From: met@ariel.ucs.unimelb.EDU.AU (Meteorology General)
Newsgroups: rec.autos.driving
Subject: Re: Can I install wider tyres than what specified by the auto-maker?
Date: 28 Jan 1993 12:19:17 +1100
Organization: University of Melbourne Dept of Meteorology
NNTP-Posting-Host: ariel.ucs.unimelb.edu.au

In article <1993Jan26.213020.24807@odin.corp.sgi.com> mikesteg@syclone.detroit.sgi.com (Mike Stegbauer) writes:
>In article <1k26mlINN7l6@ariel.ucs.unimelb.EDU.AU>, met@ariel.ucs.unimelb.EDU.AU (Meteorology General) writes:
>|> >$|> >If your going from a 195-60-15 to a 205-60-15, the 205 will be taller. 
>|> >$|> >This will affect your speedo.  You might try looking at a 205-55-15.  They
>|> 
>|> I think we are missing the point here guys. The subject line says WIDER tyres.
>
>Ok, let's think about it.  Friction (grip) only depends on two things, weight and the 
>coefficient of friction for the surfaces, NOT the area of contact.  This is because, as
>area increases, the weight per unit area decreases proportionally, and the terms cancel.
>This being the case, wider tires will not increase grip, only better tires, or more weight
>on top on them.
>This is closely related to why FWD cars drive well in snow, there is more weight over the drive
>wheels to push them into, and through the snow.
Sorry Mike, but I have to disagree with you here. The formula surely (yes, yes; dont
call me Shirly) is not that simple. (Take it from someone who did a few years of
physics - nothing is that simple.) If you want to use an analogy; then why do F1 cars,
and all racing cars for that matter, have wider tyres? Yes the tyres are "stickier", but
then the width enables more "stickiness" to be applied to the road (Well, you know what I 
mean - bit of a hand-waving argument I know). The case of the narrow tyres in snow is
a completely different matter - you're not getting your grip from the tyre sticking 
to the surface, but rather from the increased force per unit area "pushing" the tyre
into the snow and creating its own rut/groove in which the tyre travels. Sort of like
making your own railway track as you go along.

The physics that you talk about is correct, but not in the real world. Reminds me of
a joke in a recent New Scientist magazine. A genetisist, a physiologist and a physisist
were asked to compete to determine who could predict the outcome of a horse race.
After a few weeks they returned to the judging committee to tell who they thought would 
win. The genetisist said " ive looked at all the bloodlines and all the dna characteristics
of all the horses, and I am sorry to report that on the evidence before me I cannot tell 
you who will win." Next the physiologist reported "I have studied the cardio-vascular system,
the muscle strength and suppleness, the body mechanics and size of all the horses, and on
the evidence before me I am sorry to report that I cannot tell you who is going to win the
race." Finally the physisist came before the committee. "I am happy to report that after
placing all the appropriate parameters and equations into a complex mathematical
computer simulation of the horse race, and running this at 15 sec timesteps with 
centred differences in both space and time, I can tell you who will win. There are only two 
considerations to be taken into account. It must a a spherical horse in a vaccuum."



Andrew B. Watkins                    |    `To strive, to seek, to find and
awatkins@mullara.met.unimelb.edu.au  |     not to yield'  
awatkins@buster.met.unimelb.edu.au   |                  - Lord Tennyson 


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