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 variables affecting braking performance and the reluctance to provide definitive braking figures.
| Identifier | ExFiles\Box 113\2\ scan0071 | |
| Date | 19th March 1925 guessed | |
| contd :- -2- It is most difficult to accurately estimate the co-efficient of friction between the tyre and the road surface, the more so as the surface of the road varies even from yard to yard. This can be noticed when pulling up on a warm day on a tarred road - in some places the tar is softer mx{John H Maddocks - Chief Proving Officer} than others and the wheel will slide. A high average value for this co-efficient of friction is .7 for a dry macadam road. This will, of course, vary slightly with the tyre characteristics. Concrete is the best surface on which to carry out braking demonstrations of maximum stopping ability. (2) On any four-wheel brakes it is most difficult to estimate accurately the front to rear proportioning which is actually being obtained on the car. This can be altered very considerably when some large variation in the efficiency of the servo or the brake shoes, has taken place. (3) In some cases the best pull-up will be accomplished when the wheels are just not locked, in others when they are locked. In general, on a smooth surface, the former statement is true, on a rough surface, the latter. An expert might be able to obtain the best results by just preventing his wheels sliding, a thing he would be unlikely to do in an emergency, and therefore, useful only for demonstration. From The above points it will be seen there is some reason for shewing reluctance to give braking figures to be taken as an accurate representation of what the car may be expected to do. People who are not cognizant with the subject seem to be only too willing to estimate the performance of any car in one series of tests without even giving any full particulars as to how the tests were carried out. We therefore consider that the calculated curves which we are giving you are as valuable as any practical figures. We should like to draw your attention to the shape of the time velocity curves and braking curves. The time velocity curve is one in which the increase of velocity steadily falls off with increase in speed. contd :- | ||