If the frictional forces are independent of the area of the tire in contact with the road, what motivates this choice?
Answers and Replies
It's because real friction forces, unlike the ones they teach you about in physics class, do depend on contact area.
The same argument can be applied to brake pads. Why not use really tiny ones? The answer to this one is also to do with engineering considerations of the loads that a material can withstand, and the amount of heat that has to be dissipated per unit of contact area.
This is the answer I gave someone who asked a similar question, under Classical Physics :
The reason for wider tires is not that the increased contact area automatically leads to more friction (though there may be some effects that depend on the area indirectly). The design parameter that determines the coefficient of friction between the tires and the track is the something related to the molecular compressibility of the rubber. In other words, "the softer the rubber, the better the friction." However, when you use a softer rubber, you need to make the tires wide enough to give them strength. So, "the softer the tire, the wider it needs to be", to withstand design forces and moments.
If you look at a tornado in nature it spins at great speeds making tight spirals of wind constantly winding which everything in its path are quickly swept up. A small twister in the desert observed closely grabs very little. With a Racing tire maybe this is the same effect between two objects. One spinning at a great speed somehow gripping 0 space. A dragster has very wide tires but at launch they become very skinny, maybe less than half the width they started at because the tire is so soft it winds itself like a cinamon roll before it moves. I wonder if the spinning tire causes so much heat in both the tire and the ground that they both almost expand into each other like a weilder combining diff metals through an oxygen medium. Some raceways have rubber mixxed into the tracks. Dymium
Auto racing tires tend to be soft because that generates more friction. Unfortunately soft materials have weak shear strenth, 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 achive 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.
Remeber that car racing is essentially limited by the amount of friction that the tires provide. In other types of racing - like bycicle racing or track - where power is not effectively unlimited, the contact surfaces tend to be narrow and stiff for better efficiency.