Dick, et al.,
A friend of mine came across the following web site that focuses on the
Dunlop SP8000 tire and has more generally useful data on tire sizes,
diameters, radii, measured @ effective circumferences. The site is:
A little explanation may be helpful. The "Dia" and "Circ" columns are
simply related by pi as expected for circumference and radius in the ideal
case. The values for "Rad" I believe are the static compressed radii which
can be used to determine ride height when the car is not moving. The
"Actual" numbers are the effective circumferences taking into account
various factors which I presume include factors that come into play on the
highway like tire slippage and centrifugal force. The "R/mile" values and
the "Actual" numbers are consistent, based on 63,360 "/mi (5280x12).
As you can see, the effective circumference is significantly less than the
measured, unloaded, static value. On the other hand, it is larger than the
value based on the static loaded radius. In fact, the effective radii
values in this table are very close to halfway between these extremes. Not
knowing exactly what assumptions went into these numbers, I can only assume
that they represent approximate values based on factors like speed and road
conditions. For most of us, these numbers should be close enough, and are a
much better approximation than putting a tape measure around the tire.
So why is there a difference between "actual" and measured circumference.
This is a tricky question and one which I've given some thought to since
our last conversation. If one accepts that the actual tire circumference
(peripheral)dimension does not change when the tire is loaded, then how is
it that the distance the car moves is not just equal to the circumference.
The answer is to consider the tangential motion at various positions around
the perimeter of the tire. With a perfectly round tire and the center of
rotation in the center, the motion is everywhere the same. BUT, if the tire
is compressed, then the center of rotation is no longer at the center of
the tire and different parts of the tread are moving differently; more or
less depending on their respective radii from the center of rotation
(axle). If the motion over the whole circumference is integrated (summed)
then, again assuming no change in total circumference, one revolution at
the axis gives one revolution at the perimeter. BUT, the distance moved in
this ideal case is the compressed radius of the tire. Now, in practice,
this value is an underestimate of the distance travelled because of other
factors, some of which I have mentioned and others which I may not have
Based on this discussion and the numbers you can get from the above web
site, if you calibrate your odometer by measuring how many revolutions of
the speedo cable in 1/100 mile, your answer will be high by about 5%;i.e,
the difference between the distance based on compressed radius which would
apply for slowly rolling the car and the "actual" values that apply to
normal driving down the road.
Hope this at least sheds a little more light (perhaps too much) on the
UCSD, AMES Dept.