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 and comparing overdrive gear ratios, tractive effort, and performance characteristics for various vehicles.
| Identifier | ExFiles\Box 136\2\ scan0101 | |
| Date | 9th June 1939 guessed | |
| -2- I have also looked up statistics in regard to accepted practice for other Overdrive gear ratios; figures are per centage step up from direct ratio. The A.E.C. charabanc chassis is 40% with layshaft gear. The Dennis (even double decker long distance buses, I believe) is about the same. In America, for buses, etc. Brown-Lipe have different models, for example, 49%, 45%, 37% and 33%. The Fuller is 32%; the Mann 35%, and the Warner (cars and trucks) 39%. These are this year's figures, and based on extended user experience, and being (mostly) ordinary lay shaft gear trains, are not dictated by design limitations. Turning to what I consider the feature controlling the ratio question, the Tractive Effort on Top gear of my Studebaker is 341 lbs./ton, and the Humber Snipe 313 lbs/ton and both these are thoroughly O.K. with the Overdrive ratio we use, and in neither case was the rear axle ratio dropped by about 10%, as would be the case in production models. In both cases the rear axle ratio is 4.5, but for production I would recommend from 4.9 to 5.2 which would proportionately increase the range of utility of both direct and overdrive, whilst leaving ample Overdrive advantages re luxury performance and Service and operating economies. If therefore you contemplate a model, with normal top speed tractive effort approximating to 300 lbs/ton, may I submit that all the available evidence supports the suitability of a ratio around the 40% step up mark, (assuming exceptional ease and instantaneity of control) and to this range of road suitability could be added, any 10/12% rear axle ratio advantage, which could be conveniently employed. Finally, if preferred, the newdesign would render a 35.8% step up ratio available (26/24/74) or even 31.6% (24/26/76) which, with, say, 12% off rear axle ratio, would only mean an over-all step up of 23.8% from standard. Unless it is desired to give speed considerations preference to luxury of performance, I would personally consider this too small a step up. It now remains to apologise for the length of this letter; to thank you and your colleagues for the consideration already/ | ||