On (07 Oct 98) Bob Lawrence wrote to Roy Mcneill...


 BL> Advertising aside... a steel radial can have softer sidewalls
 BL> for the same distortion, which allows the tread to remain in
 BL> contract with the road rather than roll over, the way fabric
 BL> tyres with stiffer sidewalls do. The steel radial actually
 BL> distorts more... in the sidewall, to keep the tread on the road
 BL> and therefore wear less.

 RM> This is the exact argument I'm using to say that tyre pressure
 RM> doesn't affect speedo readings. If one revolution of the
 RM> circumference on the road covers the same linear distance,
 RM> regardless of the shape of the tyre, how can pressure affect
 RM> the speedo?

 BL>  Take a look at the "IF" in your statement.

Okay, let me rephrase it in four^H^H^H^H three letter words.

If, as you say, the tread stays on the road and wears less, then it
will resist being compressed lengthwise as well. This lengthwise
compression is crucial to your argument that tyre pressure
influences speedo readings, because if the tread resists being
squashed lengthwise as the tyre gets flatter, then the overall
circumference of the tyre will stay sorta constant. I said "sorta"
here because nothing's perfect, and there's bound to be a bit of
distortion in the real world. But, as I suggested in my *original*
post to you that started this flame war, I reckon this distortion
will be far less than most people think. My experiment seems to
confirm this, the result was a lot closer to my prediction than it
was to dorkbrain's prediction that The Speedo Reading Is Governed
By The Distance From The Axle To The Ground.

Clear enough, or do I need to use two letter words?

Cheers

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