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).
Comparing clutch spring loads, leverages and pedal pressures between the 25 HP and 40/50 HP models.
| Identifier | Morton\M9\ img084 | |
| Date | 21th November 1931 | |
| To Da.{Bernard Day - Chassis Design} From Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} ORIGINAL Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}1/AD21.11.31. x5480. With reference to your enquiry re the clutch spring loads and leverages Da{Bernard Day - Chassis Design}2/M7.11.31, we attach a sketch showing the existing leverages. We have very carefully checked the rating of the springs used on 25 HP. and find that the actual load per 454132 spring is almost exactly 125 lbs. Having the maximum M.E.P. of the Phantom II as 100 and the 25 HP. as 107 and allowing for the respective differences in mean radius of the friction disc, the correct ratio of the 40/50 clutch springs to those of the 25 HP. to give equal immunity from slipping is 1.42 i.e. the 25 HP. clutch spring ought to be 185 lbs. This explains why the 25 HP. clutch is always on the verge of slipping. The reason we have not put the spring load up is because we cannot do so without making the pedal pressure unduly heavy. The overall ratio of the 40/50 is 44.9 : 1, the 25 HP. only 26.5 : 1. Therefore, theoretically, the pedal pressures with 126 lbs spring in the 25 HP. and 200 lbs spring in the 40/50 are 38 and 39 lbs respectively. Actually the 40/50 mean pedal pressure does come out to approx. 40 lbs. On 18-GIV running in France, however, with spring G.54136 of which we attach a print, the pedal pressure Hancock gives us is, starting 53, finishing 66 lbs. We are checking up these figures here and they seem unnecessarily high. The general opinion is that the 40/50 clutch pressure is just about on the top limit of comfort, and with the 25 HP. being a smaller car more frequently driven by ladies, should require less effort to operate the clutch. Obviously, if we can manipulate the 40/50 clutch with 40 lbs. pedal pressure we ought to be able to work the 25 HP. with considerably less effort. For our improved performance on the 25 HP. therefore, on the lines of 18-GIV we propose to alter the overall leverage and to use the 143 lbs clutch springs - even then we may be slightly worse off than the 40/50 with regard to slipping. We will give you some more reliable figures for pedal pressures as soon as we have got an average. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||