Friction and wider wheels

I found this on the web, should be good enough to start a discussion .. Question As an engineer, I know that friction does not depend upon surface area. As a car nut, I know that wider tires have better traction. How do you explain this contradiction? Answer #1 This is a good question and one which is commonly asked by students when friction is discussed. It is true that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch, however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance. Answer #2 Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap .

fat slows the car down too

Doesn't the type of tire, Sticky/hard make a difference in friction also?

When I was a lad studying Physics, I asked my teacher the same question. He didn't know, and invited me to go and find out. So I phoned Jaguar's Group C racing team (showing my age here) and a very kind engineer talked me through it. He said that the classical rules of friction, where hills and valleys work against each other, assume that both surfaces are reasonably firm. Tyre rubber, on the other hand, is soft so that it pokes little pieces into holes in the road, just like fingers through a grating. A wider tyre equals more fingers, and so more grip! Ta-da!

The answer is really simple. Just do what Phil does..... park your car in the garage 355 days per year and every time you walk by the car, mumble to yourself, "Friction? We ain't worried 'bout no stinkin friction!"

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and I must assume that's why he wasn't a physics professor Question Why doesn't friction depend on surface area? Answer Although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together. If you were to increase the force as you increased the area to keep PRESSURE the same, then increasing the area WOULD increase the frictional force between the two surfaces.

Friction and Normal Force Part of the standard model of surface friction is the assumption that the frictional resistance force between two surfaces is proportional to the normal force pressing them together. A common exception occurs in snow where greater traction occurs by having wider tires with lower pressure. The same normal force is exerted, but the traction (friction) is greater with the wider tire since it doesn't pack the snow as much. The packing of the snow tends to depend on the pressure exerted on the snow, and supporting the same weight on a smaller area implies that more pressure is exerted. This example is also an exception to the assumption of independence of area of contact. With a given area of contact, increasing the normal force will pack the snow, decreasing the effective coefficient of friction. While exceptions such as this are easily found, the assumption that friction is proportional to the normal force is still reasonably valid in many cases and forms a useful model for many circumstances. A valid question is why does the frictional resistance increase proportionally with normal force? Jones and Childers have a thoughtful discussion of this. They point out that the area of coverage for a sliding block is actually much larger than the area of contact. Because of microscopic irregularities in the surface, the block is actually riding on a small fraction of the coverage area. Because such points of contact are deformable, an increase in normal force will actually increase the area of actual contact and increase the resistance to movement.

However the Coefficient of Friction is not constant, it varies with speed and temperature and Auto racing tires tend to be soft because that generates more friction. Unfortunately soft materials have weak shear strength, so a large area is needed to support large tangential forces. (In effect, the amount of force that friction can exert is limited by the shear strength of the materials.) Moreover, in some kinds of racing the vertical load on the tires due to wings is quite large as well, so the tires must be able to support much larger loads than the weight of the car which is also helped by the larger tire area. Wider tires provide more resistance to slippery spots or grit on the road. Race tracks have gravel, dust, rubber beads and oil on them in spots that limit traction. By covering a larger width, the tires can handle small problems like that better. Wider tires have improved wear characteristics. In order to achieve maximum traction, the tires are made with materials that ablate as the car runs through the race - which is why the tires get switched so often. Narrower tires would wear out more quickly. Remember that car racing is essentially limited by the amount of friction that the tires provide. In other types of racing - like bicycle racing or track - where power is not effectively unlimited, the contact surfaces tend to be narrow and stiff for better efficiency.

I’m pretty sure I agree with almost everything above except the parts about friction.