Ok guys, we have been having simultaneous discussions of tires and their
measurements on the Spridgets list and the MG list. The questions were about
diameters, radii and circumference -- both loaded and unloaded.
I decided to do a little experiment using a wheel off of my MGB and one off of
my Bugeye. The MGB tire is 185/70 - 14 while the BE tire is 145 - 13.
I took the wheel off the BE to measure it unloaded and found a diameter of
21.75 and a radius from wheel center to floor of 10.875 (measurements are
approximated in inches as best I could on my garage floor). The circumference
measured with the flexible measuring tape I stole out of my wife's sewing box
was 68.4.
I left the tire on the MGB and let enough air out of it to get a radius from
wheel center to floor equaling the Bugeye's 10.875. The diameter of the tire
at that point was 22.75 measured from the floor to the top of the tire and
about 24 across the horizontal (these diameter measurements are eyeballed and
probably not extremely accurate). I measured the circumference by laying the
tape on the floor and rolling the car over it and wrapping it around the wheel
to measure. The circumference was 77 inches. What I then noticed is, that by
holding the tape over the tire from the backside and rolling the wheel
backward over the tape, the tape separated well away from the tire -- like
about an inch off of it in a quarter turn of the wheel.
Next I measured the distance the tire traveled in one complete revolution of
the wheel. The Bugeye with an unloaded tire circumference of about 68.4
inches traveled about 68.25 inches. Not surprising and well within the margin
of error of my crude experiment.
The MGB with a loaded circumference of 77 inches traveled only 73.75 inches.
This is less than what I expected, but more than the Bugeye's 68.25 inches
with the same radius from the wheel center to the floor.
This "seems" to suggest that the truth lies somewhere inbetween the idea that
the radius from wheel center to the floor exactly determines distance traveled
in one revolution and the idea that circumference of the (squashed) tire under
load is equal to the distance the tire will travel in one complete
revolution.
If anyone else is willing to duplicate this, I would like to see what you come
up with. Note that I didn't repeat the whole exercise to see if the results
came out the same.
Now I need to clean off the wife's measuring tape and sneak it back into her
sewing kit.
Larry Daniels
79 MGB LE
72 Midget
58 Bugeye
"You only need two tools: WD-40 and Duct Tape. If it
doesn't move and should, use the WD-40. If it
shouldn't move and does, use the duct tape."
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