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).
Evaluation and road test of a modified Girling 2-leading shoe braking system.
Identifier | ExFiles\Box 121\1\ scan0295 | |
Date | 24th June 1940 | |
1029. By.{R.W. Bailey - Chief Engineer} from Rd. file No.J/JBB/24.6.40. RIPLETTO BRAKES. In order to reduce cost and weight, it is desired to produce a braking system with no servo motor; in this purpose a 2-leading shoe system is essential. The Girling System 7.E.Y., a 34-cwt. car, the only change from the standard all-mechanical B.V. Girling shoe layout being the removal of the shoe balancing links, and the inclusion of a stop for the "trailing" shoe fitted to the carrier plate. The car was tried on the road and it was decided that the theoretical ratio of 1.92 was too low, and that there was too much sponge, although care was taken to obtain good equalising and the pulls in the rods were kept down with the set-up as near as possible to the expanders. The suggestion of a higher ratio then 6.33 at the expanders was turned down due to the fact that for the same mileage between adjustments the wedges would need to be longer and could not be accommodated. Curves were obtained of forces at expander cones against pedal pressure for all four wheels. Hysteresis curves were obtained, the means of up and down curves in each case giving an accurate comparison with the theoretical values. Efficiencies were: N.S.F. 31.5 O.S.F. 62.0 Average Front 36.75 N.S.R. 69.5 O.S.R. 70.0 Average Rear 67.75 It should be mentioned here that both equalisers are on the offside of the centre line of the chassis. Actual Ration F/R is, therefore 1.92 x 36.75 / 67.75 = 1.275. This system has a self-wrapping effect of 1.60 for a μ of 0.38, this figure varying as μ^2. It appears that in our tests we have used too low a μ, as Girlings originally suggested. The danger of a high μ, apart from possible squeaks and drum scoring, is that self-locking is more likely although it does not seem likely with this system (μ has to reach 0.6 or 0.7), and also that a little fading causes a lot of decrease in braking. A fading characteristic is essential, as a rising one causes instability due to the hot parts doing more work due to the higher local μ, until self-locking occurs, or at least getting very hot and possibly burning out with permanent ill effects on the lining. The Pedal ratios as now fitted up are:- Front 44.8) Rear 29.6) Ratio 1.52 Total: 74.4 Continued. | ||