibia after the pedal upstroke. Perhaps the reduced acceleration profiles associated with shorter cranks could simply reduce knee strain?
I'd be really interested to see you do some analysis on cleat position (and foot size I guess) if possible. I'm the same height as you and have big feet, so cleat position in concert with crank length feels like it makes a big difference - moving them back both allowed me to move my saddle forward and down a bit (no more toe pointing). Discounting contribution to pedaling from my calf muscles, moving cleats back I think has a similar effect to shorter cranks. And on size 14 UK feet, a tiny amount of toe pointing can make a huge difference. I would say it's slightly harder to accelerate hard & sprint, especially off the saddle feels a bit odd - but it's fixed some toe numbness.
After watching the CAD demonstration, curious what effect changing seat tube offset would have on opening up the hips. Lot's of stock bike builds these days come with a 20mm offset, in my case an SW SL6 Tarmac. If the goal is to open up the hips, would it be smarter to change seat tube offset, before crank length?
very nice! have you looked at the premature wear of the tibia's meniscus wear and displacement of forces with different seat/seat angle/ fore-aft positioning?
When you said inertia, my ears pricked up. I am only 6'2" but my legs are massive (and long). I have gone from 175 to 172.5 cranks and have my seat slammed forward. I feel I get a more integrated comfortable feel. The problem is when im at battle speed, im often at 105 - 110rpm for long periods. This has got to take a tole on my huge legs. Interestingly the only time Iv been able to finish with the chain gang at the end, after 40 minute averaging 27mph, I was running 170 cranks on a single speed with a bigish gear.
@Alien On a Bike lol poor thing. It's people like you who get sheltered all the way across France, being dragged along by decent sized men up front. Only kidding I'm just jealous 🤣
I use Rotor 155 with Rotor ovals. I also have a 76.5 degree seat tube angle with my saddle 10% down..It's incredible how much more effcient that combination is.
Great insights. I've been using "trail and error" method - with my smart rollers and Zwift. Each time I tweak one piece, saddle, seat post, cleats, bars, oval rings, I was seeing up to 20 watts + on FTP. And then you have different bikes frame geometry too. I am hoping I have close to ideal for my body now - but always interested to explore more.
Subjective comment: I’m 5’9” / 175 cm and in my late 50’s. I live on a small mountain range where my climbs average 5 to 9% with gradients of 14 to 18% common. I’ve had several road bikes in size 56 cm and they all came with 172.5 mm cranks. After reading and watching about crank length I went with 170 mm on my last bike purchase. I couldn’t be happier. The difference when climbing was immediately noticeable. I wasn’t straining as much, but I was spinning faster. My times going up remained about the same, but I feel better at the top. Thanks for the informative video.
Hi James I've always thought getting the right riding position is an art, but the science is all new to me. I would have immediately guessed that a longer lever / crank would make spinning easier, but I suppose that's where ergonomics comes in! Found your comment very interesting 👍🏾
Nice video, may need to experiment with seat position more. Was set by the basic plumb line knee over the pedal method by my local bike shop and hardly altered since.
#PeakedTorque Did you account for the additional hip opening from a shorter crank because of having to lower the saddle because the foot will be higher at the bottom of the stroke?
So here’s a question if someone was running a large single speed chainring 50T would you run shorter (165-170) arms or longer for less fatigue and better power or larger for more torque?
Well present video, thank you. As a touring unicycle rider the subject of crank length is always interesting. Road unicycles always use much shorter cranks than bicycles, mostly as being fixed gear it gives the best compromise of torque and speed. Gearing can't be changed but crank length allows for at least a change of leverage for different riding conditions, styles and rider's strength/skill. I run 110 mm cranks on a 29" unicycle and 127 mm on a 36" unicycle, others go much shorter down to 89 mm. It makes hills harder but cruising far less taxing and even a little faster, perhaps because the angular velocity allows for an easier passing of the dead spot at the bottom on the crank stoke. The issue I find hard to compare is rider position. There are no "bike fits" for unicycle riders! The basic position is much more directly over the crank hub with little room to vary seat position fore or aft and a very open "hip angle". I believe it's still be same 4-bar linkage so perhaps the mechanics are the same as on a bicycle. Maybe you can comment on that? Again, thanks for interesting video.
I don't think short cranks make unicycling any easer/faster due to the dead spots, I think it's simply that you are limited more by cadence and how fast you can move your feet, than you are by the amount of force you can apply to the cranks. to do 20km/h on a 29" unicycle you're going to need to do around 145rpm. You don't need much more power to do it than you would on a bicycle. If you try and ride your bicycle at 20km/h with a 1:1 gear ratio and the same 145rpm, you would be happy to have tiny cranks there too.
Superb. V interesting. Nice CAD idea, first thing I thought was surely it is a 5 bar linkage system ( to include foot link and ankle pivot) and glad you pointed this out at 12.22 that you assumed ankle flexion is constant and I think this is the biggest flaw in this model; would have been interesting to see the numbers where the foot is kept horizontal and ankle flexion allowed - think it would reduce the minimum hip angle differences. Also note how you have the ball of foot pivot is not perpendicularly below knee pivot at the 3 o' clock position of the stroke, as bike fitters often recommend. I will try your CAD idea however - think it is great. Would love to have seen the follow up about Forces investigation but couldn't find. My experience with shorter cranks were worse - you needed to go one bigger rear cog and therefore increase rpm to match the longer crank.
Just by curiosity, have you taken into account the muscles that are engaged given the crank length? I reckon it might have some effect on power. Like muscles might flex just a bit more to get more juice out, or some crazy shit like that. Just a thought
Ive got a shorter fenur to tibia to mist oeople which had ne trying shorter cranks but in the end i went to a longer crank. I positioned my saddle forward and raised my bars 10mm and my saddle 5mm. I have a linger longer extension but leverage has increased from 12 to 4 oclock. I find my out if saddle power is greater.
just watched this ...very interesting, what effect does crank length have on the knees, just recently having problems with my left knee, well both knees but left mainly. Seem to be getting fatigue in the vastus medialis I think. I do climb a lot though . I ride Giant tcr adv sl Rotor oval 172.5. Wondered if 170 or 165 would be better im not that tall about 172-173🤔
Another excellent presentation. Can you add the effects of eccentric chain rings and perhaps, the variable crank arm phasing geometry BB? To this analysis, please?
Looking forwards to the force analysis, because I imagine within a range of gears, it needs to also be within range of motion, and in the powerband of extension relating to muscle-groups, for all of what is in this video. I think it isn't discussed, because sweetspots of combinations delivered don't lend themselves to the limited range of not only cranks, but also frame-sizes, made worse by how it sometimes in reality is just one model scaled up, down or both.
Bravo - separating fact from fiction! I'm adjusting my seat height and saddle position tomorrow! But I need your torque analysis - please. I'm 65yrs old, 186cm tall, recovering from covid pneumonia a year ago which totally sacked my lung capacity and muscle mass. About 4 months ago I changed my crank length from my traditional 170mm (sometimes 172.5mm) to 175mm crank. I use a hacked drive train (48/38/28t x 11-50t). I’m not a weight weeny by nature, but I have managed to reduce my Surly Straggler down to 21.6 lbs from 28.5 lbs with phat 38c tires. With the longer crank arm, I feel more power and can accelerate faster in shorter periods, and I can keep peddling through varied flats and steeper slopes more often and for longer periods of time. I watch my cadence and it's easier to keep it in the 85/95 PRM range. The biggest plus of all is I get outside and away from a trainer in a room watching a flat screen, which I hate. So far, I've regained about 75 percent of my former strength. My stamina is still the sh*ts though. I would really appreciate your crank length vs torque analysis. I understand what you said about crank length vs gearing, but I'm convinced a longer lever (crank arm) is a benefit across the entire gear range for sport riding vs endurance/road racing.
I have a question regarding the the change to crank arm length: Why did you not change the seat height by the same amount? It is recommended to change the seat height by the same amount you change the crank length to reach the same effective seat height again. I see the value of isolating a variable in analysis, but in this case it does not seem correct to isolate this change when you will always change seat height at the same time. Further more so as in the next step you talked about the benefit of raising the saddle and setting it further forward. This would be the next logical step after reducing crank length anyway. Seeing bikefit as a system, where one parameter changes others at the same time, it seems to me, that it is not advisable to isolate changes for analysis.
Just when i think about the acceleration peaks (in angles where there is not much force), maybe it explains the tendency to grind more with longer cranks? On the trackbike i reach higher cadance than on the roadbike with the 7,5mm longer cranks. Maybe also „rocking“ the hip left and right can be explained with this phase of low power / high acceleration. Very well made!
really interesting - also worth considering the positional change associated with shorter cranks, what i mean is; shorter cranks would mean saddle can go higher maintaining the same leg extension angles on the bottom of the pedal stroke - at 6 o'clock, and consequently compounded effect on hip angle of increase saddle high and shorter cranks as explained ?
Thanks,I shall follow with interest. Another aspect: what if you have different leg length's and/or variances in tibea/ femur? E.G: my right femur is 22 mm longer than left,while the right tibea is shorter. Overall my right leg is 37mm shorter. To complicate matters my right ankle has a fusion and alignment issues,making for complicated bio-mechanics.
5' 7" (30" inseam) and I find 170mm cranks too long. I've been experimenting with shorter kids bike cranks, 152mm to 130mm, and find that 1mm ends up equaling about 1RPM of perceived cadence. (85RPM @170mm feels like 103RPM @152mm) The cadence where I can hold my FTP at the lowest heart rate is about 95RPM. That seems to line up with the dynamic model that I should probably use 160mm cranks. Too bad they're such a PIA to find.
@@PeakTorque I managed to get some 160mm 105 Cranks. Instantly increased my average cadence by 8 rpm. Power is down a little because I'm not conditioned to the faster cadence. (Fast twitch fibers in the quads are on fire). Way more comfort, and simpler to find a good fit.
Interesting approach and conclusions. At what cadence did you do the simulations to generate the velocity and acceleration curves? Did you account for the fact that pedal speed is greater for the same cadence with longer cranks? That's probably relevant, because higher muscle contraction velocity concurrent with greater pedal speed entails lower muscle force capacity. That means, in reality, one is likely to gravitate toward a higher cadence with shorter cranks to maintain the same (subjectively optimal) balance between muscle force and contraction velocity (i.e., pedal force and pedal speed). Just wondering it you did, or could, correct for that, and if so, whether the differences in peak acceleration would remain so large.
Great engineering here. I was looking for someone to do this using Adams with all forces. Matching the forces with the muscle fiber to show really fatigue. As small fiber will be fatigue faster with similar force compared to large one. Thanks. Enjoyed the video.
I have some sort of nerve problem possibly affecting the muscles in my right foot. It needs an orthotic for support. My ankles are the only hypermobile joint. The right hip is bothered for some reason, too. What would happen with a 73° seatpost ? I would go to 165 from 175. My femurs are 52cm long. I'm 189cm tall. Any thoughts? I went from racing cat 4 and 10000km/year to lots of intermittent pain this year.
I had always wondered the effect of crank length for my neck! I'm fairly flexible but my neck has issues and wondered it crank length would help. And I'm a bit cheap on getting a proper bike fit.
Changing the crank length for neck issues is wishful thinking and expensive. First assuming that we're talking about road bikes if you have a visor on your helmet - remove it. Now. Visors are fine for mountain biking and such where you have a more upright posture but not for a road bike. After that I'd look into handlebar position. Usually this means that you want your handlebars to be moved higher and/or back.
How is your neck doing? I used to have more frequent neck aches than today. I find swimming really helps a lot to make it limber and any cycling helps the back overall if done for enough time.
@James Medina unfortunately I don't get on the bike as much as needed to properly adjust. The neck has gotten better as I adjust my fit. I've found that a zero setback sestpost and shorter stem are pretty much a must.
Nice presentation. Other key points I would consider.... One of the major reasons given for shortening cranks is the theory that it helps in more endurance focused events where power outputs tend to be relatively steady state (again, IM distance triathlon is a good example). There isn’t a huge amount of scientific evidence to back this up, as anything that involves testing whether a bike fit or equipment change increases failure time is inherently really difficult to test (there are just too many other external factors that might influence fatigue rates). It is pretty easy to dynamically measure closed hip angle (most bike fit motion capture tools do this). It’s all very well measuring the angle (or calculating what it might be with a tool like CAD) but that doesn’t tell you what hip angle any given rider can cope with over the course of their chosen event. It is possible to measure how the rider’s centre of pressure moves in the saddle throughout the pedal stroke (the pelvis can become more unstable in cases where any link in the kinetic chain (hip, knee or ankle) reaches, or is close to reaching the limit of its range of motion). Changes in pelvic stability due to any bike fit change including crank length can also be assessed using Inertial Measurement devices (mini gyroscopes and accelerometers) attached to the pelvis. These are especially handy as they can be used out on the road, not just in the lab/fit studio. Another hugely important thing to consider (and this is where using CAD or such like really starts to struggle) is that the human body (even of a highly trained elite athlete) is far from being a perfect machine. Accurately measure knee, hip and ankle angles for 100 individual pedal strokes and you won’t see two pedal strokes the same. Throw into that mix that a key component of fatigue (as well as reduction in force production) is a loss of motor skills (the ability to fire the right muscles in the right order) and it’s easy to appreciate that rider motion can be very different at the end of a long ride than at the start. This is why we feel like we’re pedalling squares when heavy fatigue sets in. A practical way of testing what crank length might work for you is to find a bike fitter with a fit bike that allows seat angle, bar position and crank length changes to be made. Once a position is found that’s approaching “optimal” (sorry for using that word as I’m a firm believer in that it’s unlikely there is a single fixed position that’s actually optimal). Pedalling one legged while varying crank length (and altering seat height to maintain the same leg extension at each selected length) will give a good guide as to what length “works”. The rider should be able to maintain a fluid pedal stroke when riding at low cadence without having to excessively rock the pelvis. Most people have one side that’s more mobile than the other. The trend in bike fitting at the moment is towards practical outdoor testing. Expect to see more fitters offering the opportunity to test crank length changes out on the road as part of their bike fit services. Sorry for rambling. Hope this helps.
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Thanks for all the comments so far. Some assumptions/approximations for the dynamic model:
1. The 'tibia' or shin length of the link is assumed to be the hypotenuse between the actual tibia length and ankle to cleat measurement. This is an approximation to make it work in the four bar analysis. Ankle flexion assumed constant.
2. Crank angular velocity assumed constant. This is imperfect as the crank vel varies with load in 1 rotation.
See this for some information on the ankle rotation during the pedal stroke: cds-0.blogspot.com/2011/09/pedaling-model-i.html?m=1
Eduardo Bueno What’s interesting is if you repeated that mathematical process with 10 different riders (or even the left hand side of the same rider) you’d more than likely get a different result. Again, it’s pretty easy with the right tools to measure the angles your study has extracted. Studies have shown that more elite riders have a tendency to drop the heel less in the first phase of the stroke. The pedal stroke dynamics of individual riders is learnt (by huge amounts of repetition) and is very individual in itself. The real world challenge for bike fitters is to allow for individual movement patterns and more often than not, asymmetries in those patterns.
@@JerrysScenicCycling really interesting results, thanks for sharing.
@@JerrysScenicCycling I would imagine this to be the case indeed. Cleats position affects muscle recruitment. Far too forward requires more calf to stabilize the foot for pedaling. Another issue is the shoe. Sometimes, larger-size shoes are purchased to deal with wide feet. unfortunately further moving the cleat screw-in position forward (as opposed to buying actual wide-style shoes).
Lyan Villacorta Such an important point. I see so many bike fit clients who turn up with big shoes because they have wide feet. There are some great wide fit shoes on the market now, so if you have wide feet it’s really worth taking the time to get the right shoe. Different brands have different hole positioning relative to their last too. The first Fizik shoes that were released 10 years or so ago had holes REALLY far forward. Bont have them pretty far back. Do your homework before buying.
As soon as I heard PowerPoint, I was waiting for 'aged 5' to appear 😂. Crackin' vid 👍🏻
pen is working
Being a shorter rider (173cm) I changed from 170 cranks to 165 a few years ago, it changed my life on the bike.
It isn't just about the 4 bar.
Shorter cranks allow the saddle to be moved up and back a few millimeters, for the same closed hip angle as the longer cranks, which unloads the weight on the hands, in my case it stopped numbness in my hands.
Same here.
Worst thing about this video was that it ended! Really informative and thoughtful analysis of something I’m currently trying to get my head around. Thanks 👍
2 key problems I have with this analysis are failure to adjust seat height with crank length and the acceleration analysis assuming consistent velocity.
When crank length is reduced by any measure, if saddle height is adjusted such that the bottom of the pedal stroke remains consistent, the effective change at the top of the pedal stroke is doubled.
Also, there are several studies that show that reducing crank length results in increased preferred cadence. This would mitigate some if not all of the acceleration difference we see in your graphs.
I also suspect that most cyclist's have a preferred/threshold of force per pedal stroke, and this is why we see higher preferred cadence for higher wattage.
Been riding for 30+ years. Always had 170s, am 5' 7", and noticed that I always had trouble accelerating from a dead stop, or ever, as compared to my taller compadres. Switched to 165s about 10 years ago and, man, what difference! Changed everything. And now I know why. Thanks so much for this.
Fascinating study you’ve done here. I’m very interested in following your progress. My academic background was in Human Factors Engineering and Ergonomics many years ago before I started cycling. I have been searching for an analysis just like yours. I like that you also mentioned leg mass as well. I am about 185 cm tall with large feet (size 47), and big legs. I also have fairly long tibias. I do not use a power meter but I do get the feeling that I cycle better with shorter cranks. The joint angle I am concerned with is the knee. There is a very powerful fulcrum there as well and I want to continue to preserve my 65 year old knees. I haven’t measure power but is seems to me I am able to generate more power at the pedal with the more open knee angle a shorter crank provides. Knee angle and hip angle are related but I haven’t paid attention to hip angle. I look forward to seeing your next video on the subject. Thanks!
Beautifully done; loved the analysis!
FFT!!! Could you add a video for all us fast-tempo long distance for-fun-pleasure riders. Less so than this video for explosive-power-racing types !!!! I'd love to think I'm fast and unlike Alehandro I was still fast at 40 and now 43 I've lost power and now just enjoy zone 4 no zone 5's
"Let's put some numbers to it" thank you! The lack of data and sound reasoning behind most discussions/debates in cycling has driven me nuts!
Thank you for this content man. This is exactly the thing I've been looking for. Keep it up!! Definitely looking forward to more analysis like this.
Oh man this brings back dynamics and kinematics studies from uni... Love it.
I touched on dynamics at high school, remember fluid dynamics in physics class. I remember we had to calculate the torque and kilowatts required for an oil pump to deliver X amount of litres per hour at a given RPM. The fluid viscosity was set and the pump volume per cycle was also set. Pretty simple with given data much harder to engineer.
The time spent reading or watching how to improve my performance normally yields a negative return on investment vs spending the time actually training. This video is an exception. It inspires informed experimentation which may indeed uncover flaws in my current setup. Being able to present significant ME theory to the point of almost sounding sexy is quite a testament to your overall skill set. 😊
I rotated my handlebars to have the drops closer to horizontal like you suggested in another video and I am now more confident and comfortable in the drops and use them far more often. Thanks, dude!
I moved fom 172.5 to 165mm to alleviate anterior knee pain. I also have te seat forward & raised. So much more comfy in the drops or TT position. Great vid!
Wonderful video. I have been considering shorter cranks because of intuition only, now I am starting to understand due to your excellent work
Very good analysis, real food for thought, thank you. looking forward to the next one.
Good analysis. I ride long cranks (old school 185 and 180mm). Recently I have a bike with shorter (175mm) cranks. The feeling is very different. My second ride with the bike resulted in "exploded" legs after 75km. You really need to adjust to it timing wise. My cadence did not change (94rpm average). In several studies they changed crank length and tested in a short period of time. That would never give a good result.
Excellent video. Love hearing the workings of someone who knows his stuff. Had a quick look through the title of previous videos to see if there's interesting stuff there. Saw the name Hambini so instantly subscribed.
Really enjoyed this, cracking explanation. Solidly de-mistifying bike fitting. Subscribed!
Tri dork here. Loved this video! I found your data on hip angle using saddle position very interesting. Thanks for making this. I bet we could make a small smart phone app doing something similar here. Or maybe there already is something on the market that's easy. 🙄
Noice! Have you considered or already done the 5 linkage version, adding in the ankle-to-cleat link? The whole "cleat under ball of foot" rule of thumb has also been subjected to actually doing the science and thinking about it from a biomechanical POV, with cleats being closer to the midfoot showing some real benefits, such as increased foot stability and reduced calf loads/fatigue.
This was hilarious AND informative. I wish I could like this twice! Please do more of these Myth-debunking analysis videos for other topics too!
Great video, very informative and I love the use of engineering to debunk cycling myths.
Did you ever make the follow up video about force? I couldn't see it
Great video ! With the reduction of crank length, should your model not also increase the seat height since the maximum extension of the leg is correspondingly shortened when the shorter crank is a the bottom of the rotation? That would open up the hips even more.
Martin Anderson very true 👍
1:30 and 16:30 mentions that.
It does but you must consider that as one raises the saddle the setback incrementally increases too if the axis point is fixed so if you bring the saddle forward your hip angle is the same.
Excellent analysis! Thank you also for the very clear presentation.
Easily the best explanation of crank length and hip angles I've ever seen. I'm gonna try these adjustments and comment in how I feel. Thanks man!
Cheers
Great video! I would like to add a few comments and thoughts though. With the shorter crank installed, the saddle should be raised at least the equivalent amount of difference. This would open up the hip and knee angle further. Additionally, the angles can be opened up with a more rearward cleat position. The other factor to consider, on a road bike, is depending how far the saddle is moved forward, this can result in pressure and discomfort in the hands, wrists, shoulders and neck and effect breathing.
There of course many other changes to be made, as a result of what I have listed above - "knock on effects"
Enjoying the videos, keep them coming!
I would love to see how an oval chainring plays into this equation and also the other interest would be how cadence speed would it should change to create the same output. Great clip.
it's neat to see the benefits of shorter crank lengths and I'd be interested to see how this could be adapted for MTB use where shorter cranks have inherent value anyways- fewer pedal strikes/lower BB for cornering aggressively. I wouldn't be entirely surprised if this is the next portion of geometry that gets pushed to the practical limit (like head angle, bar length, top tube length and stem length already have).
I'm 186cm but prefer 170mm cranks to 175mm due to bad knee (yeah, most people say I should use 172.5 - 175mm). Never thought about angles, but a friend recommended that I try shorter cranks to see if it helps with knee pain. I was sceptical at first, because in my simple mind, shorter crank means more torque and worse pain... But it worked!
Your video explains a lot why it is so. I might experiment with saddle position as well.
Thanks!
Id like to see you do a force vs. Femur/Tibia angle at the pedals. That coupled with the crank length should give you the best idea of optimal crank length to run.
Fantastic analysis, thank you for posting this.
Enlightening! This is the best analysis of crank length found thus far.
Watched this video and would like to comment on your CAD model/dynamic simulation. In that model the angle between your ankle and lower leg is held constant, which you do mention at approximately 12:25. For your CAD model what you should do is constrain the angle of the foot to the ground, not to the lower leg. If you watch riders from a side view you will see that through a full pedal rotation, the foot is held at 20-30 degrees to the road surface. This varies throughout the stroke, and if you could model this correctly in your CAD skeleton, you would see different results in hip angle especially at the top of the stroke (330°-60°). See if you can find videos of experienced cyclists on trainers from a side view and you will see what I mean.
I commissioned a custom-built frame with 80° STA and paired it with a 152mm crank. It took a little getting used to, but found no downside other than it forced my seat a bit higher, which made the seat-to-grip height difference a bit worse. I also had to switch from using RPM to using "foot velocity" instead. Thought cornering would be worse, but instead it actually felt like I got more consistent at them. In my head, I thought my legs were doing more of a rapid "piston-like" movement, like stair climbing, and speculated that this kind of position would benefit my fitness on foot (running), which made me wonder if triathletes were going after the same thing. Made me question the whole idea of going for different positions to rest overused muscles was even wise, compared to just giving the most used muscles even more training.
Great analisys, would you go shorter for road. Do you use 175 on TT as well now?
You could model the next video on a selection of frames in one series. Then mark out the heatmap of where the effective seatpost could even be, given the extension of the fore-aft, and similar for reach given available stems, and the same for crank length. What does that look like with a different colour for each frame, if all frames are stationary at the BB?
Not only do some bike models have the same geo across their range, they also sport the same seat, stem, cranks, and gears (and hopefully BB height for the comparison). That seems wanting.
Really interesting. I'm trying to square it all with the fact that I just perform better on longer cranks (180mm). Anecdotal - I've got a better jump, I don't need to get out of the saddle coming out of tight corners, under effort I think more about muscle recruitment and ankle flexion, I'm thinking round the pedal stroke, ....and I really like the wider range of motion under light load for recovery. I'm going to try the CAD modelling, there might be something else going on if you remove the foot/tibia approximation and make it a 5 bar linkage. I think the ankle acts as a fluid coupling and damps that peak acceleration. Like you say, hip ROM is a red herring, there's lots of other stuff you can do to solve that.
This was a very useful exercise - it would be great if you did a follow up or two
I'm particularly interested to see a comparison with Greg Lemond's method of setting saddle height from the BB
I went from 170mm cranks down to 165mm in an effort to improve pedal stroke smoothness when in the aero tuck position. Now I know why it works!
Floydie I did the same! I even feel more confortable breathing while riding on the drops.
I also went from 170 to 165 at 5’8”. It made a world of difference, The typical crank lengths are wrong.
When I was racing off road in the 90s, everyone was using 175+mm cranks, but being under 5.5 ft tall with short legs, I found 175 cranks just too long. I switched to 170s and never looked back.
As I aged, I noticed that my warm-up for rides to the point where I could accelerate with the group was taking longer and longer. I am 6'4" tall and have used 175 cranks forever. I decided to try 172.5's, and that warm-up period more or less disappeared. Also, my pedal stroke up very steep hills smoothed out. Since I am about to turn 79 (tomorrow) and over the last two years, my power has almost disappeared, so every little bit helps. Over the past 12 months I've ridden 5,000 miles and climbed 80,000 feet which is about one third my normal climbing (last winter destroyed many of the hill roads) I have been unable to easily find the climbs best suited to me. Plus, with my degradation of power, I am too slow to ride with the group and so have to ride solo much of the time. But the improvement in power with the shorter cranks. I am able to continue even on the worst sections, though I am crawling over the steep sections.
Great analysis. I think the main problem is that road bikes are still way too conservative on their geometry. Crank length is one point - but the much larger problem is that road bikes seem archaic when it comes to findings on what is faster on mountainbikes, think Mondraker Forward Geometry or Pole bikes - mow more or less implemented in all bikes. Ideally we want much steeper seat tubes - maybe 76-77°, maybe even 78° as on Pole bikes (though then the more you use your bike for climbing - the steeper you like your seat angle), longer reach but shorter stems, and much slacker head angles (and yeah wheel base will rocket from the paltry 99cm to 110-115cm or so - notice enduro mountainbikes have arrived at 130cm already for a size large). The main problem is the handlebar width - going 10-20cm wider would likely be really beneficial fore most going uphill, but really inefficient in the wind. This would resolve a lot of too much weight on hands/shoulders problems. Someone really needs to think forward, forget UCI norms, and look at things that made mountainbikes fast. Then get people to train on a new geometry for some months and compare speed and comfort. Some things from the mountainbike side may not work on roadbikes (for sure no 75-80cm wide handlebars) but a lot will. To counter the slacker head angle and weight distribution - the chainstay length will need to grow too - however road bikes likely need much less weight on the front wheel vs mountainbikes - so shorter CS = less weight on your arms = more comfortable. And yeah if crankarms reduce in size, the long needed evolution of chainring sizes going smaller needs to continue too. Average Joes tend to run way too big rings up front instead of spinning faster and trying to choose a small ring that allows them never to drop below 80RPM on their steepest common climbs.
It's crazy how conservate and stupid a lot of the bikefitting is on roadbikes - trying to fit people onto frame geometries that are simply wrong from the start. And yeah it needs a radical departure from the status quo. Most of bike fitting is simply an reaction to wrong bike geometries.
I suspect: What the acceleration peak allows is for you to drop the toe a bit through ankle flex. The toe drop absorbs the acceleration which works perfectly to begin the pull up cycle. If so, that could be an advantage.
This is a very interesting analysis thats is very well done, I understand the constant angle flection assumption as adding more DOF can drastically change the complexity of the analysis.
One thing I was thinking whist watching is that muscles have different force production profiles depending on the amount of flection at the joint (amount of stretch on the muscle), and although such impediments as hip impingement is a first priority, to maximise performance with a force analysis, consideration to each muscles force production profile is needed for an accurate depiction. I have an inkling that the ratio of force production between the glutes and quads at their respective angles would contribute towards a total picture.
Again great work, and thanks for the video.
Hi, thanks for the comment. You're completely correct I think. It just highlights how complicated an optimisation problem it is, there are sooo many variables.
Great work. It shed some light on crank lenghts. Thank you.
I switched from 172.5 to 165 cranks a few years back for the reasons you so ably have shown in your presentation. Your presentation is the best I have encountered so far on this topic and validates my decision to switch to shorter cranks. Good to know that moving seat forward also helps to open hip angle. Maybe you could have a look at oval chainrings too, another controversial topic. Cheers
Thanks Norman. There are lots of assumptions and its a simplified model. Mixing engineering and biological/chemcia/neurologicall functions will never truly align.
Great video and discussion thx.
When you say you raised the saddle with the shorter cranks in Case 1, how much did you raise it and why? Are your normalizing for 'feel,' hip angle at the bottom of the stroke, or leg extension? If you were doing a similar kinematic analysis on a mechanical system, you wouldn't "raise the saddle." It's only when I started fitting using digital motion capture tools a decade ago that I realized the futility of this. I had spent the previous 20 years of my bike fitting career fixated (as many fitters are) on 'optimizing' leg extension for the pedal at the bottom of the stroke. There's not much power being generated there. Takes a bit of work but we can now test for different FTPs at different saddle heights, and most of the research finds that within a ~2cm band, power output doesn't change much (just as it doesn't change much for crank length). Raising the saddle also erases the significant aerodynamic advantage you'd get from the shorter crank (and resultant ability to lower the upper body while maintaining hip angle constant).
Thank you for the video, as you shorten the crank length you have shown that there is a realized benefit of reduced peak torque, but you can shorten the crank length only so much. When does the crank length become too short such that it becomes a "negative" effect or change on the riders effectiveness?
Earlier this year, after watching this video, I went and changed the crackset on the cheap on one of my bikes from 172.5 to 165 mm (for about $40). I tried this out for a few weeks and it felt less fatiguing overall and enabled pedaling at about 10 rpm faster.
It also alleviated saddle discomfort for me, especially at higher cadences.
Hey I'm now considering the 165 for that exact reason.. may I ask how tall you are?!
@@willappleton I'm 5'10" w/ 32' inseam and my shins are longer than my tibias, 17' vs 15'. Sorry for the late reply.
stunning presentation and analysis
Great piece of work, thinking and visually attractive presentation. OK it's a model. There are assumptions. But it's interesting to see what can be derived from that, knowing that it's not a perfect repesentation of reality (yet... :-) ). Curious to see what your real world experiences are with the 165mm cranks. Look forward to a follow-up!
Interesting about the power curves around 16 mins in - my return to road bikes came via a TT rig after 25 years on MTB’s or various types - it came with oval rings - when i bought a “proper” road bike so i could group ride (did’nt happen in the end) i ended up after a painful “proper” bike fit - Retuel scrapping most of it and sort of replicating my TT setup going from a layback post to straight and a short nose saddle - $$ is only reason I’ve not swopped out the TT/road cranks from 172.5’s to 160’s but it’s on the cards for this year
Back to to the ovals - my own experience has been they smooth out the pedal action - with my mtb running a 32t up I tried a oval and immediately removed the chop chop chop on a high cadence on a flat ride - no science here just my own thoughts and experience - new subscriber too
Cheers! What do you do if you have a massive difference in femur and tibia length? I am 172 cm tall with 45.72 cm femurs and 38 cm tibiae. I use 170 cranks. On all my bikes I have to jam the seat back with set back seat posts to be comfortable or powerful. Any thoughts?
+1 for the constraint based sketches! +1 for the consideration of acceleration! I’m cringing while thinking about my femur crashing into my tibia after the pedal upstroke. Perhaps the reduced acceleration profiles associated with shorter cranks could simply reduce knee strain?
I'd be really interested to see you do some analysis on cleat position (and foot size I guess) if possible. I'm the same height as you and have big feet, so cleat position in concert with crank length feels like it makes a big difference - moving them back both allowed me to move my saddle forward and down a bit (no more toe pointing). Discounting contribution to pedaling from my calf muscles, moving cleats back I think has a similar effect to shorter cranks. And on size 14 UK feet, a tiny amount of toe pointing can make a huge difference.
I would say it's slightly harder to accelerate hard & sprint, especially off the saddle feels a bit odd - but it's fixed some toe numbness.
After watching the CAD demonstration, curious what effect changing seat tube offset would have on opening up the hips. Lot's of stock bike builds these days come with a 20mm offset, in my case an SW SL6 Tarmac. If the goal is to open up the hips, would it be smarter to change seat tube offset, before crank length?
very nice! have you looked at the premature wear of the tibia's meniscus wear and displacement of forces with different seat/seat angle/ fore-aft positioning?
Very well illustrated, love it someone shows the numbers.
Beautiful Newtonian Mechanical explainstion....
Awesome stuff mate! Makes me think about my own bike fit again.
When you said inertia, my ears pricked up. I am only 6'2" but my legs are massive (and long). I have gone from 175 to 172.5 cranks and have my seat slammed forward. I feel I get a more integrated comfortable feel. The problem is when im at battle speed, im often at 105 - 110rpm for long periods. This has got to take a tole on my huge legs. Interestingly the only time Iv been able to finish with the chain gang at the end, after 40 minute averaging 27mph, I was running 170 cranks on a single speed with a bigish gear.
@Alien On a Bike lol poor thing. It's people like you who get sheltered all the way across France, being dragged along by decent sized men up front. Only kidding I'm just jealous 🤣
This is incredibly helpful, thank you very much.
I use Rotor 155 with Rotor ovals. I also have a 76.5 degree seat tube angle with my saddle 10% down..It's incredible how much more effcient that combination is.
Great insights. I've been using "trail and error" method - with my smart rollers and Zwift. Each time I tweak one piece, saddle, seat post, cleats, bars, oval rings, I was seeing up to 20 watts + on FTP. And then you have different bikes frame geometry too. I am hoping I have close to ideal for my body now - but always interested to explore more.
Fantastic video, thanks for this detailed analysis
Subjective comment: I’m 5’9” / 175 cm and in my late 50’s. I live on a small mountain range where my climbs average 5 to 9% with gradients of 14 to 18% common. I’ve had several road bikes in size 56 cm and they all came with 172.5 mm cranks. After reading and watching about crank length I went with 170 mm on my last bike purchase. I couldn’t be happier. The difference when climbing was immediately noticeable. I wasn’t straining as much, but I was spinning faster. My times going up remained about the same, but I feel better at the top. Thanks for the informative video.
Hi James I've always thought getting the right riding position is an art, but the science is all new to me. I would have immediately guessed that a longer lever / crank would make spinning easier, but I suppose that's where ergonomics comes in! Found your comment very interesting 👍🏾
yours video is still the best today , real data always stands time
Nice video, may need to experiment with seat position more. Was set by the basic plumb line knee over the pedal method by my local bike shop and hardly altered since.
#PeakedTorque Did you account for the additional hip opening from a shorter crank because of having to lower the saddle because the foot will be higher at the bottom of the stroke?
So here’s a question if someone was running a large single speed chainring 50T would you run shorter (165-170) arms or longer for less fatigue and better power or larger for more torque?
Well present video, thank you. As a touring unicycle rider the subject of crank length is always interesting. Road unicycles always use much shorter cranks than bicycles, mostly as being fixed gear it gives the best compromise of torque and speed. Gearing can't be changed but crank length allows for at least a change of leverage for different riding conditions, styles and rider's strength/skill. I run 110 mm cranks on a 29" unicycle and 127 mm on a 36" unicycle, others go much shorter down to 89 mm. It makes hills harder but cruising far less taxing and even a little faster, perhaps because the angular velocity allows for an easier passing of the dead spot at the bottom on the crank stoke. The issue I find hard to compare is rider position. There are no "bike fits" for unicycle riders! The basic position is much more directly over the crank hub with little room to vary seat position fore or aft and a very open "hip angle". I believe it's still be same 4-bar linkage so perhaps the mechanics are the same as on a bicycle. Maybe you can comment on that? Again, thanks for interesting video.
I don't think short cranks make unicycling any easer/faster due to the dead spots, I think it's simply that you are limited more by cadence and how fast you can move your feet, than you are by the amount of force you can apply to the cranks.
to do 20km/h on a 29" unicycle you're going to need to do around 145rpm.
You don't need much more power to do it than you would on a bicycle. If you try and ride your bicycle at 20km/h with a 1:1 gear ratio and the same 145rpm, you would be happy to have tiny cranks there too.
Superb. V interesting. Nice CAD idea, first thing I thought was surely it is a 5 bar linkage system ( to include foot link and ankle pivot) and glad you pointed this out at 12.22 that you assumed ankle flexion is constant and I think this is the biggest flaw in this model; would have been interesting to see the numbers where the foot is kept horizontal and ankle flexion allowed - think it would reduce the minimum hip angle differences. Also note how you have the ball of foot pivot is not perpendicularly below knee pivot at the 3 o' clock position of the stroke, as bike fitters often recommend. I will try your CAD idea however - think it is great. Would love to have seen the follow up about Forces investigation but couldn't find. My experience with shorter cranks were worse - you needed to go one bigger rear cog and therefore increase rpm to match the longer crank.
Just by curiosity, have you taken into account the muscles that are engaged given the crank length? I reckon it might have some effect on power. Like muscles might flex just a bit more to get more juice out, or some crazy shit like that. Just a thought
Ive got a shorter fenur to tibia to mist oeople which had ne trying shorter cranks but in the end i went to a longer crank. I positioned my saddle forward and raised my bars 10mm and my saddle 5mm. I have a linger longer extension but leverage has increased from 12 to 4 oclock. I find my out if saddle power is greater.
just watched this ...very interesting, what effect does crank length have on the knees, just recently having problems with my left knee, well both knees but left mainly. Seem to be getting fatigue in the vastus medialis I think. I do climb a lot though . I ride Giant tcr adv sl Rotor oval 172.5. Wondered if 170 or 165 would be better im not that tall about 172-173🤔
Another excellent presentation. Can you add the effects of eccentric chain rings and perhaps, the variable crank arm phasing geometry BB? To this analysis, please?
Don't forget the beer belly effect! For a lot of riders I see, reducing the hip angle means that the knees don't hit the gut LOL.
Very helpful video. Much appreciated
Looking forwards to the force analysis, because I imagine within a range of gears, it needs to also be within range of motion, and in the powerband of extension relating to muscle-groups, for all of what is in this video. I think it isn't discussed, because sweetspots of combinations delivered don't lend themselves to the limited range of not only cranks, but also frame-sizes, made worse by how it sometimes in reality is just one model scaled up, down or both.
Bravo - separating fact from fiction! I'm adjusting my seat height and saddle position tomorrow! But I need your torque analysis - please. I'm 65yrs old, 186cm tall, recovering from covid pneumonia a year ago which totally sacked my lung capacity and muscle mass. About 4 months ago I changed my crank length from my traditional 170mm (sometimes 172.5mm) to 175mm crank. I use a hacked drive train (48/38/28t x 11-50t). I’m not a weight weeny by nature, but I have managed to reduce my Surly Straggler down to 21.6 lbs from 28.5 lbs with phat 38c tires. With the longer crank arm, I feel more power and can accelerate faster in shorter periods, and I can keep peddling through varied flats and steeper slopes more often and for longer periods of time. I watch my cadence and it's easier to keep it in the 85/95 PRM range. The biggest plus of all is I get outside and away from a trainer in a room watching a flat screen, which I hate. So far, I've regained about 75 percent of my former strength. My stamina is still the sh*ts though. I would really appreciate your crank length vs torque analysis. I understand what you said about crank length vs gearing, but I'm convinced a longer lever (crank arm) is a benefit across the entire gear range for sport riding vs endurance/road racing.
Looking at oval chainrings in conjunction with this analysis would also be quite interesting.
Looking forward to this as well
I have a question regarding the the change to crank arm length: Why did you not change the seat height by the same amount?
It is recommended to change the seat height by the same amount you change the crank length to reach the same effective seat height again. I see the value of isolating a variable in analysis, but in this case it does not seem correct to isolate this change when you will always change seat height at the same time. Further more so as in the next step you talked about the benefit of raising the saddle and setting it further forward. This would be the next logical step after reducing crank length anyway. Seeing bikefit as a system, where one parameter changes others at the same time, it seems to me, that it is not advisable to isolate changes for analysis.
The first online evidence based biomechanical analysis I met online. Cheers dude. Great job. The tool seems really interesting.
Just when i think about the acceleration peaks (in angles where there is not much force), maybe it explains the tendency to grind more with longer cranks? On the trackbike i reach higher cadance than on the roadbike with the 7,5mm longer cranks. Maybe also „rocking“ the hip left and right can be explained with this phase of low power / high acceleration.
Very well made!
Love this channel, deserve more more subs!
really interesting - also worth considering the positional change associated with shorter cranks, what i mean is; shorter cranks would mean saddle can go higher maintaining the same leg extension angles on the bottom of the pedal stroke - at 6 o'clock, and consequently compounded effect on hip angle of increase saddle high and shorter cranks as explained ?
When is part 2 coming? - Personally what would you go with if you had the choice. 170mm or 165mm? GREAT video btw! Best wishes Malte
could you run where power produced is constant, meaning as the the crank decreases in size, pedal speed will go up. accelerations may become the same
Thanks,I shall follow with interest. Another aspect: what if you have different leg length's and/or variances in tibea/ femur? E.G: my right femur is 22 mm longer than left,while the right tibea is shorter. Overall my right leg is 37mm shorter. To complicate matters my right ankle has a fusion and alignment issues,making for complicated bio-mechanics.
5' 7" (30" inseam) and I find 170mm cranks too long. I've been experimenting with shorter kids bike cranks, 152mm to 130mm, and find that 1mm ends up equaling about 1RPM of perceived cadence. (85RPM @170mm feels like 103RPM @152mm)
The cadence where I can hold my FTP at the lowest heart rate is about 95RPM. That seems to line up with the dynamic model that I should probably use 160mm cranks. Too bad they're such a PIA to find.
Very interesting experiments. Good to see someone trying the extremes. Cheers
@@PeakTorque I managed to get some 160mm 105 Cranks. Instantly increased my average cadence by 8 rpm. Power is down a little because I'm not conditioned to the faster cadence. (Fast twitch fibers in the quads are on fire). Way more comfort, and simpler to find a good fit.
Interesting approach and conclusions. At what cadence did you do the simulations to generate the velocity and acceleration curves? Did you account for the fact that pedal speed is greater for the same cadence with longer cranks? That's probably relevant, because higher muscle contraction velocity concurrent with greater pedal speed entails lower muscle force capacity. That means, in reality, one is likely to gravitate toward a higher cadence with shorter cranks to maintain the same (subjectively optimal) balance between muscle force and contraction velocity (i.e., pedal force and pedal speed). Just wondering it you did, or could, correct for that, and if so, whether the differences in peak acceleration would remain so large.
Is there some way for the femur to move in simple harmonic motion?
Nice one, cheers. Interesting software used to demonstrate, is this something more than the standard solid edge?
Great engineering here.
I was looking for someone to do this using Adams with all forces.
Matching the forces with the muscle fiber to show really fatigue. As small fiber will be fatigue faster with similar force compared to large one.
Thanks. Enjoyed the video.
Adams! Gosh. Not used that for years. Forgot it existed. For the motion plots i put the equations into python. Cheers
Outstanding presentation.
Waiting patiently for Part 2 :)
I have some sort of nerve problem possibly affecting the muscles in my right foot. It needs an orthotic for support.
My ankles are the only hypermobile joint. The right hip is bothered for some reason, too.
What would happen with a 73° seatpost ? I would go to 165 from 175. My femurs are 52cm long. I'm 189cm tall.
Any thoughts?
I went from racing cat 4 and 10000km/year to lots of intermittent pain this year.
Great video, as always. Thak You
I had always wondered the effect of crank length for my neck! I'm fairly flexible but my neck has issues and wondered it crank length would help. And I'm a bit cheap on getting a proper bike fit.
Changing the crank length for neck issues is wishful thinking and expensive. First assuming that we're talking about road bikes if you have a visor on your helmet - remove it. Now. Visors are fine for mountain biking and such where you have a more upright posture but not for a road bike. After that I'd look into handlebar position. Usually this means that you want your handlebars to be moved higher and/or back.
How is your neck doing? I used to have more frequent neck aches than today. I find swimming really helps a lot to make it limber and any cycling helps the back overall if done for enough time.
@James Medina unfortunately I don't get on the bike as much as needed to properly adjust. The neck has gotten better as I adjust my fit. I've found that a zero setback sestpost and shorter stem are pretty much a must.
@@markmarlatt1105 give things time. You could get more limber or tighter, but one things for sure : you could probably ride more👍
Nice presentation. Other key points I would consider....
One of the major reasons given for shortening cranks is the theory that it helps in more endurance focused events where power outputs tend to be relatively steady state (again, IM distance triathlon is a good example). There isn’t a huge amount of scientific evidence to back this up, as anything that involves testing whether a bike fit or equipment change increases failure time is inherently really difficult to test (there are just too many other external factors that might influence fatigue rates). It is pretty easy to dynamically measure closed hip angle (most bike fit motion capture tools do this). It’s all very well measuring the angle (or calculating what it might be with a tool like CAD) but that doesn’t tell you what hip angle any given rider can cope with over the course of their chosen event. It is possible to measure how the rider’s centre of pressure moves in the saddle throughout the pedal stroke (the pelvis can become more unstable in cases where any link in the kinetic chain (hip, knee or ankle) reaches, or is close to reaching the limit of its range of motion). Changes in pelvic stability due to any bike fit change including crank length can also be assessed using Inertial Measurement devices (mini gyroscopes and accelerometers) attached to the pelvis. These are especially handy as they can be used out on the road, not just in the lab/fit studio.
Another hugely important thing to consider (and this is where using CAD or such like really starts to struggle) is that the human body (even of a highly trained elite athlete) is far from being a perfect machine. Accurately measure knee, hip and ankle angles for 100 individual pedal strokes and you won’t see two pedal strokes the same. Throw into that mix that a key component of fatigue (as well as reduction in force production) is a loss of motor skills (the ability to fire the right muscles in the right order) and it’s easy to appreciate that rider motion can be very different at the end of a long ride than at the start. This is why we feel like we’re pedalling squares when heavy fatigue sets in.
A practical way of testing what crank length might work for you is to find a bike fitter with a fit bike that allows seat angle, bar position and crank length changes to be made. Once a position is found that’s approaching “optimal” (sorry for using that word as I’m a firm believer in that it’s unlikely there is a single fixed position that’s actually optimal). Pedalling one legged while varying crank length (and altering seat height to maintain the same leg extension at each selected length) will give a good guide as to what length “works”. The rider should be able to maintain a fluid pedal stroke when riding at low cadence without having to excessively rock the pelvis. Most people have one side that’s more mobile than the other. The trend in bike fitting at the moment is towards practical outdoor testing. Expect to see more fitters offering the opportunity to test crank length changes out on the road as part of their bike fit services.
Sorry for rambling. Hope this helps.
Cheers, great comment and insight.
A guru bike fit can adjust all these metrics.
bryn green Choose a fitter as opposed to a branded system when you’re looking at bike fit options.