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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The calculation of the effective circumference and diameter for a 20 HP car's balloon tyre.

Identifier  ExFiles\Box 53\3\  Scan049
Date  5th April 1927 guessed
  
A.J.L. Please <s>file</s>

<s>Could you let me have the effective rolling dia of the 33 x 5 1/4 20 H.P. balloon tyres at 35 lbs / □"</s>

20 H.P. 33 x 5 1/4 tyres
35 LBS / □"

Effective circumference = 96.4375"
.. diameter = 30.70"

(New type; weight of rear of car on which tyre was fitted = 1 TON
Weight of N.S. rear of car, side on which wheel was fitted for measurement = 10 cwt)
  
  


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