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).
Test rig specifications for Borg & Beck clutch thrust springs on 25/30 and 4¼ Litre Bentley models.
| Identifier | ExFiles\Box 129\4\ scan0252 | |
| Date | 6th July 1938 | |
| W/N. W/K.{Mr Kilner} o.{Mr Oldham} Hd.{Mr Hayward/Mr Huddy} o.{Mr Oldham} GWH.{George W. Hancock - Head Chateauroux} 110 Rm{William Robotham - Chief Engineer}/NRC.{N. R. Chandler}10/JH.6.7.38. TEST RIG FOR BORG & BECK THRUST SPRINGS 25/30 and 4¼ LITRE BENTLEY. Some complaints of clutch operation may be the results of unsuitable thrust spring poundage, and this factor may have considerable influence on the behaviour of the clutch even when the facing is the best type we know, and free from oil. Failure to make a quick get-away may be the result of insufficient thrust poundage, while a tendency to grab, or difficulty in making a really smooth start, may be the result of excessive thrust, assuming the driven plate is the best we know in both cases, and free from any trace of oil. While no "best" poundage for the thrust springs of any particular clutch can be stated definitely, as experience shows that considerable licence is permissible with different cars and drivers, but the following readings on the rig, (with the finger raised, so as to render the anti-rattle springs in operative) will be found to be a good average. 11" clutch 25/30 1300 to 1400 lbs. 10" " 4¼ Bentley 1350 to 1500 lbs. Clutches showing thrust poundages both above and below the above figures may work quite satisfactorily, but broardly speaking, if there is a complaint of failure to make a quick get-away, and the rig indicates a thrust below the low limit, fit another clutch unit with stronger springs, but if the complaint is a tendency to grab, and the rig shows the thrust above the average, fit another clutch with lighter springs. It will be appreciated that the particular value of the rig is that it quickly gives a very good indication as to whether or not the spring thrust is at fault or whether the trouble is to be looked for elsewhere, such is the state and type of the facings finger levers not correctly set, undue friction in the pedal mechanism, excessive end movement of the engine or slack in the tie-rod | ||