If Friction is independent of Contact Surface Area, then why do 'performance tyres' generally wider?

Friction changes with temperature, and temperature increases less when the heat is spread over a wider area. If you spin narrow tires with a heavy load on them, they will wear down faster than wide tires; they may also melt and catch fire.

There at least two different things going on here.

First, if the mass of the car and the tire pressures are the same, the contact area between the tires and the road will be the same whatever the width of the tires. The force to support the weight of the car = pressure x area.

If your performance tires run at higher pressure than standard tires, the contact area will be LESS even though they are wider, because pressure x area = constant.

The SHAPE of the contact area will be different. If the tire is wider, the area will be smaller in the circumferential direction, therefore the tire walls have to flex less, which gives less rolling resistance and less heat generated in the tire. It also affects the stability of the car to bumps on the road surface etc, and the ability to shift rain water off the road surface through the "holes" between the tire and the road created by the tread pattern.

Second, Coulomb's "law" of friction that you learn in school physics is only approximate. It works pretty well for hard surfaces like wood or metal with low relative velocities between them. It doesn't work well for soft materials like tires.

Calling it a "law" is misleading, because people can get the idea that it is universally true in the same way that Newton's laws of motion, or the law of gravity or the ideal gas laws are "true". It should really be called Coulomb's MODEL of friction, and it is called that in real-life engineering situations, where other more complicated models of friction are also used.

One reason why you don't learn the more complcated models in school is because you can't do simple hand calculations using them. You have to do computer simulations instead. Unfortunately, that can leave people thinking that they know all there is to know about "friction", when they don't. There is still a lot of serious research going on, to understand better how friction REALLY works. I know that, because I've done some of it!

Under normal conditions, friction force can be found from F = kN; where k is the coefficient and N is the normal force of the body onto the surface. But those are normal conditions where contact area is not explicitly indicated.

But "friction" comes from a variety of sources: adhesion between two surfaces, roughness between two surfaces, and so on. In the case of smooth, wide racing tires (tyres) the additional friction force comes from adhesion where more area in contact between the surfaces yields more friction force.

When determining k in F = kN, that is usually done experimentally. In which case, the smooth wide tyres are tested against whatever surface they are expected to run over to find the coefficient. And that k value will include the additional effect of adhesion; so it will be typically higher than a grooved tire using the same materials.

Good question! It was on my freshman physic exam. Tires include not only friction but also adhesion. Performance tires, the rubber melts and wears a little making the contact surface "sticky". t rakes energy (work) to break the adhesive bonds between the road and the tire. The Number of bonds (area) where the tire is bonded to the road is a function of area. This is why racing tires wear fast. The adhesion is very important. Basic friction does not include the energy expended to wear the surfaces in contact.

Several factors affect the magnitude of rolling resistance a tire generate: wheel radius, forward speed, surface adhesion, and relative micro-sliding.

* Material - different fillers and polymers in tire composition can improve traction while reducing hysteresis. The replacement of some carbon black with higher-priced silica–silane is one common way of reducing rolling resistance.

* Dimensions - rolling resistance in tires is related to the flex of sidewalls and the contact area of the tire.For example, at the same pressure, wider bicycle tires flex less in sidewalls as they roll and thus have lower rolling resistance (although higher air resistance).

* Extent of inflation - Lower pressure in tires results in more flexing of sidewalls and higher rolling resistance. This energy conversion in the sidewalls increases resistance and can also lead to overheating and may have played a part in the infamous Ford Explorer rollover accidents.

* Over inflating tires (such a bicycle tires) may not lower the overall rolling resistance as the tire may skip and hop over the road surface. Traction is sacrificed, and overall rolling friction may not be reduced as the wheel rotational speed changes and slippage increases.

* Sidewall deflection is not a direct measurement of rolling friction. A high quality tire with a high quality (and supple) casing will allow for more flex per energy loss than a cheap tire with a stiff sidewall.Again, on a bicycle, a quality tire with a supple casing will still roll easier than a cheap tire with a stiff casing. Similarly, as noted by Goodyear truck tires, a tire with a "fuel saving" casing will benefit the fuel economy through many tread lives (i.e. retreading), while a tire with a "fuel saving" tread design will only benefit until the tread wears down.

* In tires, tread thickness has much to do with rolling resistance. The thicker the tread, the higher the rolling resistance.Thus, the "fastest" bicycle tires have very little tread and heavy duty trucks get the best fuel economy as the tire tread wears out.

* Hard steel rails last longer but also have lower static friction (traction) than rubber tires. They may also suffer fatigue cracking because the cracked area is not worn away by the passing trains.

* Smaller wheels, all else being equal, have higher rolling resistance than larger wheels. In some laboratory tests, smaller wheels appeared to have similar or lower losses than large wheels, but these tests were done rolling the wheels against a small-diameter drum, which would theoretically remove the advantage of large-diameter wheels, thus making the tests irrelevant for resolving this issue. Virtually all world speed records have been set on relatively narrow wheels,[citation needed] probably because of their aerodynamic advantage at high speed, which is much less important at normal speeds.

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