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).
Proposal to replace the third speed with an overdrive on the Bentley III Gearbox.
| Identifier | ExFiles\Box 86\4\ scan0167 | |
| Date | 20th April 1937 | |
| 228 Der C. Sgs. Ins. Ex. Rm.{William Robotham - Chief Engineer} E/HDY.{William Hardy}2/HP.20.4.37 Bentley III Gearbox. With regard to the proposal to rearrange our four speeds to give an overdrive in place of the third speed as for the Phantom, we are doubtful whether this will be appreciated in the case of the Bentley, for use in this Country, or on Alpine Passes on the Continent, for the following reasons:- (1) On an average run in this Country a car with the over-drive ratios is likely to put up a lower average speed than one with the standard gears. Although on a level road and in still air the maximum speed is increased 3.5 m.p.h. (see N.Sch.5120 attached) and down hill with a wind behind an extra 10 m.p.h. can no doubt be obtained, the actual use for such speeds to increase the average speed is nil. The difference in acceleration between the over-drive and top gear is the same as between the use of a 3½ litre and a 4¼ litre engine. (2) Our present 3rd speed gives a comfortable 60 m.p.h. This will become 52.5 m.p.h. with the proposed .7 overdrive box second speed at the same engine speed. There will be however an impression of greater noise because of a lower car speed. The 25/30 fitted with 3 speeds for trial by Sg.{Arthur F. Sidgreaves - MD} had a 2nd speed ratio of .67. (3) The proposed 2nd speed ratio of .7 to 1 should enable the car, when loaded to 40 cwts, to climb 1 in 5.8 continuously. A long hill steeper than this or with steep hair pin corners may require a change into 1st speed. At 3,500 r.p.m. engine speed the speed on a .4 1st speed is 29 m.p.h. At the same engine speed on our present 2nd speed the car speed is 42 m.p.h.. Any long hills which are at present climbable on 2nd but require 1st on the new box, will cause an objectionable increase of noise and loss of speed with the over-drive box. | ||