On Wednesday, July 17, 1996 at 3:00:00 AM UTC-4, Jim Nelson wrote:
Samuel C. Blackman wrote:
My Ph.D. advisor and I are trying to find the answer to the following question, wholly unrelated to our field (pharmacology):
"Why do drag racers use wide rear tires, if the coefficient of friction
is independant of surface area?"
We got to discussing this question, but were not able to come up with a scientific explanation - intuitively, it seems that wider rear tires would increase traction, but that's not a function of friction, is it? If a narrow tire were used, the force vector perpendicular to the ground would be increased because point of contact was smaller (higher pounds per square inch).
My guess is that the answer is more mundane - namely, that the pressure
on a narrow tire, combined with the friction, would make blow-outs more likely. But could you use a thick-walled, narrow tired and achieve the same effects as a wide, but thinner tire?
If someone could help explain this mathematically and provide the relevant physical concepts, we would greatly appreciate it (because we'd be able to get back to our work).
Thanks!
-- Sam Blackman
Boy--wait 'til you get to wrinkle-walls. *That* will really
make things interesting in your department.
You are correct, the Coefficient of Friction of any object, including a tire, is independent of surface area. Strange, huh? But from Physics 101, this is a true statement. It is not the Coefficient of Friction that is in play here.
When a tire spins while being supported by the track, the rubber is placed into shear failure; the sound and smell you experience is the result of rubber shearing from internal forces provided by external forces. The rubber left on the tracks is that
rubber which has been sheared and left behind. When the dragster performs a "burn out", s/he is changing the chemistry of the tire's "tread" surface. The attraction between the tire rubber and the track rubber is actually molecular attraction via Van der
Waals forces. Yes, the rubber on the track and the rubber on the tire become "one" ... if you perform the equation of motion for a body accelerated through a distance of 1,000 ft (the reduced distance of drag racing), you will find that today's speeds
and times give up to 4.7 g's of acceleration. There is no way that the Coefficient of Friction can provide that type of "grip" for these g's. It's a chemical thing (a tough pill to swallow for a mechanical engineer!!).
Bill Bryan PE
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