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).
Tyre wear, mileage, and potential modifications to tyre size and gear ratios for the Wraith limousine.
| Identifier | ExFiles\Box 157\2\ scan0292 | |
| Date | 5th August 1938 | |
| -2- Rm.{William Robotham - Chief Engineer}15/MH.{M. Huckerby}5.8.38. You have really had only one change as you ran no more than 453 miles on a set that was fitted temporarily while your hubs were changed for a different cone angle. On our experimental cars we admit mileages have been disappointing, 5,000 front and 4,100 rear; on the other hand we have been driving the cars hard and have had summer weather. From the above our conclusions are that if the standard limousine Wraith is going to weigh (as seems probable) 42 or 43 cwts. empty we may get complaints of excessive tyre wear in service. We ought probably, therefore, to consider going to a 7 x 17 tyre. All the chassis clearances are satisfactory, but the wheel arch drawings would want modifying as they were reduced when a 6½" tyre was adopted. The rolling circumference of the 7" tyre is 3% greater than the 6½" and we can hardly stand a higher gear, particularly in view of potential weight increases. One would therefore suggest reverting to the 9/41 axle which is 7% lower than the present 8/34, the nett result being a 4% lower overall gear ratio. In view of the progress that we have made in eliminating timing gear wear we think that we could now stand a 4% lower ratio. A set of five 7" tyres adds 26½ lbs. to the chassis weight. Rm.{William Robotham - Chief Engineer} | ||