[T3] Tire inflation pressures

[Jim Adney]

On 15 Jul 2011 at 13:28, Daniel K. Du Vall wrote:

> Which brings up a question are you recommendations for radial or bias
> ply tires? Mine are radial and on 5.5X15 rims with p186/65 R15 tires.

The numbers I gave are for stock 165SR15 radial tires, although I 
think they would apply equally well to stock bias 165-15 tires. VW 
did indeed recommend a couple psi less for bias ply tires, but I 
never saw any good reason for this and haven't driven on bias ply 
tires since about 1970.

Larger tires, like yours, would do best with even less (yes, really) 
pressure.

Think of it this way: Pressure is force per unit area. You're trying 
to support a certain amount of weight on a certain amount of ground 
contact area. The inflation pressure of the tire, times the area of 
the contact patch, needs to be about equal to the weight supported by 
the tire.   

So if you use a larger tire, with a larger contact patch, you need 
less pressure.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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