Is tyre slip caused by power or torque?

[Jersey Tom]

...still goin with torque on this one.

[Belatti]

I will give you something to think about:

"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"


BTW, Tom, can you tell us something about this?

Edit - Was originally gonna include some thoughts on the claims of some engines to "give tires time to rest between cylinder pulses and regain grip" but I'm saving that for another day.

[safeaschuck]

Torque. Same for tyres as every other part of the drive train. And it seems as though Tom is alluding to the big bang theory currently finding much favour in Moto GP. If you are interested in engines there is plenty of print out there on the subject and although couter intuitive it seems to have been proven in real world conditions, some of it must have made it into these forums. Requies much more guts to implement on a V8 than a straight 4 though. Who is man enough? and more appropriatly have the real gains een proven post F1 engine freeze? I think so. Up to that point it was just a Yamaha fluke.

[Jersey Tom]

BTW, Tom, can you tell us something about this?

No

[gioma]

Hi all,

in my opinion is Torque, no doubt!

Power = (Torque) x (rpm)

This means that without torque there is no power at all!

Making it easy:
• Torque is what makes acceleration possible.
• Power is what makes possible to run at a certain speed without accelerating (not increasing the speed).

Why does a wheel spin? Because it “wants” to turn faster then its actual turning speed which means it wants to accelerate and therefore is Torque.

PS: sorry for my poor English

[Belatti]

BTW, Tom, can you tell us something about this?

No

Is that because the frequencies of an engine power pulse in a wide rpm band is far away from the natural frequency of the tyre longitudinal sliping mechanism?

[Roland Ehnström]

• Torque is what makes acceleration possible.
• Power is what makes possible to run at a certain speed without accelerating (not increasing the speed).

Well, you need power to accelerate, and to have any power you need torque, so in that senese of the word you need torque to accelerate. But what is really accelerating the car is the kinetic energy that is tranferred to the car's mass from the chemical energy in the fuel by applying power over time.

Torque is what initially makes the tires spin, but power is what makes them keep spinning faster and faster (=acceleration).

[RH1300S]

Surely it's a case of thrust measured at the wheels, which is a function of gearing. I am pretty sure you arrive at thrust by multiplying torque*primary reduction ratio*transmission gear ratio*final drive (which would include tyre diamter BTW).

From that, clearly you have the potential for same thrust at the wheels either at the fat part of a torque curve (most torque) or at higher revs where torque is falling away, but power building.

Which means it's torque dependent.

As already posted, power is derived from torque.

[fizzer]

Or just assume 'all else being equal'. Say you have an rx-8:

232 hp @ 8500 rpm
159 lb-ft @ 5500 rpm

Put it on 1st gear. Is it more likely to break traction at 8.5k or 5.5k rpm? Of course you'll say, it'll spin the tyres in both cases, and you'd probably be right.
So say you start putting weight over the rear axle, 100 Kg at a time, will it stop spinning the tyres at 8.5 or 5.5 first?
And yes, assume you put new tyres and clutch every run, and engine temp, and air pressure, and any other potential variables remain the same. Driver is a robot of course.

Well this is purely anecdotal but...

I have an RX-8. I autocross it on very sticky AutoX specific compound tires (Hoosier A6 285/30/R-18s).

Clutch drop at 5.5k - engine bogs, tires hop, ugly launch.

Clutch drop at 8k - immediate spin up, nice predictable launch.

I'm sure there's plenty of other factors, but the tires break traction way easier near peak hp. Same thing in turns, I can slam the throttle at 5k without upsetting the chassis nearly as much if I was steady state and went full throttle at 7k.

[rjsa]

When you start from highier revs you have much more kinetic energy stored in the crank and flywheel, which is used to break traction. It´s a much different thing than going into spint with the cluch already fully engaged.

At any given gear, with the clutch engaged, wheel spin will most likelly happen at peak torque, or whenever torque is enough.

People fail to see that power is a measure of force X speed, and that torque is force and is what pushes the car forward. Power is not directly measurable, you must measure speed and force to obtain it.

[engineguru00]

Figure I would actually register to answer this, I have been lurking around for a while. I did a good amount of work with wheel slip when gearing an FSAE car last year around a CVT transmission.

The amount of longitudinal force relies on a few things. Tires produce their maximum force at a certain slip ratio, which is not to be confused with runaway slip. Tires also vary their longitudinal thrust with load, although it isn't a linear relationship. Temperature and inclination angle both play a big role as well and neither are constant. Most SR vs LF vs Load plots you see are on a belt setup which doesn't accurately predict tire capacity in real life during operation, so you normally have to add a correction factor to take this into account and scale down all the values. A good driver can modulate the throttle to keep you at this peak of force, so you take that multiplied by your correction factor to get your maximum longitudinal force.

We all know torque is force times the distance it is being applied. With tires this distance is the loaded tire radius, which is not a constant value as it changes with load transfer. You now know how much torque you need to provide. You then work back through the half-shafts, differential, and transmission and take into account all the gear ratios and transmission losses to get the amount of engine torque required to get this longitudinal acceleration.

Now that all that ground work is laid out, to the question. The answer is "Yes". Your SR vs LF vs Load plot looks like a mountain on most tires, so once you exceed the peak force you actually have less tractive capacity and the wheel will spin faster and easier while actually providing less force. This is because you have no transitioned from static friction to kinetic friction at the contact patch. If you notice through all of this I have said nothing about HP and that is because it has nothing to do with wheel spin. You could spin the tires at peak torque or peak horsepower if the car has enough torque to pass the peak on your tire plot. Sorry if its a bit of a long winded answer, but thats the engineering behind it.

"Horsepower may get the car off the showroom floor, but torque is what gets it off the line" -C.S.

EDIT: I didn't go into the whole inertial side in this, but thats another story for another day. I just got out of my class on system modeling and don't really want to go through that again.

[autogyro]

Figure I would actually register to answer this, I have been lurking around for a while. I did a good amount of work with wheel slip when gearing an FSAE car last year around a CVT transmission.

The amount of longitudinal force relies on a few things. Tires produce their maximum force at a certain slip ratio, which is not to be confused with runaway slip. Tires also vary their longitudinal thrust with load, although it isn't a linear relationship. Temperature and inclination angle both play a big role as well and neither are constant. Most SR vs LF vs Load plots you see are on a belt setup which doesn't accurately predict tire capacity in real life during operation, so you normally have to add a correction factor to take this into account and scale down all the values. A good driver can modulate the throttle to keep you at this peak of force, so you take that multiplied by your correction factor to get your maximum longitudinal force.


We all know torque is force times the distance it is being applied. With tires this distance is the loaded tire radius, which is not a constant value as it changes with load transfer. You now know how much torque you need to provide. You then work back through the half-shafts, differential, and transmission and take into account all the gear ratios and transmission losses to get the amount of engine torque required to get this longitudinal acceleration.

Now that all that ground work is laid out, to the question. The answer is "Yes". Your SR vs LF vs Load plot looks like a mountain on most tires, so once you exceed the peak force you actually have less tractive capacity and the wheel will spin faster and easier while actually providing less force. This is because you have no transitioned from static friction to kinetic friction at the contact patch. If you notice through all of this I have said nothing about HP and that is because it has nothing to do with wheel spin. You could spin the tires at peak torque or peak horsepower if the car has enough torque to pass the peak on your tire plot. Sorry if its a bit of a long winded answer, but thats the engineering behind it.

"Horsepower may get the car off the showroom floor, but torque is what gets it off the line" -C.S.

EDIT: I didn't go into the whole inertial side in this, but thats another story for another day. I just got out of my class on system modeling and don't really want to go through that again.

I think going through all the complex Math is exactly what the question was posted for. It remains impossible to answer.

[Jersey Tom]

I think going through all the complex Math is exactly what the question was posted for. It remains impossible to answer.

...I'm still goin with plain and simple 'torque.'

[rjsa]

^^^^^^ This! It is, in fact, that simple.

[Mystery Steve]

I think going through all the complex Math is exactly what the question was posted for. It remains impossible to answer.

...I'm still goin with plain and simple 'torque.'

Yeah, I thought your blog entry did an excellent job of answering this question, but I guess I was wrong...

Plain and simple: power is the measure of the time rate at which work is performed. Torque is the result of a force acting on a moment arm. A tire is only capable of providing some finite level of longitudinal force. This critical force is multiplied by the tire radius to get a critical torque that the tire can handle. If the torque applied to the wheel by the engine is greater than this critical torque, you've got slip. Power is related to the torque by the gearing. You could have a one-million horsepower engine, but for some gearing ratio (granted it would be ridiculous, but just being theoretical..) you won't get enough torque applied to the wheel.

The answer is torque.