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Old 05-24-2011, 01:58 AM   #62
mkodama
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Quote:
Originally Posted by TerraPhantm View Post
This has always bugged me - if friction is independent of contact area, then why do wider tires grip better than narrow tires? I would think the coefficient itself would vary a bit with the contact area
For the most part, larger tires don't grip a whole lot better than smaller tires, and either increasing the load(like with weight or downforce) on the tire or using a stickier tire will make the biggest difference and is the safest way to think of things.

The grip does "vary a bit" though. As you increase the load on the tire, the coefficient of friction goes down. An example is if you have a tire that with 500 pounds resting on it, the coefficient of friction will be 1.0 and the amount of "grip" will be 500*1.0= 500 pounds of "grip" in some direction. If that same tire has 1000 pounds on it, the coefficient of friction might drop to 0.9, so the "grip" will be 1000*.9= 900 pounds of "grip" in some direction. The load was doubled but the grip was not. The more load, the more total grip you will have, but the lower the coefficient of friction of the tire will be. So tires with a larger contact patch spread out the load more and help, but the difference is probably not as big as most people expect.

There is a very good explanation of this in Carroll Smith's book, Tune to Win.

In the case of a brake pad, I don't think the coefficient of friction changes nearly as much as a tire with the change in load.
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