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).
Technical memorandum discussing the viability of using cable-operated Girling brakes on a Bentley III.
| Identifier | ExFiles\Box 85\1\ scan0252 | |
| Date | 16th November 1936 | |
| 206 File To E.{Mr Elliott - Chief Engineer} from Rm{William Robotham - Chief Engineer}/AFM.{Anthony F. Martindale} c. to Hs.{Lord Ernest Hives - Chair} Hy.{Tom Haldenby - Plant Engineer} Ds. Rm{William Robotham - Chief Engineer}/AFM.{Anthony F. Martindale}31/Ar.16.11.36. BENTLEY III. BRAKES. Now that you have examined and tried 1.B.17. fitted with Girling brakes, we understand that you consider it worth while to prepare a scheme for these brakes for Bentley III, using cable operation on the front brakes, this being the standard Girling layout for independently sprung cars. We agree that this arrangement is worth testing, but we must say at once that we do not like the Bowden cables, and if it is possible to replace them by universally jointed solid rods, we consider that this should be done. (Fig.1. shews the type of arrangement we mean.) We attach a graph which shews the efficiency of a Bowden cable, fully lubricated, when bent through various angles. The values for μ were calculated from the formula T₁/T₂ = e^μθ (where T₁/T₂ is the ratio of the tensions at either end of the cable) You will notice that although the cable does not behave consistently the values of μ so obtained are reasonably constant and of a reasonable order of magnitude. As we do not see how the total of the bends of the cable could be less than 80° or 90° with the wheels in the straight ahead position if the cable is mounted normally, this means that the combined efficiency of cable and wedge is only 65% and one of the advantages of the Girling brakes is lost. A much superior arrangement, in our opinion, which necessitates a good deal of original design work, is a non-floating wedge arrangement which is pulled through the steering pivot. The wedge can have a long travel, so that adjustment can be effected by shortening the vertical pull rod. The off stop is then on the bell crank. Alternatively, the Girling expander can be used for adjustment. Deep groove ball bearings take the side thrust on the wedge, the unit being encased as in the Girling or Bendix wedges. The efficiency of this wedge should be 6% higher than the Girling wedge as the plunger axes are at right angles to the wedge faces. Fig. 2 shews what we mean. The brakes are of course unaffected by steering lock. Rm{William Robotham - Chief Engineer}/AFM.{Anthony F. Martindale} | ||