Re: Automobile tires and friction

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Another factor, at least when looking at 'drag racing' as an example of the
use of large, wide tires.  There the size of the tires helps dissipate the
'heat' generated by the frictional forces and keeps the tires from melting.
You don't want a liquid between the road and the tire.

Also note sometime when watching these races how once the rear tires start
turning, the conformation of the tire changes, from being wide and low, to
being more narrow and high.  The rear of the car rises appreciably as the
spinning tires become more narrow disks of greater radius.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, February 25, 2002 8:13 AM
Subject: Re: Automobile tires and friction


> Michael Bowen wrote:
> ...
> > 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.
>
> Good question.
>
> > Is the superior traction of wide tires a truth or a misconception?
>
> Yes and no.
>
> Note that other things being equal, a wider tire corresponds to lower
> pressure.  Obvious facts include:
>
> 1) On a road with standing water, you will get more traction with a
> _higher_ pressure.  Other Things Being Equal (OTBE) this favors narrower
> tires.  The rule is that the speed at which you will hydroplane scales
> like sqrt(pressure).
>
> 2) On a road with a small amount of snow, you will do better with a
> higher pressure, so you can punch through the snow to the underlying
> road.
>
> 3) Off-road, on e.g. sand, you will do better with a lower pressure.
> OTBE this favors wider tires.  This is not primarily a traction issue;
> it's just that you don't want to sink in.
>
> 4) On a thick layer of snow, you will do better with the lowest possible
> pressure, so you can float on top of it.  At some point caterpillar
> treads are what you want.
>
> 5) In cases where you want a high pressure, at some point traction is
> limited by the yield strength of the rubber.  A hard tire gives poor
> traction.  A soft tire gives good traction up to the point where you
> start erasing the tire.  Spreading the force out over a larger area
> helps prevent erasing.  This is obviously outside the elastic regime and
> obviously nonlinear, so one would not expect the usual linear models of
> friction (coefficient of friction etc.) to apply.
>
> 6) Last but not least..... For any given tire, over-inflating it or
> under-inflating it will do bad things.  On my car, the tire-tread is
> designed to pump water out from underneath the tire.  Over-inflating it
> would distort the tread and probably impair not improve the
> anti-hydroplaning performance.  If you want to investigate the effect of
> width (or pressure) you need to change tires, not just monkey with the
> air-content.
>
> 7) There are presumably other factors that didn't immediately occur to
> me.