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).
Analysis of vehicle performance gains based on variations in tyre type and pressure, comparing Dunlop and Avon tyres.
| Identifier | ExFiles\Box 97\3\ scan0177 | |
| Date | 3rd July 1939 | |
| -2- From this table it is seen that the twin exhaust system is worth approx. 1½ m.p.h. in max. lap speed. By far the largest speed gains have been obtained from tyre variations, the change from India at road pressure to the higher pressured racing tyre giving a straight gain of 10½ m.p.h. 4.7 m.p.h. is obtained by merely pumping up the Avon tyres 10 lb/in.2. all round. In order to check up on the statement made in a previous memo. RM{William Robotham - Chief Engineer}/TA.13/JH.17.5.39. that the power absorbed by tyres on a four foot steel drum is approx. 1½ times that used on the road, the following figures have been worked out. From the power curves already obtained on the chassis dyno. and the max. lap speed figures it is seen that at 99 m.p.h. in overdrive on Avon tyres at high pressure the hub H.P. is approx. 110. If the frontal area = 24 sq.ft. total weight = 1.83 tons. (on road). rear axle load = 0.9 tons (on drums). The air resistance coefficient K works out at .00137. From this figure the Windage H.P. for lap speeds on racing tyres and Avon tyres at road pressures may be calculated, and by deduction from the hub horse power the actual horse power absorbed by the tyres on the road is obtained. [TABLE] ||Max. M.P.H.|Hub HP. available.|Wind-age H.P.|HP. Remain-ing for tyres.|Tyre HP. from curves taken on drums.|2/3|(Tyres HP.) (from curves) |---|---|---|---|---|---|---|---| |Dunlop Racing Tyres. 45 - 47 lb/in.2|107|122|107½|14½|20|13⅓| |Avon tyres. 31 - 33 lb.in.2.|97|116|80|36|58|38⅔| | ||