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Old 05-24-2011, 02:16 AM   #63
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Join Date: Feb 2008
Location: Philadelphia
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My Ride: M3 & ZHP
Originally Posted by mkodama View Post
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.
Maybe it's placebo, but I could swear that even relatively minor changes in tire width seem to have a profound effect on the traction available in the lower gears. In 1st gear, my car would squirm all over the place with 255 width rear tires. After upgrading to 275s, the rear end is planted unless I purposely abuse the clutch a bit.

Probably a dumb question, but does total load effect static friction differently than kinetic friction? And I know "vary a bit" isn't exactly a scientific term lol. I have no idea what all goes into calculating the friction coefficients, so I couldn't use a more precise term

I'll have to look that book up
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