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Static Friction
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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Update: June 2012
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