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It is well known in racing that driving the car on the ideal "slip angle" of the tire where it is crabbing slightly from the pointed direction produces more cornering speed than a lower slip angle or a higher one.

(More explanation as requested) I'm considering two main effects on the tire when in a turn:

  1. The tread of the tire is twisted from the angle of the wheel it is mounted to. There is more force as speed increases, and generally, more twisting.

  2. The tire slides somewhat at an angle on the road surface rather than rolling.

At low speeds, the angle between the pointed direction of the wheel (90 degrees to the axis of rotation) and the direction of travel is nearly 0. When the speed increases to the point the angle reaches about 10 degrees, the tire generate more grip and the car goes faster around the turn. (Higher angles produce lower grip)

So the grip is higher at 10 degrees of slip than at 0 or 20 degrees.

What is the physical effect that causes this increase in grip?

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Not to say there's anything wrong with your question, but since a lot of people here may not be familiar with the mechanics of car racing (at least I think it's not just me :P), it would make your question better if you add some more details and/or perhaps a picture to explain the situation you're describing. –  David Z Feb 25 '11 at 2:53
    
I assume he means that a tire can extert more lateral force, if its direction of motion includes some lateral slip. I don't know if this is true, but it is my take on the physics of the question. –  Omega Centauri Feb 25 '11 at 3:10
    
@David Sure, see above. –  james creasy Feb 25 '11 at 22:33

4 Answers 4

At small slip angles, the tire deforms linearly with the side-force demanded of it. This effect saturates when the rubber is essentially fully deformed. The tire then starts to slide over the surface of the road, which reduces the side-force somewhat. Now, this force acts perpendicular to the wheel plane, which is not quite perpendicular to the car's trajectory. This effect reduces the force available to turn the car and causes an "induced drag" that slows it down. The combination of these effects gives rise to the concept of an optimum slip angle.

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You describe why a high slip angle reduces grip, but not why a small slip angle increases grip over zero slip angle. –  james creasy Feb 25 '11 at 19:32
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The tire is not rigid, so you can't have a zero slip angle with nonzero force. It's not really that the slip is producing a force so much as the force is producing the slip. Increasing the force increases the deformation. –  Stingray Feb 25 '11 at 20:56

Not familiar with racing but my initial thought is that the vectors are in a more ideal direction. Maybe the kinetic friction plays a small role (as opposed to static friction)?

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The effect sounds the same as braking with ABS. Maximal deceleration is achieved when a wheel rolls slightly slower than the speed of the car. The lateral effect in corners works the same; both are friction effects between rubber and road surface. Both are isotropic materials, i.e. the direction of the friction force doesn't matter.

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I believe this to be true. But why is this true? Is it true for a non-pneumatic tire as well, i.e. a property of the tire material? Or are there other forces at work? –  james creasy Mar 2 '11 at 0:43

The simple answer

The tyre is deformed sideways due to the sideways force like Stingray describes, but only around the contact with the road, the rest of the tyre is not deformed so.

The tyre constantly rotates. As the contact pad is lifted off the road the force is taken off this pad and thus it undeforms, meanwhile a new part of the tyre get into contact with the road and is therefore deformed instead. A new deformation happening always moves the car a little sideways, and as new deformations have to happen as the tyre rotates this sideways motion depend on the speed of the car.

The more exact event description

My description has been stepwise, but this is of course a fluid motion. Lets consider a small part of the tyre on its way through the contact area with the road. During the contact phase the tyre part gradually deform more and more, this process must be linear, as the car is moving sideways at a certain speed any point of the tyre that is stationary relative to the road must deform at this speed. The lateral force on the tyre part increase with this deformation.

For the first part of the contact phase this actually looks like a pretty neat pattern, the vertical force increase from the start of the contact pad to the middle of it, so the same coefficient of friction can gradually support more lateral force. Unfortunately towards the back of the contact area the vertical force decreases while the lateral force continue to increase, at some point before the end of contact area the coefficient of friction can't keep up and that part of the tyre slips.

Conclusion

Part of the tyre is slipping, and thus causing degradation, disproportionately more so the close you get to the friction limit. But the grip come from the part of the tyre that is not sliding, this part is however in a constant motion of deformation that effectively makes this part of the tyre rotate in a slightly different direction than the rest of the tyre.

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