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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 effect of centrifugal force on tyre revolutions and mile counter calibration.

Identifier  ExFiles\Box 59\2\  Scan053
Date  9th August 1930
  
EFC1/ADL9.8.30. contd.

-2-

tyres most highly inflated and at the higher speeds at which the car is run on a level road.

It appears from the numerical results mentioned in Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}2/ADL4.8.30 that the effect of centrifugal force on the tyres is to decrease the figure of the revolutions per mile at the very high speeds as compared with that at low speeds by about 2½%.

It would therefore appear reasonable, after having arrived at the minimum figure of the drive revolutions per mile as affected by size of tyre and degree of inflation, to further reduce this by an amount of 2½% in order to arrive at the absolute minimum figure which is to be taken into consideration in specifying the revolutions per counter mile of the instrument itself (see below).

The Instrument.

Mile Counter.

The number of revolutions of the instrument spindle corresponding to the indication of one mile on the counter can be arranged by the makers at certain figures, the steps between which are not, however, so large as to introduce any serious error in selection. The selected figure will be as near as possible to the minimum drive revolutions per mile referred to under "The Drive" with however, a bias in favour of making this figure smaller than that other to ensure that in no circumstances will the mile counter
  
  


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