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## Tire data

 To: tigercoupe@aol.com Tire data Bob Palmer Thu, 29 Jan 1998 13:39:54
 ```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 thought of. 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 subject. Best regards, Bob Bob Palmer UCSD, AMES Dept. rpalmer@ames.ucsd.edu ```
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