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).
The calibration error of a 5th wheel speedometer and its history.
| Identifier | ExFiles\Box 165\2\ img302 | |
| Date | 1st January 1941 | |
| 6065 To Rm{William Robotham - Chief Engineer}/Wym.{G. Harold Whyman - Experimental Manager} from Rm{William Robotham - Chief Engineer}/GB. Copy to Rm.{William Robotham - Chief Engineer} Rm{William Robotham - Chief Engineer}/RC.{R. Childs} 5TH WHEEL ERROR. =============== When the 5th wheel was first put into service, it was inaccurate for two reasons. Its history is therefore recorded here, together with recommendations designed to maintain its accuracy in the future. HISTORY. ======= The first calibration of the speedometer head was as follows:- 10 m.p.h. - 128.5 r.p.m. 50 m.p.h. - 643 r.p.m. 20 " - 257 " 60 " - 771 " 30 " - 386 " 70 " - 900 " 40 " - 515 " This was based on a rolling radius of 13.1" obtained by stretching a wire round the wheel and subtracting the wire diameter from the rolling diameter. The above method of arriving at the rolling radius suffers from the difficulty of getting the wire free from unwanted bends. A number of cars were tested with the above calibrations. Rm{William Robotham - Chief Engineer}/Wym.{G. Harold Whyman - Experimental Manager} then discovered that the rolling circumference, measured by rolling the wheel along the shop floor, could be varied from 79¼" to 80½" by means of changes in tyre pressures. The rolling circumference with a medium hard pressure was 80½". As near as could be judged this was the pressure used in the earlier tests. 80.5" circumference corresponds to 12.81" radius. This would make the calibrations given above 2% slow. A check made with a second Hasler on 31.12.40 showed only 746 r.p.m. for 60 m.p.h. The two Haslers have been checked by Rm{William Robotham - Chief Engineer}/RC.{R. Childs} on an induction motor and the first instrument was reading 3% fast and the second one correctly. | ||