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From: Tegger on 7 Nov 2009 17:44
"C. E. White" <cewhite3(a)mindspring.com> wrote in news:hd18tv$dr6$1
> I am sure you are greatly over emphasizing the contribution of the
> loaded radius on the rolling diamter of tires, but don't know of any
> other effective arguements. Maybe an experiment would convince you. If
> you have the time, measure the loaded radius, mark the tire, move the
> car for 100 revolutions of the tire, and then measure the distance
> moved...You will find that it moved a significantly greater distance
> than 2 x pi x loaded radius x 100.
I must be nuts, because I actually went out and tested your theory (and
mine). I hope you do me the grace of actually reading this, because I did
perform the test instead of being Usenet-snarky and telling you to go and
do the test yourself.
After making a gauge with corrugated cardboard, I discovered that the
unloaded diameter of both front and rear tires is 23.25" dead-on.
23.25" x 3.14159 = 73.04" unloaded circumference.
Tire pressures at time of test were all the same, 31 lbs (checked hot).
On a flat, newly paved industrial parking lot, I marked the tires (and the
lot) with chalk. Leaning out the window, I then slowly rolled the car so
that the mark on the left tire described ten revolutions, coming down to
the very bottom again. That covered almost 60 feet (100 revolutions was not
practical for me.)
I did this four times each, for the front and then for the rear tires (both
sides). The results were very consistent.
Actual distance covered for the fronts: 704.5"
Actual distance covered for the rears: 708.5"
Now, how about the "loaded" radius?
For the front left, it's 11.75", which gives a circumference of 67.54"
However, that tire actually covered 70.45" per rev, not 67.54". But at the
same time it was not covering 73.04" either.
For the rear left, the loaded radius is 11.0625". This gives a
circumference of 69.51". That tire actually rolled 70.85" in the test.
Let's summarize, as percent reduction from unloaded to loaded:
Front hypothetical rolling circum based on loaded radius: 7.5% less
Front actual rolling circum based on test result: 3.55% less
Rear hypothetical rolling circum based on loaded radius: 4.8% less
Rear actual rolling circum based on test result: 3% less
Looks like the fronts split the difference between unloaded and loaded, and
the rears were affected pretty close to what I theorized.