<VV> Re:Tirez

Kirby Smith kirbyasmith at gwi.net
Thu Oct 20 16:57:35 EDT 2005


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.


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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