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).
Technical memorandum discussing vehicle handling dynamics, steering angles, and Ackermann geometry.
| Identifier | ExFiles\Box 170\3\ img126 | |
| Date | 17th November 1937 | |
| Mr. Grylls, Rolls Royce Limited, Derby. The alternative is to set the front wheels to 3.1/2°, giving only .22g at the start and allow the car to drift into the turn rather slowly. But, on English roads, I cant do this because there are no approach curves to the turns, the roads are narrow and I shall be across the road before I settle into the turn. This boils down to saying that below the "critical point" the handling is stable in the transient state, and above this point it is unstable. The critical point is where Attitude cross Ackermann, also where Front Slip angle crosses steering angle. It is the point where the centre of turn has moved forward into line with the front wheels. Note that at high speeds the centre of any turn, even the slightest, is ahead of the front wheels, so the car is always nosed in, on turns. This is so far all theory. I dont know whether there is a grain of truth in it. Thought you might like to comment on it with the idea of checking it against practice. If it means anything it is that long wheelbases, giving steeper Ackermann lines, and rear ends with the least possible slip angles, improve stability in entering and leaving turns. Hence a prejudice against rear stabilizers and for front stabilizers instead. Hence also the need for rear axles well enough controlled to allow high rear tyre pressures. I think an interesting thing about the chart is to suggest why a strict "Ackermann" geometry, which really means that front slip and Attitude angles are identical, is undesirable. Suppose the car on the chart has the front slip line brought down to the attitude line. Then try to produce .5g at 30 m.p.h. The correct steering angle is 3/4 degrees, but to start the acceleration right away requires 2.1/4 degrees. So the guy has got to turn 3/4 degrees and sit there and wait, or grab for two or three degrees and get back to 3/4 degrees quickly. -2- | ||