Power to Weight Ratio - Is it all that affects acceleration?
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- čas přidán 28. 05. 2024
- Today, we look at the physics of car acceleration, and if power/weight is the only thing that changes acceleration times. The problem is that the simplistic explanation of power to weight neglects aerodynamic forces, such as downforce and drag. Typically cars are the same sort of size, with similiar drag, however a heavier car with more power will be affected less by the drag component, hence the high speed acceleration will be higher.
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Having flashbacks of physics class
Thank you for this! I've been explaining this for years... Especially to people that hate on the Veyron. And of course one can get much more in depth, but the principles are pretty solid.
Koenigsegg is laughing at you right now
I'm trying to watch this at night when I'm tired. That's like trying to read with a bag of your head, it's frustrating and I can't do it
Nice Explanation! I've done very similar write-ups for the RC car speedrun community. It's good to see someone else preaching the same Gospel. :-)
power to weight wont make two cars at the same weight to power ratio accelerate the same , cause of where the power bands are on their motor and configuration of drive train , theirs a lot more that go into making cars faster and performing , like unsprung weight , RPMs where its in its most efficient power and how fast it can get to those rpms of efficiency , tires of how sticky they are or not depending if you a little bit of slip to jump rpms before they reach optimal temp for gripping , spring rates of your car to provide best traction for you configuration , damper rates , gear ratios , tire size in height and width , cooling system to keep your engine at its best temp for proper detonation of gases , and minimizing heat friction , areo dynamics , traction to you drive wheels , shift times , power to weight is great thing to to have higher HPvsWeight it helps your car a lot but to compare to very different cars off the same number is not something to really do and can be ideal for gustamating but it comes down to side by side , on the black top find out
Excellent!
Well said; I knew someone would have spotted that.
if the power to weight ratio at a given instance is the same. then, the acceleration would be identical. since the energy input to move a certain weight is the same. it cant break the laws of thermal dynamics. when you discuss gearing, drive loss and traction, then you aint talking about the same power output at all times, thus making the original question VOID
Nice explanation, thanks!
Very interesting! This explains what we see at Bathurst with the GT3 cars. They're balanced to similar power-to-weight ratios but we see the powerful, beefy Bentley and Nissan GT-R punching their way up and down the hill while the nimble Audi R8 gains time in the corners.
Great video, thank you
When doing this calculation do you use crank hp or wheel hp?
6:48 "very low top speed". I'm reading 270km/h. More than double the speed that is allowed in many countries and wayy more you can drive most of the time here on german Autobahn.
Great job keep going
Basically Traction rules off the line and aero dynamic drag rules top end.
That's why the AWD Chiron Beats the RWD Koenigsegg Agera RS up to about 250kph and after the High drag Bugatti is toast.
Need to see them run on a drag strip which will help the RWD Only Agera RS more....never seen that video Yet.
Traction is Key given the same hp/Lbs. That's how the $85k 840hp Dodge Demon out runs a $1,2million 987 hp Veryon that weighs the same.
RWD with Drag radials wins over AWD when wheels come of the ground.
Also why the Nissan GTR falls on it's face after 180kph. Traction isn't a concern and the Low HP/lbs slows it compared to it's competitors.
@@JerseyMikeP the gtr doesn't fall on it's face at all lol
so if im understanding this right and had a theory of it in the past by the way that a to light car in a dragrace will have less traction at start and therfore get slower acceleration then the heavyer car with the same amount of engine power due to weigt aiding in to get friction on the tyres and grip?
and the heavy car can accelerate faster att speed due to that drag and air resistance is more a factor to overcome with brute engine force then weight of the car?
In low speed they are basically the same.
In medium speed the lighter car will get more effect from downforce, only if the tires are still slipping.
In higher speed, the heavier car will get less effect from air drag, so they will accelerate faster.
Is that question really that "simple" to answer? I think you also have to consider the tyre load sensitivity which indicates that with higher tyre loads the µ decreases slightly. So this would mean that a lighter car would have a very basic advantage over the heavy car in longitudinal (as well as in lateral) performance. But on the other side (assuming RWD), a heavier car would have more longitudinal weight transfer in acceleration so maybe this would partially compensate for that (but at this point I don't know how the CG heights compare between a light and heavy car...). So don't you have to wrap your head around a lot more stuff on the suspension side?
P.S.: I like your videos very much, really helped me understand the functionality of some aerodynamic devices. Keep up the good work! ;)
Very good.
Awesome video. I was wondering if you could do one about the effects of increasing tyre size on vehicle dynamics.
For example 3rd gen rx7 comes with 235 front and 255 rear width tyres. How would putting 265 sections all around effect the balance of the car leaving everything else equal and assuming 50/50 weight distribution.
Side note I've read several times 50/50 distribution isn't ideal, and a rearward balance is better. What are your thoughts?
+bertanderny I'll add it to the video list, it's largely related to the contact patch dynamics, but deserves standalone video from my video on tyres!
Ideal weight distribution depends completely on drive type, and what you want out of the car. If we are talking fastest around a circuit, RWD should be slightly rear biased, FWD should be slightly front, and AWD can be anywhere around the middle. You can modify your suspension setup to then work around pretty much any weight distribution with reasonable results. Perhaps this deserves a video as well, as it gets much more complicated than what I've described here...
KYLE.DRIVES Videos on these subjects would be much appreciated. Thanks for the brief explanation though, makes sense when you think about it.
@@KYLEENGINEERS I have to disagree with your logic sir. Try adding 2,000 pounds (907 kilograms) of concrete in your OWN car. Tell me how fast you still can accelerate at higher speeds. And tell me how fast of top speed you still can reach. I can guarantee not only you would have slower acceleration BUT ALSO you would have lower top speed as well. More weight also means more ground friction and rolling resistance.
@@prandomable Please correct me if I'm wrong, but regarding your assumptions:
1. True only if you are not traction limited.
2. This can be compensated with tire pressure, top speed would be equal.
@@miguelmurua96 not true completely try. More weight = more rolling resistance as well. More friction from the road.
Weight won't affect top speed much correct HOWEVER it most likely will still decrease your top speed, but maybe only like 3, 4, 5 MPH. Which isn't even much at all
So it only significantly affects it mostly at higher speeds?
I always wondered why mini's fall off at high speed.
Very few "normal" cars are traction limited in "mid" range speeds.... So inertia is the slowing component. Ofcourse car with higher output even when heavy will attain bigger top speed, but it takes longer to get there.
you're a real teacher aren't you.....
i like you :DDD
How to calculate the speed of motor related to weight
You say that increasing weight won't change acceleration, but i've been said that tyre friction coefficient goes down when weight goes up. I'm speaking of "true" weight here, not wind downforce.
Is that a marginal phenomenon, maybe ?
Perfecty explained. I'm european, but I've understood everything. Nice job, Man! :)
can someone explain to me will a 2500lb integra with 450hp keep up with a 4300lb hellcat with 730hp? like someone break down the math
I am really looking for the same answer !!
I think it will win. My dad has a 500whp mr2 and beat a demon. It weighs 2800.
my bad the hellcat is 4500lbs
@@josiahbutler2254 I think if the integra gets traction it can win
Why did you join the two equations together in the first part of the video?
What combining these 2 equations really say is: The force accelerating the car is equal to the grip the tires provide. Incase of traction limited acceleration this is true: You will accelerate as fast as the traction of the wheels allow.
I have a question on my mind for some time that I can't really get my head around:
How do brake-fans work?
Like on the old 935 Porsches -or Ken Block had them on his car too.
And why are the no longer used/what are the problems with it?
Do they suck out air trough centrifugal force (how does that work on the front side where the air is pushing against it?)
Do they guide air into the wheel?
+Kalimerakis Looking at the smoke patterns on Ken Block's ones, it appears that they are using the centripetal force to draw the air from the centre of the wheel outward (much like a turbo compressor). Such a setup would likely be draggy at high speed, but is useful for brake cooling when the wheel rotation speed is high with respect to the total car speed (rally, gymkhana etc.), hence why it is only used in some scenarios. Would you like a video?
I would love to see a video on that topic!
Hi! Can You do a video on thrust curves, power and torque?
+Boiko Aladjemov I remember you had one on power and torque but people still get confused...
+Boiko Aladjemov You know, I don't think I do have one actually... I think Engineering Explained has one, but I can do one in my style if you would prefer?
+KYLE.DRIVES Yes, I guess I got mixed up. I think it will tie up good with this video.
Since you are saying a heavier and more powerful car will have greater Drag Coefficient than a lighter and less powerful one with the same p/w ratio, it should have an increased drag force according to the drag equation. Then how come it accelerates faster instead ?
And also, what is the "Drag Penalty associated with cooling" ?
That's a Lotus Exige you showed us 😉
You mean a Bugatti?
Weight doesn’t increase top speed, it’s the higher power that does.
If two cars have the same drag, same power to weight, only diffrence is one weighs more and makes more power
At 60mph when drag starts to take effect it will have a smaller impact on the car that makes more power.
And gearing of course...
power to weight ratio = the same at a given instance, then the acceleration is the same. ( IN A VACUUM)
the ratio can change, like a 10lb rc car with 1hp vs a 3000lb car with 300hp. the acceleration might be the same, but only on paper. once you introduce gears and air resistance, clearly this same ratio tells you more than just a ratio. the rc car with the same ratio will not perform the same as the car since the available energy is not the same.
Ok, I have watched it 3 times and I see what you mean. But sure the answer to this is not one dimensional. However when drag is significat, sure - more power will be better even if car is heavier.
Sir, I disagree. Lighter vehicles will ALWAYS be quicker and faster IF the power output at the wheel is exactly the same. More weight means less acceleration is because heavier and more massive object is harder to accelerate, heavier vehicles will also have more ground friction/rolling resistance. Try adding 2,000 pounds (907 kilogram) in YOUR OWN car and tell me how fast you still can accelerate and how fast you can reach, and try it with your vehicle empty. I can guarantee you without the added weight your OWN car will have a FASTER top speed as well.
The aerodynamic drag is all the SAME, whether you have extra weight in your own car or not. Except weight will probably make your car more stable at higher speeds. However weight will also reduce your top speed. More weight only means more rolling resistance/ground friction.
Exactly and sandbags are always available
@@Whatguywhat and concrete mix bags ;-)
For non-believers...
@@prandomable 😂yep! I got a 77 Vega I don’t use bags unless the track is poorly prepped or I’m on a street no running slicks but this guy wasn’t make a lick of sense with his nonsense you hit it right on the head with roll resistance and friction factor
Not sure how pedantic you wish to be but there are some very basic errors in the physics that you present.
1. Newton's 2nd Law is not F=ma: it is Fnet=ma where F is the "unbalanced" force. The engine does not provide Fnet. A lot of the engine's force is lost in the drivetrain friction, rolling resistance, etc, even at low speed when aerodynamic drag is negligibly small.
2. The simple expression for friction (mu x normal reaction force) is very inaccurate when applied to car tyres. If that expression was true then wider tyres wouldn't give greater traction than narrow tyres as the classic friction force is independent of contact area (there is no "A" in the simple friction equation that you refer to and use). What is critical for car traction is "mechanical grip", which is not just friction but represents the sum of forces acting between the tyre and the road surface.
3. "The mass of the car pulling us back"! That is a real howler. The mass does not exert a force or "pull" opposing the car's motion. The mass is simply a measure of inertia or resistance to acceleration.
4. "As we increase the speed the normal force increases". No. You have already defined normal force as mg. As the values of mass and g do not change with speed then N does not change (in the absence of aerodynamic downforce. If there is aerodynamic downforce then it needs to be added as an additional force term).
5. "The car that is lighter with less power has a higher component of drag on it than the car that is heavier"?? Drag is a force that is only determined by the coefficient of drag (Cd) *and* cross-sectional area (at equal speeds). Hence drag depends entirely on the shape of the car and not on whether the car is "lighter" or "heavier" or how much power it has. It is entirely possible that a smaller, lighter, less powerful car will have lower drag, or as you put it a lower "component" of drag and a higher top speed. "We assume the Cd is the same" - again, not the full story because the vehicles' cross-sectional areas also need to be taken into account.
6. The top speed comparison between the Lotus and the Bugatti is misleading: top speed is purely determined by when the net force provided by the engine reaches equilibrium with the total drag force (aerodynamic and other drag forces) - it has nothing to do with the weight of the car, and hence the power to weight ratio either.
Your conclusions are broadly true but I find that the reasoning does not rigorously support them. If physics is to be used - as it should/must be - then it should be without gross simplifications and half-truths. Or at least some qualification should be provided stating that some simplifications have been made which will not always give the results asserted in your video.
Apologies for sounding negative - I think that your goals and effort are nevertheless generous and praiseworthy.
NHRA drag racing ,Pro stock car and motorcycle. The car with 1500 HP weighs 2500 lbs is faster MPH & ET then the motorcycle that weighs 550 lbs and has 400 HP .Car 215 MPH @6.55ET Cycle 199 MPH @6.85 ET 1/4 mile
Why does a lighter car accelerate faster with downforce than a heavier one?
This only applies when the car is limited by traction. When the downforce starts pushing down on the car it will increase the traction of the wheels of the car, so the acceleration will increase.
Because it has less mass to counter to get up to speed
why do you put that F is equal to Fr? The friction is less than the force of the car, otherwise the car won't accelerate
He doesn’t address the issue of torque and the need to get revolving mass up to speed. It depends on where the weight is distributed so that if a car has very heavy wheels compared to a car of similar weight but with light wheels and a heavy body, it will accelerate slower.
Hmm. Interesting (no doubt) but incomplete...
Lighter cars (with same p/w ratio) are often narrower and overall smaller than heavier cars (I've seen a Veyron once at Geneva Auto Salon, as well as the 4 turbo'd W-16 engine, That's huge!) so the lighter cars could have less drag coefficient than heavier cars. But the light ones are not aero-designed for maximum top-speed, but for maximum agility. More downforce results in more drag, results in lower top speed. Above 300 km/h You'd need a huge amount of extra power to get a few km more top speed and there are not many streets or tracks to go flat out.
There is a good documentary about the Veyron, if I remember well he said that above 350km/h the Veyron need about 10hp more for every single km/h more.
That's why "usual" sports cars or supercars are seldom faster than 300...330 km/h, where the territory of Hypercars begins.
I’m in physics
U disregarded "force of engine applied via tyres as reaction force on car=F" altogether....wow
Eq of motion of car is
ma = F -F friction - F drag If we calculate force of engine from engine power then F friction = all power train friction + road tyres kinematic rolling friction
Power/Weight is the reason a nissan 240sx only 205hp can keep up with a v8 with much more hp
When he says traction limited does he just mean the engine's power capability is higher than the max possible frictional force? And then if so how do you eventually become NOT traction limited as you go faster?
When you change gears and have less wheel torque.
Ok so when i reduce the gear ratio, less torque is being applied to the wheels? Then I am just applying less force into the ground and moving more slowly isn't that right? I might be misunderstanding something.
It's confusing if you put it that way.
When you upshift, you will get less wheel torque but more speed. When you downshift, you will get more wheel torque but less speed.
You can test this with a bicycle.
Since in low gear you have more wheel torque, it's more likely get 'overpowered' so the tires will slip.
so if you could keep upshifting to reduce torque you could just keep going faster and faster as you can put more power into the ground before the wheels slip then? Why don't race cars do this? Is it just for reasons of cost/feasibility that car manufacturers don't allow you to upshift farther then you currently can?
Is all power/weight equal? Today we find out!
Well i'd rather accelerate sideways towards the axis of an arc than forwards :P
To make it simple, 400hp 4000lbs car is going to out accelerate 200hp 2000lbs car for 1/8, 1/4, 1/2mi runs and more. 200hp 2000lbs may or may not have quicker 0-60mph and maybe quicker to lower speeds. 200hp 2000lbs car will have better lap times on almost all low-mid speed tracks (circuits), 400hp 4000lbs car may have better lap times on tracks with many straights. 400hp 4000lbs car will have much higher maintenance as fluids, tire and pads wear out much faster. This is assuming you keep most other variables constant (driver skill and weight, same drive train configuration, similar front:rear weight ratio, naturally aspirated with similar power and torque curve, same or similar tires, same or similar gearing, same or similar drag coefficient, no driving aids interfering or assisting and so on).
Power-to-weight ratio is just a rough ball park measurement for circuit lap times, and I believe it is even less relevant in drag racing (probably torque, gearing and traction more important), well unless you are comparing near stock production cars for street light drag racing.
Interesting you say torque is more important than horsepower, especially after mentioning gearing is important hahahaha
Underrated comment
i dont touch other than one teme in your comment 400 hp 4000 lbs car will not neccesery have weake rtime on short track it depend on tire load sensistivity so size and componnent more or less and suspension setup period .
heavier car could transfer weight in the ssame way like lighter with proper more stiff etc suspension now with good tire on it watch TLS and compound lighter car have nothing better agains heavier on short track on powerlaps .
you lost me at f=ma
force=mass x acceleration
Yes
AMG - as in AMG Mercedes Petronas F1 team 👀
Would 500kg to 500hp be insane?
That's basically what Koenigsegg Says the One:One has. (1:1 hp/kg)
By comparison, A NHRA Prostock car is better...about 1,400hp, 1,066 kg. (1.31 to 1 hp/kg)
Top fuel is 1,052Kg...and currently said to be Over 10,000 hp. (9.5hp/kg)
BMW 1000RR by comparison is 199hp/204kg ( .975hp/kg)
Nah, that's just 2.5 supersport motorcycles. Honda, bmw, Ducati, whatever, they all got over 200hp at 200ish kg with fuel. Allthough granted, the rider ruins that way more than with a 500kg car.
0-100 km/h at 1g is 2.83. How can the Bugatti go faster than that?
5:24 is not an accurate statement
but isnt displacement a very importante factor here ??
It is, in the fact that you will probably make more power or P. BUT, Whether it's naturally aspirated big displacement or high boost smaller engine the overall Power number is what he's looking at. Look at large displacement of an engine more as potential to make even more power given that same mods as small engine.
No, With proper gearing a car with a smaller engine with the same HP that weighs the same will run the same time.
Depends. Imagine a super high revving engine, that does 150hp. 1200kg car. Versus a 150hp engine that has more cc, much more. Slightly heavier car. The heavier car will be faster off the line, because gearing wont help you gotta start at low revs, unless you got a particular torture resistant clutch. The torque at low rpm comes into play. ..... Buuuuuuuuut! Repeat the game with 1000hp. You cannot actually get it all on the ground from 0 anyways, so the torque the weaker low cc engine gets on the ground at low revs might still be more than possible to actually turn into acceleration, now the lower weight it has is better
Hey guys with the lifted truck/“sports car” watch this it’ll help you understand why your losing 😂😂😂😂.
With heavy car you will have more friction. You will loss more power.
You're not Jason
What kind od Engineering explained is this? Well at least this guy explains with maths. But he talks very fast
6:40
6:44
is it just me or the guy starts talking in another language 2:10
is that a joke or you don't know what the Greek symbol Mu is?
Mochi-san pretty sure it was a joke saying that he didn't understand
Hi.
What the fuck are you saying.
From,
Interested viewer
Thank you