Re: Automobile tires and friction

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Michael Bowen wrote:

> A student asked me last week, during a friction lab, why, if the
> coefficient of friction is roughly the same regardless of the contact area
> between two surfaces, wide automobile tires are commonly considered
> to have "better traction" than narrow ones. I couldn't come up with a
> good answer. Is the superior traction of wide tires a truth or a
> misconception? Does "traction" for tires depend, at least in part, on
> factors other than mu? Your comments, please.

What does your friction lab consist of? The activity which
comes to my mind is to measure static friction on the inclined
plane (mu=tanA, where A is the angle at which the object
starts sliding). I would ask the student to prepare several
wooden sliders which have different widths and to explore
the effect of the width on the angle A. I would suggest to
use sliders whose masses are identical. Yes the mass is not
expected to be important but one should not lean on
expectations, if possible. It would be a good idea to explore
the effect of the mass as well. I would also try at least two
very different materials. Why not?

Reproducibility is likely not to be great and this offers an
opportunity to use statistics. For example, if each A is the
average of ten measurements then is the observed difference
in A (for two objects of different width) significant at the
confidence level of 95% or not? At least they will learn
what the standard deviation is good for. Please share
your findings with the rest of us.
Ludwik Kowalski