In a resonse to another thread on another subject I described the difference between friciton on 2 hard surfaces and that between a tire and the road in order to make a point. There was some disagreement with my tire traction comments and I'd like some further discussion without taking over the other unrelated thread.

I said "because the tire is soft and squigy and the effective coefficient of friction developed is not the same as two hard surfaces in contact with each other. The soft tire compound envelops the small imperfections in the road surface somewhat like 2 gears meshing which increases the effective coefficient of friction. Also a tire tread deforms in multiple dimensions under lateral and acceleration and braking loads which further complicates the shape of the contact patch. It was the final realization of this principle in the 60's that partly lead to wider tires on race cars and subsequently passenger tires. A wider tire may have a larger contact patch but it may also have a more stable contact patch which may be even more important. Sometimes you just have to think outside of the box."

I beg to disagree. There is some truth to what you say but wider tires of equal rubber compound do indeed make stickier tires which is why we like wider tires on performance cars. You are right that the coeffcient of friciton of a tires rubber does not change with width but the "stickage" (edited above to effective coefficient) does because the "stickage" is dependent on multiple variables and not just the coefficient of friction of the the particular rubber compound on the tarmac. Tire construction is all about keeping the footprint as large and stable as possible under differing conditions such as acceleration, deceleration and lateral cornering loads. Because tires do deform in complex and multiple ways they do not perform like steel train wheels on a railroad track.

Also wider tires do no always have softer compounds although they do have different construction to deal with the width.

I think my point of disagreement with dills above is that going back to my original point one of the fundamental differences about tire traction is that rubber is squidgy and does not behave like 2 firm solids rubbing against one another. Whatever the coefficient of friction (mu) of some rubber compound is when a flat smooth piece of rubber is slowly drawn across a flat smooth (ie glass or metal) object, that is not what is happening between the tire and the road. True there is the friction of rubber against the other material (asphalt). But the road is not smooth, there are pointy surfaces reaching up that deform the flat surface (lets use a slick race tire for simplicity) of the tire. This completely changes the character of the friction surface. The situation now becomes somewhat more like 2 gear meshing. And I think that we would agree that the description of the torque available to a meshed gear exceeds whatever the friction coefficient of lubricated steel on steel is. It is this more complex situation of combined friction of rubber and the deformation of the rubber both in the micro and macro environmental that sums to equal the tires traction.

 

To continue are discussion i stated....

I'm interested in hearing your thoughts Jim.

 

I was editing while you were posting so I will repost.
I think my point of disagreement with dills above is that going back to my original point one of the fundamental differences about tire traction is that rubber is squidgy and does not behave like 2 firm solids rubbing against one another. Whatever the coefficient of friction (mu) of some rubber compound is when a flat smooth piece of rubber is slowly drawn across a flat smooth (ie glass or metal) object, that is not what is happening between the tire and the road. True there is the friction of rubber against the other material (asphalt). But the road is not smooth, there are pointy surfaces reaching up that deform the flat surface (lets use a slick race tire for simplicity) of the tire. This completely changes the character of the friction surface. The situation now becomes somewhat more like 2 gear meshing. And I think that we would agree that the description of the torque available to a meshed gear exceeds whatever the friction coefficient of lubricated steel on steel is. It is this more complex situation of combined friction of rubber and the deformation of the rubber both in the micro and macro environmental that sums to equal the tires traction.

 

True there are more variables present it real time situations, but it truly is simpler than described. The shape of the surface is irrelavate to the material as long as the surface is solid. A tire spread across a racetrack and a tire spread across a highway will have the same friction, if the road has macroscopic grooves but is flat relative to the tire will not alter the friction between the tire and the road.

It is not like 2 gears meshing because gears present no valid friction. There are two times of friction, constant and slipping. Constant friction is when you push on a box and it does not slide across the floor, slipping is when you push hard enough you break the constant friction, it begins to slide, and slipping friction takes over. When you brake hard, turn hard, accelerate hard you strive to hold the vehicle as close to the constant friction as possible, because as soon as you put to much energy into it, it breaks lose and reverts to slipping friction, burnouts, drifting, etc.

 

If you guys would like to read up on this topic (and many other interesting ones related) try these two paperback books. I got my on Amazon. I'm sure there are others but these cover this topics well...

Going Faster - Mastering the Art of Race Driving; Chpt 13, Tires; By the Skip Barber Racing School, 278 pgs

Tune to Win - The Art and Science of Race Car Development and Tuning; Chpt 2, The Racing Tire; Carrol Smith, 172 pgs

I've read these over and over thru the seasons and I'm still learning and comprehending all the infomation they contain. A valueable addition to a sports car lover's library.

 

I have "Tune to Win" and "Drive to Win" by Carroll Smith. Perhaps I'll have another look before I go further. Dill you are referring to kinetic and static coefficients of friction, both of which I am familiar with. The point I keep trying to make, perhaps inaccurately or incompletely, is a fundamental one that you seem and I seem to disagree on. So I will try once more.

As you mention there are 2 coefficients of friction for any 2 particular flat surfaces in contact with each other. The static coefficient occurs while the objects are not moving, that is you are pushing on one and it hasn't moved yet. Once the object starts sliding the coefficient switches to the kinetic coefficient which is less than the static. This is why a rolling tire has more traction that a sliding tire and why we like ABS brakes.

But I maintain, that although the static and kinetic coefficients play a significant role in tire traction, tire traction and the resultant slip angles are also heavily influenced by the nature of the surfaces involved which are not smooth. This is why different roads have varying degrees of traction even though they are composed of the same material.

An extreme example would be the difference between the tire traction on a plate of polished steel and a plate of steel with short nail points on it (short so that they don't puncture the tire) or something similar to a file. Both situations are rubber against steel but I believe the rough steel one (within limits as there is a point where too rough would decrease the traction) would have significanty more traction than the polished steel.