## Is tyre slip caused by power or torque?

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Ciro Pabón wrote:So, another answer to Roland clever point could be that you can have large torque and produce a large force but if your wheels doesn't increase the rotation rate more quickly you will lose. The original question is about slip, not acceleration (although I know the former brings the later, so...).

Again

Belatti wrote:"The power injected by a torque depends only on the instantaneous angular speed – not on whether the angular speed increases, decreases, or remains constant while the torque is being applied"

I think everyone is messing all up with this.

Power and torque being in the same lineal ecuation and cars having gears, who gives a damn about one or the other?

A tyre, like everyting else in this world is a mass-spring-damper system. Or a combination of several of those systems if you will. Well, the longitudinal "slip" component of the M-S-D in the middle between rim and the ground has coefficients in its ecuation that can describe friction characteristics. The whole system, like any other M-S-D one, is sensible to speed and acceleration.
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
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Joined: 10 Jul 2007
Location: Argentina

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Thanks, Belatti, very clear, my mistake. So, what's the answer to Roland point? I did not understand.
Ciro
Ciro Pabón
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Joined: 10 May 2005

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Firstly I would tend to give you an "Autogyro" kind of answer: its a very complex system and there are a lot of assumptions made to linearize and simplyfy the problem.

But, as I dont want to receive any kind of telling off from our respected moderator, Ill add that Friction factor changes with rotation rate changes and a 1000 of other things are happening a that moment, like clutch slippage, rpm falling and so engine power delivering fluctuating.

Does an electrical system fails because of over current, over tension or over power(wich is both of them) ?
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
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Joined: 10 Jul 2007
Location: Argentina

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rjsa wrote:Not it doesn't. Two absolutelly different things. From 5K rmp to 7K rpm on the same gear you are going 40% faster on the same turn, so it's much easier to upset the car when you are faster.

Hmmm, quite right

Ciro Pabón wrote:
Alejandro, I'm changing the host of the image and editing your quote of my post. My host wasn't working when I posted and the quote was too long. Thanks for your kind words, I might be wrong, I'm not a professional, let's wait for the "big guys" judgement, it's just an opinion.

Thanks, can't tell you if it worked cause now i'm at home though
So looking at the graph i'm back to no longer being so sure if I understand the whole thing, i guess i'll chew on it for a while. The one thing that i thought i should point out is:

Ciro Pabón wrote:That is, when your car is in first gear, for every turn of the engine, the axle at the end of the gearbox turns 3 times and your differential box also turns 3 times per each revolution of the gearbox, for a final relationship of 9 to 1.

I think you meant "for every turn of the engine, the axle at the end of the gearbox turns 1/3 times and your differential box also turns 1/3 times per each revolution of the gearbox", which would then match your next paragraph which of course is correct

Ciro Pabón wrote:In English: every time your engine turns 9 times, your wheels turn 1 time. That would be more or less typical for the kind of cars I drive.

engineguru00 wrote:I will try to put up some of the calculations and plots that I have done to show what my post said earlier in more than tire dynamics fundamentals.

Thanks,along with Ciro's reply and some others' it seems my impressive failure at grasping the relationship between force, speed and power may bring interesting contributions to the forum from other members
Alejandro L.
alelanza
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Joined: 16 Jun 2008
Location: San José, Costa Rica

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Unfortunately, I couldn't find any of my tire plots to throw up here. I'll have to call my suspension guy later to get them. As for the relationship of torque, force, vehicle speed, and power, here is my stab on this. Before I get started, it should be clear this is a very simplified look at the system with a lot of assumptions and your physical vehicle won't behave nearly as well as this would suggest.

Just to lay out the units:

Force (F=ma): N
Torque (T=Fr): N-m
Power (P=W/t):J/s = kg(m^2)/(s^3)

Through the Work-Energy Method. Assuming the internal energy of the car and potential energy are constant and that losses are negligible(cleans up calculations for this example).

Work=Change in Energy

Power*Time=.5*Mass*(Change in Velocity^2)
(Change in Velocity)^2=(2*Power*Time)/Mass

Change in Velocity=sqrt[(2*Power*Time)/Mass]

So from this, More Power=Better Acceleration. This is true, but it only works once the inertia of the vehicle has been overcome and that is where the longitudinal force from the tires comes into play. This is why you very often see high torque cars beat a high power car off a line, but the high power car tends to have the advantage once the cars are both moving. Hope that helps clear some of that up.

EDIT: Just to do the similar equation for Torque.

Force=Mass*Acceleration
Change in Velocity/Time=Force/Mass
Change in Velocity=Force*Time/Mass

Yet again way oversimplified, but you get the idea. You can calculate the change in velocity from both values as they are actually both the same unit (HP=RPM*Torque/5250). One is simply the actual force applied while the other is the rate of work done to the system.
Last edited by engineguru00 on Sat Mar 20, 2010 12:57 pm, edited 1 time in total.
Chris
UB Motorsports: Formula SAE '08-'10
Powertrain Team Leader '08-'10
Captain '09-'10
engineguru00
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Joined: 18 Mar 2010
Location: Buffalo,NY

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For the high power car add some lower gear ratios or an electric motor.
Sorry
autogyro
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Joined: 4 Oct 2009

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max accel G's and max force applied to the rear tires will always happen at peak torque. torque is a measure of force. hp is a measure of power (force over time)
drivinhard
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Joined: 24 Mar 2010

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drivinhard wrote:max accel G's and max force applied to the rear tires will always happen at peak torque. torque is a measure of force. hp is a measure of power (force over time)

Yes but it depends on when this occurs relative to engine rpm and the gear ratios between the crank and the tyre. Low ratios magnify torque and high ratios reduce it.
Sometimes maximum bhp at the engine results in maximum gear effected torque at the tyre. So in this case is that max torque or max bhp?
autogyro
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Joined: 4 Oct 2009

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autogyro wrote:
drivinhard wrote:max accel G's and max force applied to the rear tires will always happen at peak torque. torque is a measure of force. hp is a measure of power (force over time)

Yes but it depends on when this occurs relative to engine rpm and the gear ratios between the crank and the tyre. Low ratios magnify torque and high ratios reduce it.
Sometimes maximum bhp at the engine results in maximum gear effected torque at the tyre. So in this case is that max torque or max bhp?

Sorry, I was looking at it in the case of using a given gear (any gear).
drivinhard
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Joined: 24 Mar 2010

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There seems to be a lot of confusion as to the role the engine and drivetrain relate to tyre slip. But the original question doesnt mention either.

Therefore I suggest we look the the torque/power at the WHEEL and forget about the engine.

Here, the slip is related ONLY to the torque.
1. If you change the wheel torque independant of the power, the tyre slip will change.
2. If you change power but keep the torque the same, the wheel slip will NOT change. (neglecting speed dependancy effects)

Therefore, the wheel slip is only dependant on the torque at the wheel.

The operating point of the engine will then be totally dependant on the gearing of the drivetrain.

Tim
Tim.Wright
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Joined: 13 Feb 2009

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It seems the same thing is being said by a lot of people, yet we are still having this debate. The answer is easy, tire slip is caused by forces that are caused by....

TORQUE!
Chris
UB Motorsports: Formula SAE '08-'10
Powertrain Team Leader '08-'10
Captain '09-'10
engineguru00
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Joined: 18 Mar 2010
Location: Buffalo,NY

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I agree but unfortunately that was not the question.
It seems we are in the difficult and unusual position of knowing most of the answers but failing to know the question.
autogyro
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Joined: 4 Oct 2009

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Torque is what causes the tires to lose grip. But as speed increases, it will take more power to achieve the levels of torque required.
A proud Canadian, and yes, HOCKEY is our game.
DaveKillens
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Joined: 20 Jan 2005

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autogyro wrote:I agree but unfortunately that was not the question.
It seems we are in the difficult and unusual position of knowing most of the answers but failing to know the question.

"Is tyre slip caused by power or torque?"

Seems straightforward enough to me.

Then again, I'm a tire professional.. tyres are a bit unknown to me.
Grip is a four letter word.

2 is the new #1.
Jersey Tom
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Joined: 29 May 2006
Location: Huntersville, NC

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Jersey Tom wrote:"Is tyre slip caused by power or torque?"

Seems straightforward enough to me.

Then again, I'm a tire professional.. tyres are a bit unknown to me.

Then do you sleep a lot, JT? (Apologies, I couldn't resist).
DaveW
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Joined: 14 Apr 2009

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