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).
Engine vibrations ('boom') in the 40/50 engine and the effects of modifying connecting rod length.
| Identifier | ExFiles\Box 182\M19\ img095 | |
| Date | 9th January 1931 | |
| To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson} c. to Ss.{S. Smith} Wor.{Arthur Wormald - General Works Manager} c. to Da.{Bernard Day - Chassis Design} E.{Mr Elliott - Chief Engineer} c. to RY. ORIGINAL ENGINE VIBRATIONS. Tests have been carried out on the 40/50 engine reported in Hs{Lord Ernest Hives - Chair}/Gryl/MJ.7.1.31 which convince us that the flywheel is largely responsible for the over-run and forward boom above 55 M.P.H. When taking measurements of the flywheel movement on the test bed, the boom can actually be heard and felt at the moment that the recording apparatus shews a displacement. We therefore recommend that immediate design steps should be taken to eliminate this disturbance. We still consider that the 'boom' is the worst feature of the Phantom ll car, and suggest that it is more manifest on this car than on Phantom l because of the amplification of the gearbox. We were discussing the optimum connecting rod crank ratio for car engines with E.{Mr Elliott - Chief Engineer} in view of the proposal to lengthen the new 25 HP. rods to the same ratio as the Phantom ll. It is a fact that the Expl. Dept. recommended a change from 10" to 11.5" connecting rods when the Phantom l was first introduced. The object was to eliminate the over-run rattle in the servo gears but it was eventually found that a fabric coupling had to be fitted even with the longer rods. Therefore in this case we had no positive information that the longer rods were an improvement. Japan had slightly longer rods than China but again we cannot say that we have any positive results shewing them to be better. We admit the Chrysler and Graham Paige both have the same con rod/crank ratio as Phantom ll but we suspect that they may have blindly copied us because actually adding 1" to the existing 20 HP. rod only decreases the 3rd. harmonic of the inertia torque by about 12% according to Tsn. The fact remains that we cannot say that we have reduced over-run vibrations with longer connecting rods though naturally they reduce the side thrust on the piston. Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson} | ||