From: loki@nazgul.physics.mcgill.ca (Loki Jorgenson)
Newsgroups: rec.motorcycles
Subject: Re: physics 101 (was Re: Centripetal !!)
Date: 12 Sep 92 15:15:45 GMT
Organization: Centre for the Physics of Materials

In article <79571@ut-emx.uucp> captain@ccwf.cc.utexas.edu (Bran Muffin) writes:
>
>Several people have noted that a car can stop faster because it has a
>larger contact patch.  According to my understanding of Newtonian 
 .... Newtonian stuff deleted....
>friction (the one that will slow you down) has nothing to do with "contact
>patch" or llamas.
>

	Of course, we could go to relativistic moto-mechanics and see that
the larger contact patch actually increases the cubic displacement at near
light speeds but..... I'll settle with just quoting this passage:

	"Why do dragracers have tires so much wider than normal?

	....but frictional forces do not depend on the contact area.  When a
dragracer accelerates his/her vehicle, starting from rest, the tires can spin
furiously.  The best way to accelerate would be to maintain a drive power
level that produces frictional forces just short of slipping.  But this is
technically very difficult to achieve, so the dragracer simply applies
maximum power and allows the tires to spin until the car's velocity
increases to the point that slipping ceases.  While the tires are slipping,
considerable heat is generated.  This causes actual melting of the tire
rubber (and possibly also the road surface).  The presence of liquid rubber
(and asphalt) reduces the friction coefficient and the frictional force.  By
using dragster tires that are very wide, the increased contact area
distributes the heat load and maintains the temperature sufficiently low
that excessive melting does not occur.  A greater frictional force and
therefor a greater acceleration are thereby achieved by using wide tires
instead of narrow ones."   (Marion & Hornyak, p153).

	Of course, this is rubbish.  Leave it to a physicist to try to
work out a problem theoretically; theory is so much more interesting
when you leave out the facts.  I believe some of you could comment better
on the dragracer analogy but I believe that the idea is to spin the tires
before the race to *increase* the coefficient of friction with sticky rubber
and then *avoid* spinning the tires on the actual take-off.  So how can
you quote from these books without feeling a little silly?

	As for ABS-equipped cars and bikes, the point is still relevant
that the size (and number) of the contact patches doesn't change the
coefficient of friction.  However, one point that was raised above is
the idea of heat generation and static friction breakdown; with the
larger area and multiple contact points, it might be reasonable to 
say that the better distribution of the heat and stress on a four-wheeled
vehicle permits improved braking.  Certainly, one can imagine that dive and
subsequent loading of the front-end of a motocycle during braking
really concentrates all the energy dissipation on one contact patch.
For a car with lower centre of gravity and two front tires, the energy
can be more effectively bled off without relying on just one patch; in
fact, I would bet that the rear tires are relatively more useful on a 
low-slung car than a taller bike bike.  Thus it's a four-to-one
advantage in energy distribution for a car.

	In other words, its not contact size that matters; it's geometry.
Mind you, I'm a simulationist.

	Now, let's get back to the usual non-motorcycle related topices.

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