-=> Quoting Roy McNeill to Bob Lawrence <=-

 RM> 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.

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

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

Going against established wisdom and it "is so simple stupid"
arguements is paramount to uphill battles and windmill fighting.

Since playing Don Q is one of the things I enjoy doing to keep other
peoples brains flexible I decided to throw my bit of science in the pot 
as well and give you a hand (sort of).

If you are right there obviously has to be a compensating mechanism,
I.e. on the surface it seems since the radius is obviously reduced there 
have to be more axle revs for a given distance.
However, the tyre is flattened over a wider area, not just at the point 
of minimum radius.

For you to be right these extremities of contact with the road have to be 
greater then the original radius, i.e. the tire needs to be squared of a 
bit.
That this is the case is easily seen, since it is your arguement you
can measure and report if the bulge at (two) ends with increased radius
will indeed compensate for the radius reduction in the (one) center.

Simply pullling the integral over the linear distance would solve it
conclusively.

QED, 
don't thank me:-)

Cheers,

Theo

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