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 spring construction, rear axle behaviour including sideways shake and tramping, and roll centre dynamics.
| Identifier | ExFiles\Box 153\1\ scan0063 | |
| Date | 23th September 1937 | |
| -9- We were shown an interesting construction which may be used to replace a spring for finding out the various movements. This we reproduce in Fig.6., where the upper drawing shows a spring with controlling eye central with the main leaf, and the linkage replacing this consists of two end links whose length is three quarters of the spring half length, plus a central portion rigidly fixed to the axle. The dotted lines show the position of the pivot pins between the end links and central portion, and in chain-dotted lines are shown the spring and axle in a position when some rebound movement has taken place. The lower drawing shows a spring with offset eye, the pivot for the end link is then made as far below the main leaf centre line as the eye centre is above - dimension 'a' in the drawing. One of the most interesting and possibly useful things we heard was in connection with rear end sideways shake. Olley says this is due to flat rear springs set below the axle. They had carried out experiments on the bump rig with their cams running parallel i.e. striking the two rear wheels at the same time, and invariably they found that the axle would not move up and down parallel to itself, when they came to the hop frequency of the wheels it would always tramp and any slight error in the placing of the cams would result in the trailing cam making that end of the axle tramp about the other. If the cams hit the rear wheels exactly at the same time, then any slight difference in damping would result in the side with less damping tramping about the other side. Moreover, if a few men hang out the rear bumper on the tramping side of the car, the tramp would go over to the other side, and usually stay there if the bumper was then released. The roll centre of the rear axle is in the plane of the four spring eyes, and with the flat rear springs below the centre of gravity of the axle. Fig.7. shows an axle in the normal position with roll centre R.{Sir Henry Royce} and centre of Gravity G.{Mr Griffiths - Chief Accountant / Mr Gnapp} This appears to be the reason for sideways shake, for when an axle arranged like this tramps, a point on the axle above the springs describes a loop similar to that shown in Fig.7. where the chain-dotted lines show the axle in one extreme position. The resulting axle movement suggests a figure-of-eight shape. The cure is to have positive camber on the spring in the normal position, the roll axis being then moved up. We were shown a mathematical analysis of what was happening, but did not have time to go through this at all in detail, however, the result deduced was that the best position for the roll centre was about ½" above the centre of gravity of the axle. We were shown graphs where the horizontal scale was frequency of exciting impulses and the vertical scales were amplitude of rear wheel top and of rear end sideways shake, and the peak of the sideways shake curve under the wheel hop frequency peak was very obvious, also the improvement obtained continued | ||