On (11 Oct 98) Terry Smith wrote to Roy McNeill...

 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 the
 RM> speedo?

 TS> As those who have defiled the classic Greek ideal of science have found,

Que? What's that?

 TS> your question needs to be "As pressure affects the speedo reading,

Pressure certainly affects the speedo reading. What I'm arguing is
that the speedo reading is *not* proportional to the distance from
the centre of the axle to the ground, as everyone else appears to
think. The experiment I did demonstrated this (see msgs to Rod S
and Bob L, dated 30 May, subject line "real speed" - for a radius
change of 4.8%, the speedo reading changed by 0.7%)

Btw, haven't we been here before?

 TS> how can the `circumference on the road' [whatever that gobbly-gook
 TS> means - you'd need a rather curved road to get such a thing] cover
 TS> the same linear distance?".

Think of a caterpillar track on a bulldozer. It's curved in all
sorts of funny ways, but its circumference (perimeter, if you like)
is constant. Let's say its drive gear has 8 teeth, and the inside
of the track has the equivalent of 80 teeth. Rotate the drive wheel
10 times, and the vehicle will move forward 1 track circumference
forward. Bend the track into all sorts of funny shapes, including
circular, and the distance travelled forwards for 10 drive wheel
revs won't change.

Now think of a steel radial tyre: I'm suggesting that the steel in
it will keep the tyre's circumference reasonably constant as it
distorts with pressure changes. Assume that it stays exactly
constant, then one rev of the axle will carry the vehicle one
circumference forward. Relax the assumption, and allow the tyre to
deform a little bit (as it will in the real world) and the distance
will reduce a bit, but certainly not by the same fraction that the
axle to ground distance has changed.

 TS> There is a short and simple answer to your long and simple question.

Everyone says it happens, but I seem to be the only one who's
quoted actual figures. 0.7% vs 4.8% .



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