<VV> :Tire size

BBRT chsadek at adelphia.net
Thu Oct 20 17:38:45 EDT 2005

I suspect, for a given design tire, the effective rolling radius is a 
function of load, temperature, air pressure, and rotational speed. At load 
speeds, seems to me the effective radius is less than the circular radius 
resulting in a rotatating "circumference" less than an unloaded tire 
circumference, but  with increasing rpm, then growing with rpm and exceeding 
the unloaded circumference at some point due to rotational forces with 
increasing rpm. In other words, the tire becomes circular and bigger than 
the unloaded radius, thereby giving a larger circumference.

So, when you get to a130 -150 mph, there probably isn't much distortion of 
the circular radius at speed. From then on, you are going even faster than a 
calculated rpm-derived speed.

Chuck S

----- Original Message ----- 
From: "Kirby Smith" <kirbyasmith at gwi.net>

> Bruce:
> Do you consider the static loaded radius a useful (proper?) means of 
> calculating revs per mile from which speedometer calibration can be 
> inferred?  The reason I ask is this historical tidbit.
> Once upon a time there was a large deck tape recorder series manufactured 
> by Ampex (which I noticed last week still have a building in the Valley). 
> The tape speed was determined by a capstan that pinched the tape between 
> itself and a rubber idler so that tape speed equalled the surface speed of 
> the capstan.  Below the deck the capstan axle was attached to a steel 
> wheel with a rubber band around it about 1/4 inch thick.  This wheel was 
> driven by a precision diameter metal wheel attached to the motor.  The 
> motor wheel was smaller in diameter than the capstan wheel by a factor of, 
> say, five.  The tape speed could be tweaked by changing the motor wheel to 
> capstan wheel pressure.  The interesting thing was that if one wanted the 
> tape to go slower, the pressure was increased, pushing the motor wheel 
> into the capstan drive wheel, reducing the rolling radius (which one might 
> think would make it spin faster).  What was happening was that the motor 
> wheel had to travel along the entire circumference of the capstan wheel to 
> generate one capstan revolution, and this circumference was slighly 
> _lenghtened_ by distorting the rubber inward.
> I have always wondered whether the rolling circumference of a tire is pi 
> times the loaded radius, or something larger that the tire companies 
> determine by experiment.
> kirby
> Bruce Schug wrote:
>> Here's another figure of interest, especially to Bill who is trying to 
>> figure hub to wheel opening figures. The static loaded radius for the 
>> 6.50-13's ranged from 11.1 - 11.5. For 7.00-13's; 11.5 - 11.9 and for 
>> 7.00-14's 12.1 - 12.2.
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