I believe I have a clearer answer to this "apparent" discrepancy.
At 09:45 AM 6/6/05 +0100, Paul Hunt wrote:
>....
>The biggest discrepancy is that my tyre measured 1940mm circumference
>whether inflated to 24psi, 10psi, laden or unladen. But it only travelled
>1885mm at 24psi and 1876mm at 10psi.
The inner plies of the tire, especially for a belted tire, will not stretch
or compress much under any circumstances. This means the steel belt
remains the same length regardless of air pressure or deformation of the
tread. In the measured example above, the actual measured travel distance
divided by PI equates to the diameter of the steel belt, which is at about
3/8 inch less radius from the outer tread. The rolling travel distance is
therefore very nearly equal to the circumference of the steel belt.
As the tire tread is deformed to be flat against the ground, the outtermost
part of the tread is compressed slightly to be shortened to the same length
as the steel belt over the length of that flat area. This then accounts
for the slightly reduce travel distance relative to the tire circumference,
not the length of the flat relative to the initial arc length.
The much smaller variation in rolling distance with the large reduction of
air pressure is accounted for by more squirm in the tread as the sidewalls
flex considerably. In essence, when you have a gross amount of squirm the
tread slips some on the pavement, and the total travel distance is slightly
reduced. In the example above the travel difference between 24 PSI and 10
PSI is only 0.5% (representing a lot of squirm on a half flat tire).
>However the rolling radius (which is what is used in speedo calculations)
>indicated that it should have travelled 1823mm at 24psi and 1615mm at 10psi.
No, that's small point of confusion. The rolling radius noted is likely to
be the distance of the wheel center from the pavement, which changes with
air pressure but has not much to do with travel distance. If you look at
tire stats you will often find a specification for Revs Per Mile, which is
the number needed for speedometer calibration. This would correlate closer
with the actual measured travel distance in the example above.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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