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).
Letter discussing the shimmy effect, moment of inertia, and specifications for 20-inch wheels and tires.
| Identifier | ExFiles\Box 71\2\ scan0057 | |
| Date | 17th September 1928 | |
| X88107 ROLLS-ROYCE OF AMERICA, INC. SPRINGFIELD, MASS. OY3.R.9.17.28 September 17th, 1928. Mr. Robotham, Rolls Royce Ltd., Derby, England. Dear Mr. Robotham: WHEELS & TIRES Regarding your recent memo referring to probable experience of greater shimmy effect with 20" wheels, our experience so far is rather the reverse. Shimmy, tramping and "snatching" at the steering wheel appear to be less. Our tests so far with the smaller wheels however have been with well-base wheels and tires on which three important factors are different from the S.S. wheels. (1) The Moment of inertia of the wheel is possibly slightly less due to a lighter rim. (2) The "base" of the tire between flanges is 4" wide as against 3-3/4" on a standard S.S. wheel. (3) Non-skids have been used on the front wheels. The change we are being forced to make is not in any way comparable with the change you have made on the 20 hp. where a 31" x 5-1/4" tire has been changed to a 31" x 6", using a much larger tire section. We are using exactly the same tire-section wrapped round a 20" wheel instead of a 21" wheel. (32" x 6.75" in-stead of 33" x 6.75".) We shall have 20" S.S. wheels and tires about October 6th and will then make tests and write you. Meanwhile following figures may interest you:- Moment of Inertia Wire Wheel and Tire. 33x5 Dunlop SS{S. Smith} cord. Derby figure - 2.73 units 34x7.3 Dunlop Balloon " " - 2.33 units - continued - | ||