From: ulrich@canberra (Nathan Ulrich)
Subject: A few simple tire facts
Reply-To: ulrich@canberra.whoi.edu
Organization: Woods Hole Oceanographic Institution
Date: Sun, 11 Oct 92 00:29:17 GMT

Gene Van Buren writes:
> In article <1992Oct9.044552.11107@thumper> meier@thumper (Christopher Meier)  
writes:
> ] Wrong.  Racers use slicks for better traction (in dry conditions).
> ] More surface area on pavement transfers more power to pavement,
> ] ie, less or no wheelspin.  Gas mileage has nothing to do with it.
> 
> Well, in racing, they don't usually worry about fule mileage when it comes to
> tire choice. But fuel mileage is quite related to tires. The price of higher
> traction is fuel mileage. Higher traction results from higher friction.
> Higher friction increases drag. There are two things that keep a car from
> getting infinite fuel mileage on a flat road: tire friction and air friction.
> Fuel mileage would have a relatively dramatic boost if we could only make
> cars float effortlessly...:-) ...but that would lead to a whole slew
> of other problems (propulsion).

Not quite.

Mileage can be important in racing, especially in Formula 1 races or in  
regulated fuel consumption races like Le Mans.

But although in street cars and closed-wheel race cars the rolling resistance  
of the tire contributes to overall resistance, at highway speeds and above  
aerodynamic drag dominates (except in special low-drag vehicles, like GM's  
Impact electric car).  In open-wheeled cars, especially Formula 1 cars, the  
drag created by those big tires spinning against the direction of motion is  
huge.  Reducing that drag and the turbulence off the tires is one goal of the  
aerodynamicist.  In cars with wings, the rolling resistance of the tires is  
pretty insignificant compared to the drag of the tires and the body (which has  
a very high coefficient of drag compared to a street car in order to produce  
very high downforce).

But the "grip" of a tire, or how well it generates forces tangential to the  
track surface, is not necessarily proportional to rolling resistance.  Rolling  
resistance is mostly caused by flexure of the tire sidewall and tread and to  
some degree by slip at the tire/road interface.  Rolling resistance does tend  
to go up with tire width.  Rolling resistance can be reduced dramatically by  
running extremely narrow, low-profile tires at a very high pressure (which is  
what many experimental electric vehicles and solar-powered cars use).  Trains  
running on railroad tracks are this principle taken to the extreme.  Because  
flexure of their sidewalls does not require as much energy input, radials  
generally have significantly lower rolling resistance than bias-ply tires.

On dry pavement, the grip of a tire in acceleration, braking, or cornering is  
mostly a function of the rubber compound and the size of the tire contact  
patch.  Tires do follow the classic law that frictional force is equal to the  
product of normal force (the sum of the weight on the tire and the lift force  
on the tire) and the coefficient of friction, usually called mu.  If mu were  
constant for a specific tire compound, there would be no advantage to running  
with wider tires.  But it turns out that mu decreases with increasing  
pressure--not inflation pressure, but the normal force on the tire divided by  
the area of the contact patch.  This phenomenon is a result of the localized  
temperature distribution across the tire contact patch:  the higher the  
pressure, the more frictional energy that has to be dissipated per unit area,  
and therefore the higher the local temperature.  For tire compounds, above a  
certain temperature grip starts decreasing.  Wider tires simply provide a  
larger contact patch, which in turn decreases contact pressure, which increases  
mu.  

In theory, there is an optimum tire width, which is a function of the amount of  
increased drag contributed by the tire compared to the increased grip afforded  
by the larger contact patch.  I can't think of any racing series where that  
optimum can be reached within the rules.

Rain tires are subject to the same principles, but a wet track dramatically  
cools a tire and allows use of much stickier compounds (which melt if driven  
hard on a dry track).  The only additional confusion is the sometime use of  
narrower tires to _increase_ contact patch pressure; the higher pressure forces  
water from between the tire and the pavement.  Similarly, the grooves are to  
channel water away from the contact patch and allow the tire to touch the track  
surface.  Formula 1 rain tires are not narrower because they are _extremely_  
sticky and have very effective tread patterns (this also shows how far the dry  
tires are from optimum tire width).

I'm not familiar with the mechanics of snow, ice or dirt-track tires.  Anyone  
know?

--
Nathan Ulrich
Deep Submergence Laboratory
Woods Hole Oceanographic Institution
ulrich@canberra.whoi.edu
Newsgroups: rec.autos.sport