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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).
Explaining different formulas used for calculating car wind resistance coefficients.

Identifier  ExFiles\Box 161\1\  scan0244
Date  1st December 1938
  
To By.{R.W. Bailey - Chief Engineer}
c. to Da.{Bernard Day - Chassis Design}
c. to Da{Bernard Day - Chassis Design}/Ev.{Ivan Evernden - coachwork}
c. to Rm{William Robotham - Chief Engineer}/IMW.{Ivan M. Waller - Head of Chateauroux}

WINDAGE.

Attached is a list of cars with their wind resistances coefficients 'K' calculated, for the sake of comparison on different bases.

A.{Mr Adams} Rolling resistance is assumed to remain constant at 30 lb/ton, and K is obtained from first principles -
H.P.∝ RV
R = KAV²

B. As A, except that rolling resistance is obtained from the Andreau formula.
RT = (1 / p 0.64) { 245.6 + v 3.7 / 397p 1.44 }
Where p = tyre pressure lb/in²

C. The Andreau rolling resistance figure is used and K is derived from the formula.
K = (W / 2255V₁V₂A) { (101.7 (V₁ - V₂) / t) - RT }

Since this is a 'deceleration' formula and only horse power at max. lap speed figures were to hand,
(V₁ - V₂) / t
was calculated direct from the pull exerted by the hub horse power, and (max. lap speed)² was substituted for V₁V₂.
  
  


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