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).
Investigating the under or over-steering properties of the Bentley III.
| Identifier | ExFiles\Box 100\2\ scan0020 | |
| Date | 15th October 1937 | |
| Da{Bernard Day - Chassis Design}/DB.{Donald Bastow - Suspensions}/N.15.10.37. UNDER OR OVER-STEERING PROPERTIES OF BENTLEY III. In view of the reduced understeering geometry of the rear axle of the "anti-shake" project for the Bentley III compared with the original Bentley III layout, and also that of the present standard Bentley, evident from a study of PD.35 it was considered desirable to endeavour to investigate the oversteering or under-steering properties of the complete cars. When it was discovered, from Rm{William Robotham - Chief Engineer}/Gry{Shadwell Grylls}, that the standard Bentley car, in conditions of cornering giving an inward acceleration of 0.5g. has a final oversteer of 2°, it was immediately obvious that the most important factor was the tyre behaviour, since the geometric understeer for those conditions is slightly over 1° and therefore the tyres are contributing an oversteer of approximately 2½°. In the S.A.E. transactions for 1935, pp 41-49 there is a paper on tyres in which information is given about tyres of section 7.500"-17.00". The information required is the alteration in cornering power with variation in load and alteration in camber of the tyre, and this is shown by the graph Fig.4. of that paper, which is reproduced in modified form with this memo. The modification consists in the horizontal scale being converted from radial load in lbs. to radial load as a percentage of the rated load. The cornering coefficient is the cornering force developed for a 1° slip angle of the tyre in question divided by the actual load on the tyre, and for the purpose of our calculations we have assumed that this relationship of cornering coefficient and percentage radial load will hold good for other tyre sizes, but that a slip angle for that relation must be found by experiment. From Rm{William Robotham - Chief Engineer}/Gry{Shadwell Grylls}, the results of experiments with 6.50-16.00 tyres on the present standard Bentley car were obtained, where for an inward acceleration of .35g. tyre pressures of 30 lbs/sq.in. and 25 lbs/Sq.in. gave oversteer angles of 0.535° and 0.600° respectively. | ||