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 suspension linkage forces, coil spring rating, and the bending effect of the spring.
| Identifier | ExFiles\Box 154a\2\ scan0279 | |
| Date | 30th March 1939 | |
| - 2 - C can be resolved into components D & E respectively, perpendicular and parallel to the lower triangle, and the perpendicular one is known from the spring characteristics, and so C is known. B is then found by taking moments about the yoke piece upper bearing or by completing the triangle of forces. I presume Olley calls this the toggle effect because of the inward force on the end of the upper triangle lever. Figs. 1 & 2 on Sheet 2 of PD.404 demonstrate why there is alteration of rate from the "leverage squared" figure; Fig. 1 is the triangle of forces for the full bump position in Fig. 1, the road reaction B is less than the spring reaction D at the triangle lever and principally because the force A is not parallel to E, i.e., because the two triangle levers are not parallel. In Fig.2, where E and A are parallel, B is slightly different from D because D and B are not parallel, but this difference is small. Sheet 1 of PD.404 shows the resulting rating curve for B.V. and sheet 2 for B.50 with the yoke piece giving increased track. It will be seen that in the normal position of the linkage, the B.V. rating is .85 of the "leverage squared" rating, and the B.50 is .823 of that rating. The standing height of the suspension is also altered i.e., the point at which the desired load occurs. (3) The bending effect of the spring was the third and only remaining cause, and has been determined experimentally by a remainder method. Rating curves have been taken on various cars for various coil springs, see Rm{William Robotham - Chief Engineer}/FWK.5/JH.1.3.39 and for each coil spring a rating at the wheel has been established. From this rating the 23 lbs/in. due to the rubber bushes is subtracted, and the resulting rate is plotted against the rate derived from PD.404 for the spring in question. A line is then drawn from the origin through these points. Considering that the actual coil spring rate has to be assumed as correct, because of the absence of any means of measuring it, the points fall very well on the line - the graph is attached to this memo. The slope of the line represents the ratio between the theoretical rate without bending and that with bending, and this will be a definite ratio for a given linkage because the ratio of spring bending to deflection will remain constant. This ratio is seen to be 1.13. The original graphs of PD.404 of rating at the wheel are now modified, starting from the spring straight position in each case, by means of this ratio, and finally the rubber bush rating is added in and the final curves of PD.404 are obtained. | ||