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).
Clutch performance, slipping, and experimental testing methods for smooth pick-up.
| Identifier | ExFiles\Box 129\4\ scan0123 | |
| Date | 8th January 1938 | |
| -2- and may be dismissed so far as the average customer is concerned. (c) At high rubbing velocity, such as when attempting to start from rest with the engine racing, an air film may be maintained between the friction surfaces and prevent effective contact, which would in part account for the apparent loss of /u at high rubbing speeds. (d) Most facing materials have a lower /u at high temperatures, and high local temperature may be quickly absorbed by the mass of metal so long as slipping is not unduly prolonged, and this fact may also account in part for a clutch picking up after a brief easing of the throttle. Experience shows that in order to ensure a smooth pick-up with no suggestion of snatch, it is necessary to avoid excessively strong thrust springs such as are necessary for a 'stunt' such as starting on top with the engine racing all out, in other words, with the maximum rubbing velocity. Our experimental method of testing for slip is as follows Drive up an average main road hill at 30 to 35 m.p.h. open throttle fully by accelerator pedal, and declutch, allowing the engine to race to 4,000 - 4,500 r.p.m. then re-engage the clutch keeping the engine flat out all the time if the clutch picks up the drive completely in 2 to 3 seconds, no fear of excessive slipping need be anticipated under any normal driving conditions, if however this test is repeated but from about 10 m.p.h. instead of 30, it is probable that the clutch may continue to slip till the throttle is momentarily eased, after which it should pick up the drive. It is not suggested that the above rather drastic tests are applied indiscriminately to customers cars, and certainly not to the 20/25 H.P. or the 3 1/2 litre Bentley, and our reason for quoting this experimental test is to make it quite clear that the Thermoid faced clutch on the 25/30 and 4 1/4 litre may be expected to slip under abnormal conditions. We turn out the car with a spring strength suitable for all normal driving conditions, but not for stunt starts, so as to ensure a smooth pick-up with minimum wear, tear on the transmission and tyres, and maximum passenger comfort, while providing ample margin for high speed gear changing or starting on the steepest hills. | ||