|MadSci Network: Physics|
Greetings Daniel You have asked a very interesting question and one that can be very tricky to answer. So I'll do my best. There are two approaches to answering your question. One is the Physics approach, the other is the Automotive Engineering approach. First, on the Physics side, the friction and surface area argument could be one of semantics. I refer you to the answers given to Markus Engelhardt and James Reid, users who asked the same question as yourself, by our scientists. To find their questions go to our search engine (circumnavigator) and type in their names as a the keyword. You can also use this search engine in the future to find if your specific question has been asked before. Anyway, back to the answer. In addition to the answers given to the aforementioned users, I'm going to give you another physics answer, one which I found in a physics book (I'm an engineer, not a physicist): "Intuitively, you might have expected the frictional force increases with increasing contact area. On a microscopic scale, your intutition is correct: the greater the area of actual contact, the greater the friction. But microscopically, only a small fraction of the area you measure macroscopically is actually in contact with the other surface. As the normal force between the surfaces increases, the surface irregularities are crushed together and the actual contact area increases, so that the frictional force increases." So is that a confusing wording game or what! Aha!, do not worry my friend, because I got the answer you are looking for! Keep reading, I'm almost done confusing you. The answer to your question about big fat bold tires on race cars (I'm disappointed you picked Indy car as an example since I am a fan of Formula 1 racing instead), we must look outside of the realm of Physics and friction, and look instead into Automotive Engineering. Guess what, it is not about friction, it's about STIFFNESS. Here's the deal: Your choices as a Formula 1 (or Indy if you really must) race car mechanic are skinny tires or fat tires. You put on the skinny tires and let the driver loose in the circuit. As he/she accelerates thru the first turn, the car spins a hundred times and smacks the wall. Your driver and team owner are pretty upset at you. The car simply did not have the "grip" necessary to remain on the turn at high speed. Here's the trick! "Grip" is NOT necessarily "Friction". Grip is really how well a tire can remain in contact with the road surface. It does have to do a little with the friction between tire and road surface. But it has to do a lot more with the stiffness of the tire and how much it deforms under the forces of acceleration. As your driver accelerated through the turn the skinny tires deformed excessively and therefore were not able to remain in contact with the surface. If you had chosen the wider tires, however, the tire would of deformed less under pressure because of its greater stiffness and therefore you, your driver and the guy who is paying for the car would be on the road to winner's circle. So, as the car commercial says, "Widder IS Better". Oh, but wait a minute, what about our scientist friends in thermodynamics? Do you see the can of worms you have opened? There is another reason for the wider tires. Tires heat up very fast. This is good, for a while. But you do not want them to overheat easily. That would cost you precious time in the pits changing tires! So you put on a wider tire to increase the surface area by which the tire transfers some of it's heat into the pavement. So my friend, the answer to your question is not so simple. But if you want a one sentence answer (you're are thinking why I didn't just give you that in the first place), the reason is this: Fat tires are not about friction. Increase width, increase stiffness, decrease deformation, increase grip. That's the ticket! You want the formulas? I didn't think so. He,he. Thanks for your interest. Take care of yourself. Yours in Mad Science, Luis
Try the links in the MadSci Library for more information on Physics.