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).
Comparison of suspension characteristics, oscillation, and pitching speed between different car models like the Silver Ghost and Phantom.
| Identifier | ExFiles\Box 43\2\ Scan177 | |
| Date | 11th October 1926 guessed | |
| contd :- -2- and of the Silver Ghost 4080 lbs. this result surprised us at first. But if one draws a circle of 4 ft. radius from the c.g. on the outline drg. of the Silver Ghost car, one can see that the added weight of the Phantom in engine, and servo gear, occurs chiefly inside this circle. (The front axle, being unsprung, does not enter into it.) For bodies of similar type the Phantom would therefore have a smaller radius of gyration than the Silver Ghost because of the greater concentration of weight near the c.g. (4) We have seen it stated in journals etc. that the frequency of any spring suspension was proportional only to the deflection of the spring and not to the masses, and that therefore the spring should have the same deflection on any car without regard to size. (The formula being t = 2π√(l/g) . the same as for a simple pendulum.) We have therefore not understood why the 20 HP, with much smaller spring deflection than the larger car, could actually ride better. Also we have never understood why the front end of the Silver Ghost which should oscillate at 90 per minute (based on the above formula) actually oscillates about 120 per minute, and the rear end which should make about 63 oscillations/min. actually makes 70 to 80. From the results it is obvious that if all the mass of the car were concentrated at a point about the centre of the car it would have a very high pitching speed indeed irrespective of the springs used. Also if the masses were spread out beyond the axles it would have a very low pitching speed. We conclude that the larger car requires much softer springs than the smaller car, for a given pitching speed because the radius of gyration is less in proportion to its wheelbase. More particularly, if 'k' is the radius of gyration, 'a' the length from front axle to c.g., 'b' the length from rear axle to c.g., we suppose that the ratio k²/ab is less for the larger car than it is for the 20 HP. contd :- | ||