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).
Letter from Cadillac Motor Car Company regarding the results of Bumping Rig tests on springs.
| Identifier | ExFiles\Box 170\2\ img105 | |
| Date | 19th October 1931 | |
| for Hrs 150 : 60Hrs CADILLAC MOTOR CAR COMPANY DETROIT, MICHIGAN CADILLAC and LASALLE MOTOR CARS RM{William Robotham - Chief Engineer} GRY{Shadwell Grylls} October 19, 1931 Mr. E.{Mr Elliott - Chief Engineer} W. Hives Rolls Royce Ltd. Derby, England Subj: Bumping Rig Dear Sir: So far we do not seem to break anything important except front springs and shimmy shackle springs. Shimmy shackle springs undoubtedly suffer worse than on the road because it is not practical to use as long a bump as one actually meets on even a rough road. They seldom break in actual use, neither do the springs fail at the front wrapped eye on the road nearly as frequently as occurs on the bumper. The indication therefore seems to be that we need to smooth out our cams as you have done rather than using circular arcs about 12" long and 1", 2", and 3" high as we have done. (Other makers, Chrysler for example, use just a square steel or wood bar instead of a cam). Our destruction of front springs and shimmy shackle springs occurred at 7 mph with 3" cams. Springs failed at about 20-25 hours and shimmy shackle springs more frequently. The steering wheel was not held in these tests but was left free, the castor angle and a front guide on the frame being relied on to keep the wheels running straight. Since the wheel reaction is small anyway I thought this was a fairer test than clamping the steering wheel rigidly. We use a steel body shell mounted in the proper way on the frame, and ballasted to give 4 passenger equivalent, so as to reproduce road conditions as closely as possible. At 15 mph with 2" cams the destructive effects seem very much less though we only have about 20 hours so far. At 5 mph with 3" cams the car rolls a lot and puts work into the rear springs and shock absorbers, but the effect does not seem destructive as there is less twist of the frame than at 7 mph. (In giving these speeds in mph they are strictly comparable with your results since we are using 48" drums, with 2 cams on each, and cams set 90° apart on the two drums). | ||