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 tests, covering effects on speed, tractive resistance, and fuel consumption.
| Identifier | ExFiles\Box 160\2\ scan0023 | |
| Date | 1st June 1923 | |
| CHECK ON ROAD FIGURES. TYRE TESTS. 1.6.23. (a) Speedometer reading. Increase in recorded speed 5% . . increase in r.p.m. of wheels 5% " decrease in effective circumference of tyres 5% (nearly). This is approximately the figure obtained by measurement. (b) Maximum speed. From tractive resistance curves obtained 26.2.23, we calculate that on the 40/50 HP. it requires 1 1/4 HP. to increase the car speed by 1 m.p.h. at 60 m.p.h. Now deflated tyres increase tractive resistance by 14 lbs. per ton (from print) or 29.7 lbs. on the weight of the car. To overcome this extra resistance at 60 m.p.h. requires 4.75 HP. so that the maximum speed should therefore drop by 2.7 m.p.h. at 60 m.p.h. This compares reasonably well with our figure of 3 m.p.h. (c) Tyre Tractive Resistance. From the formula for tyre resistance given in Prof. Lockwood paper in the S.A.E. Journal for Nov. 1922 (it is wrongly printed there) - R = (L - 200)/100 (1.06 + (90 - P)2/7,000) R -= rolling resistance in lbs. L -= load in lbs. P = pressure (lbs. per sq. in.) This formula gives the increase in T.R. occasioned by deflating tyres in this case to be about 10 lbs. per ton. The material difference between this and the 14 lbs. per ton we got on the road, may possibly be explained by the fact that Lockwood obtained his figures from Fisk, Goodyear and other U.S.A. tyres which may materially differ from those we used. (d) Consumption Test. These figures magnify the effect of soft tyres on consumption because they are taken at the low speed of 20 m.p.h. - at higher speeds, the tractive resistance of the whole car increases enormously due to windage, and so the difference of 14 lbs. per ton becomes a very small percentage of the total tractive resistance, and consequently contd:- | ||