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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 expected speed increase from a 35% increase in power.

Identifier  ExFiles\Box 73\3\  scan0368
Date  6th February 1924
  
R.R. 403A (40 H) (SL 42 12-7-23). J.H., D.{John DeLooze - Company Secretary}

EXPERIMENTAL REPORT.

Expl. No. 4578

REF: Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}/LG6.2.24.

INCREASE OF SPEED TO BE EXPECTED FROM
A 35% INCREASE IN POWER.

The tractive resistance of a car consists of two
main factors, a constant which may roughly be said to consist
of the bearing friction, tyre rolling resistances etc. and a
factor which varies as the square of the velocity which is
mainly due to the wind resisting frontal area of the car.
At high speeds, by far the most important factor
is therefore the nature and dimensions of the frontal area
of the body. In an ordinary touring body with big wings,
the tractive resistance increases enormously at speeds above
60 m.p.h. the extent of this increase is shown on the
attached graph compiled from tests taken 26.2.23 on 6.EX car,
a car with very fairly sporting outlines.
The method of plotting the curve above 60 m.p.h.
at which speed we cannot measure the tractive resistance
practically, is shown and we should say the result is a very
fair approximation of actual conditions.

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