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).
Analysis of a disastrous 'jagger' failure on a 4-B-IV vehicle, investigating causes and proposing improvements.
| Identifier | ExFiles\Box 88\2\ scan0167 | |
| Date | 2nd March 1936 | |
| -2- Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}10/KW.2.3.36. We think that the reasons why 4-B-IV suffered from this disastrous jagger were - (1) Experimental servo giving more efficient front brakes. We took the servo complete off the wrecked chassis and found that we could by trick driving produce brake jagger at will on another car that had previously been free from the trouble. (2) Experimental front road springs having less inter-leaf friction. We have no positive evidence on this point as yet, but it seems probable that these influenced the behaviour of the axle. We have to admit that, doing the most desperate things we know of, we have so far failed to twist off the end of a Bentley front axle. In the case of 4-B-IV, the partly broken end of the axle was twisted 20°. We have twisted one 9° by trick driving on the road, but we cannot create the phenomenon at 80 m.p.h. without endangering life. However, we intend to persevere on our road wheel drums because we feel that up to date the reproduction of the violence of what must have happened in this case has eluded us. The bumper bar falling off, combined with a jagger, may have been responsible for the result, all we can say is that we have made tests at 75 m.p.h. dropping off the bumper bar and little or no damage of any sort has resulted. We say that results show that the standard car does not suffer from jaggers that damage the front axle. We have never been satisfied with the margin of safety, however. The two experiments in hand to increase the margin of safety are - (1) Improved swinging arm friction device to prevent thumps. Run 7000 miles on 21-G-IV, fitted to B-56-BN.{W.O. Bentley / Mr Barrington} A car which does not thump does not jagger. (2) Reduce shoe arc of embrace. Fitted on 14" drums on 6-B-IV. 14" drum undesirable from tramp point of view so we have developed light alloy 12" drums (a report on which will follow) which have the same heat capacity as the 14" drums but weigh no morethan the 12". These we hope will enable us to standardise reduced arc of embrace. | ||