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Name: Lucas
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Why does the formula for static friction (F = [mu]N) seem to disagree with rules of traction in, for example, High Performance Automobile. Manuals on tuning the tires for a race car suggest that Camber (the angle between the tire and perpendicular to the road surface) should be as close to 0 as possible when the most traction is required (heavy acceleration such as tight turns, heavy breaking, or acceleration out of a turn). Although the formula suggests that the contact area of the tire has nothing to do with the friction, races won because of better tire tuning is pretty much irrefutable proof

Are the frictional laws flawed in this sense, or am I simply assuming too much to say that this rather obscure concept of "Traction" described in tuning manuals is the same as static friction?


Replies:
The formula doesn't contain everything there is to know about friction. It's just one law that describes many materials well enough to be useful. If you take a microscopic look at friction you'll find it gets contributions from roughness, stickiness, viscosity, and probably lots of other stuff. Certainly the stickiness contribution would not behave according to the idealized formula (think of glue as an extreme instance of static friction).

Also, the tuning manuals certainly cannot be ignoring the effects of sliding friction, because they are based on real experience which includes skidding.

There's a whole field of science devoted to friction. It's called tribology, and it's a beautiful mess of idealized representations of real-world effects -- just like the rest of physics.

Tim Mooney


The formula doesn't contain everything there is to know about friction. It's just one law that describes many materials well enough to be useful. If you take a microscopic look at friction you'll find it gets contributions from roughness, stickiness, viscosity, and probably lots of other stuff. Certainly the stickiness contribution would not behave according to the idealized formula (think of glue as an extreme instance of static friction).

Also, the tuning manuals certainly cannot be ignoring the effects of sliding friction, because they are based on real experience which includes skidding.

There's a whole field of science devoted to friction. It's called tribology, and it's a beautiful mess of idealized representations of real-world effects -- just like the rest of physics.

Tim Mooney


The physics and chemistry of a racing tire is a LONG WAY from just F=[mu]N, The tread design, flexibility/strength at high operating temperature, the type and structure of the tire chords, side wall design, oxidation resistance which causes micro-cracks, and the list goes on. So whether F=[mu]N holds or doesn't becomes swamped by many other parameters.

Vince Calder



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