Rolls-Royce Archives
         « Prev  Box Series  Next »        

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).
Comparative analysis of car performance metrics including wheel size, weight, and engine displacement.

Identifier  ExFiles\Box 160\2\  scan0088
Date  18th July 1925
  
Oy1-E-71825
-2-
July 18th, 1925.

(4) Wheel size. The nominal tire size is taken.

All American tires are bigger than the nominal size, but the actual rolling diameter of the tire, due to its compression at the road, is always very near to the nominal size.

(5) Weights.
These cannot be ascertained very accurately. They refer in each case to the weight of the standard touring car, with tanks full but without passengers.

They do not refer to custom-body jobs which are nearly always much heavier than the standard car of the same type.

(6) Displacement per foot.
This is the actual volume of firing mixture (in cubic inches) taken in by the engine while the car travels one foot along the road in high gear.

It is arrived at thus:-

Engine displacement x axle ratio x 12
------------------------------------------
2 wheel size x Π

It is this figure which gives a comparative estimate of the real "push" which should be delivered at the rear tires (with equal friction losses and equal explosion pressures).

(7) The seventh column is arrived at by dividing the figures of the sixth column by the weight of the car in tons.

This figure is an indication of the acceleration or hill-climbing power of the car, but not of its maximum speed on the level.

The latter depends more on the frontal area of the car than on its weight, and quite as much on the design of the valve mechanism and intake system as on the power of the engine.

In general also a higher ratio in the rear axle will give a higher maximum speed on the level, but less hill-climbing power.

The remarkable feature is the little variation in the Displacement per ton-foot on the large variety of cars represented, the highest being Ford with 49.5 cubic inches per ton-foot, and the lowest Rolls-Royce with 39 cubic inches, and Cadillac with 38.9 cubic inches.

The average of the 21 cars is 42.84.
  
  


Copyright Sustain 2025, All Rights Reserved.    whatever is rightly done, however humble, is noble
An unhandled error has occurred. Reload 🗙