I've often wondered this same thing myself Jon,
is a closed form solution for adjusting pressures when changing tire
sizes? since most tires are marked with a load capacity (the 93H, 87V or
103S, etc. number that follows the tire size) that indicate the maximum
load weights at max pressure, one would expect there to be arelation to
adjust pressure based on the vehicle's max load capacity as defined on
the door jamb sticker.
from my space shuttle tire days, minimum pressure was determined by
protecting the limit of sidewall deflection. thus more load, more
pressure. although a 40 inch diameter-16 ply-triple wire bead aircraft
tire has little in common with passenger car tires other than being black
and round, I'm sure the passenger car tire manufacturers want to limit
sidewall deflection on car tires as well since it is a contributing
factor of internal delamination and eventual failure. thus for a given
load, there is a minimum pressure but that is usually not the optimum.
Rob may have a more precise answer but here's an old off-road4x4 trick I
recall used to determine hi-way pressure for oversized tires on trucks.
Start with the max inflation pressure on the sidewall, mark the treadface
of the tires with chalk or similar and drive for a few yards to determine
the "wear" pattern. adjust air pressure up or down as if the tread had
actually worn.
anyone else?
Dave Yeung
Houston Region
On Tue, 31 Oct 2000 12:43:47 -0500 Jon Rush <jonathan.rush@sdrc.com>
writes:
>This is an open question for Rob Pickrell.
>
>Rob,
>
>I have always appreciated your openness about sharing your knowledge
>of
>technical information about tires. And being in the tire business,
>I'm
>sure you encounter this frequently. How do you set the pressures for
>street tires when the customer has upgraded in tire size or
>performance?
>What if the customer has modified the suspension. Do you stick to the
>car manufacturer's recommended pressures to avoid liability? Do you
>use
>some rules of thumb for pressure changes, because obviously you can't
>test every combination in a controlled environment like we do at
>autox.
>
>The tradeoffs for the street are different than the demands for
>autox.
>Safety is important and long life is usually a prime concern. By the
>time you discover you've over or underinflated a tire by the rate of
>wear across the tread, you've worn away a good bit of useful rubber.
>
>How do the tire manufactures optimize their tire designs? Do they run
>all their tests at a standard pressure? Or do they try to design a
>tire
>that will work at a whole range of pressures? Can we ever find out
>what
>the design pressure range is?
>--
>Jon Rush
>Consultant, Engineering Solutions
>610-966-0923
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