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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).
Comparison of competitor vehicle specifications regarding wheel sizes, axle ratios, and engine performance, discussing potential design changes.

Identifier  ExFiles\Box 19\4\  Scan319
Date  25th May 1931 guessed
  
- 4 -

Engine
Revs. per mile.

Packard- 7" x 19 2670
Pierce Arrow 7" x 18 (all models) 2784
Elcar 7" x 18 3339
Kissel 7" x 18 3228
Stutz 7" x 20 2748
PII 7" x 20 2250

The Stutz is the only car using a 7" x 20" tire - besides ourselves - and then the axle ratio compensates for this, giving 2748 revolutions per mile as against our 2250.

We are aware that as a result of some experimental running, you did not find 19" wheels very satisfactory, in that they resulted in unsatisfactory steering, but we consider some trouble should be taken to verify this - for these reasons:-

1. With 19" wheels on MX{John H Maddocks - Chief Proving Officer}-5 we have obtained better riding, and steadier steering than on any other PI car. This car is equipped with Delcos, set with the rebound loading approximately two times the compression loading.

2. It is generally conceded that "badness" of steering is a function of the weight of the axle and particularly of the wheels. Now we cannot see how a smaller wheel can be anything but lighter and have a smaller moment of inertia than a large one, and feel that it was not the size of the wheels but some other feature that brought forth this adverse report.

Now our position is this - our management, represented by Mr. Fuller and Mr. Inskip, has several times, without any prompting from us, raised the question of a high speed engine, meaning by this a low gear ratio, and when the time comes along that we get complaints of sluggishness in acceleration, which we feel certain is inevitable, in our own justification we shall be forced to try out a lower gear ratio, either by the use of 18" wheels or a 10/41 axle ratio or even both. (Note that both together would give us a ratio of 2750, almost the same as Cadillac 16).

Now we have felt that we ought not to do any of this class of experimenting here leaving it all to yourselves and hope we shall not have to change this arrangement at all in view of the complications that would be created. In other words, we don't want to try to improve your chassis, but we honestly believe it can be done in this respect and are afraid that before very long we shall be forced to do something. We know from our chart SK-416 that we have the engine capacity and there is no reason why we should not get equal or better performance than the others. Of course, it may be claimed that this will detract from the maximum speed, but our answer to that is:

Firstly - Maximum speeds of 85 m.p.h. and over are really not required, and lack of this extreme speed at present is not by any means such a serious trouble as lack of acceleration.
  
  


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