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).
Article from 'The Autocar' detailing R.A.C. horse-power tests conducted at Brooklands using a Wimperis accelerometer.
| Identifier | ExFiles\Box 113\2\ scan0004 | |
| Date | 28th September 1912 | |
| R.R. 285A (150 H) (S.A. 301. 19-2-15) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 0950. THE AUTOCAR, September 28th, 1912. X1909 R.A.C. Horse-power Tests at Brooklands. The Wimperis Accelerometer in Conjunction with Accurate Timing and Weighing. A HIGHLY interesting series of tests was carried out on the track at Brooklands on July 19th with a Wimperis accelerometer. By means of this instrument, it is claimed, the brake horse-power of an engine at the flywheel may be obtained without any of the paraphernalia that such an undertaking would ordinarily involve. The horse-power reading results from the simplification of a formula composed of two easily determinable factors, a figure supplied by the instrument in question, and a constant. For the use of the Technical Committee between twenty-five and thirty cars were loaned for the trials, the results of which are given below. Starting from the paddock, each car, furnished with a Wimperis accelerometer, entered the oval of the track at the “fork,” and was driven at its maximum speed over the measured half-mile alongside the railway. About ten yards beyond the measured stretch, with the car still going at its maximum speed, the clutch was disengaged, thereby causing the car to slow down, and the rate at which deceleration took place supplied the important factor of the test. The Wimperis instrument is graduated to show the deceleration or retardation of the vehicle in pounds per ton, and, if the figure indicated by the accelerometer pointer be employed in the formula Velocity (m.p.h.) x Retardation (lb. per ton) x Total weight of vehicle ————————————————————————————————————— 375 the simplification of the latter will give the horsepower developed by the engine when propelling the car at its highest speed. The cars used for the experiments differed widely in horse-power and weight, and the results as given below should prove distinctly educational. The ease with which each car was tested may be gathered from the fact that all of them were dealt with in about four hours. No delicate adjustments were necessary beyond the levelling of the accelerometer on the vehicle. The instrument is of a handy size; the circular dial is about 3½in. in diameter, with graduations indicating acceleration or deceleration on each side of the central zero mark. Inside the instrument there is a vertical spindle, which carries a pointer on the end that projects through the dial. At the opposite end, and inside the casing, a copper disc, having an eccentric centre of gravity, is mounted. This disc is controlled by a light spiral hair-spring, and is damped magnetically. In its natural state the position of the disc is such that the pointer (which is carried on the same spindle as the disc) is over the zero mark on the dial. When the instrument is caused to move forward in the direction of the arrow shown on the dial, the centre of gravity of the disc tends to lag behind, and thereby rotates the disc and the pointer through an angle controlled by the spring. For example, we will assume that the instrument is placed on a car; when the first speed is engaged and the clutch let in, the pointer will fly back over the acceleration scale; as acceleration decreases, the needle will approach the zero mark, which will be reached as soon as the car travels at a constant speed. Similarly, when slowing down, the magnitude of the retardation is measured on the opposite scale. Method of Tests. Twenty-seven cars were tested. It will be seen that a number of old cars were included, and for the first time it has been possible to ascertain the relative horse-power of a number of new and old cars of GENERAL RESULTS. TABLE I. Car No. | Maker's H.P. | Seats. | Date. | Bore and Stroke. mm. | No. of Cyls. | R.A.C. Rating | Displacement. cc. | W = Weight as Run. tons | V = Maximum Speed. m.p.h. | R = Resistance. lbs per ton | B.H.P. | Gear Ratio. | Rear Tyres. mm. 1 | — | 4 | 1905 | 127 x 127 | 1 | 9.9 | 1583 | 1.04 | 24.0 | 90 | 6 1/2 | 4.0 | 760 2 | 15 | 4 | 1908 | 98 x 114 | 4 | 23.8 | 3440 | 1.32 | 36.44 | 135 | 17 1/2 | — | 810 3 | 12-16 | 4 | 1912 | 79 x 121 | 4 | 15.5 | 2352 | 1.36 | 42.45 | 155 | 24 | 4.37 | 810 4 | 14-20 | 4 | 1912 | 80 x 130 | 4 | 15.9 | 2610 | 1.44 | 36.73 | 135 | 19 | — | 810 5 | 10-12 | 2 | 1912 | 90 x 120 | 2 | 10.0 | 1526 | 1.30 | 38.14 | 150 | 20 | — | 760 6 | 18-24 | 4 | 1911 | 90 x 130 | 4 | 20.1 | 3307 | 1.88 | 40.72 | 120 | 24 1/2 | 3.78 | 920 7 | 15 | 4 | 1911 | 70 x 170 | 4 | 12.1 | 2611 | 1.20 | 44.33 | 185 | 26 | — | 810 8 | 12-16 | 4 | 1910 | 79 x 114 | 4 | 15.5 | 2235 | 1.25 | 40.36 | 118 | 16 | 4.4 | 810 9 | 10-12 | 4 | 1909 | 102 x 110 | 2 | 12.9 | 1797 | 1.18 | 28.75 | 92 | 8 1/2 | 4.12 | 760 10 | 10-12 | 2 | 1912 | 75 x 110 | 4 | 13.96 | 1940 | 1.08 | 37.19 | 175 | 18 1/2 | 4.285 | 760 11 | 16-20 | 4 | 1907 | 95 x 135 | 4 | 22.4 | 3828 | 1.46 | 37.97 | 130 | 19 | 2.88 | 880 12 | 20-28 | 4 | 1911 | 102 x 130 | 4 | 25.8 | 4248 | 1.67 | 44.07 | 165 | 33 | 3.3 | 820 14 | 15 | 4 | 1912-13 | 89 x 127 | 4 | 19.6 | 3170 | 1.62 | 38.96 | 145 | 24 1/2 | 4.0 | 820 15 | 18-30 | 4 | 1911 | 100 x 130 | 4 | 24.8 | 4082 | 1.91 | 39.47 | 110 | 22 | 3.37 | 880 16 | 26 | 4 | 1912 | 102 x 178 | 4 | 25.8 | 5818 | 1.98 | 49.72 | 165 | 43 1/2 | 3.25 | 880 17 | 20 | 4 | 1910 | 102 x 140 | 4 | 25.8 | 4575 | 1.45 | 56.25 | 220 | 48 | — | — 18 | 12-16 | 2 | 1907 | 82 x 120 | 4 | 16.7 | 2535 | 1.00 | 41.10 | 105 | 11 1/2 | 3.0 | 760 19 | 15 | 4 | 1910 | 85 x 110 | 4 | 17.9 | 2497 | 1.36 | 34.09 | 125 | 15 1/2 | 4.0 | 815 20 | 18 | 4 | 1912 | 90 x 130 | 4 | 20.1 | 3307 | 1.67 | 44.78 | 175 | 33 1/2 | 4.0 | 820 21 | 12 | 4 | 1912 | 75 x 130 | 4 | 13.96 | 2298 | 1.21 | 40.00 | 140 | 18 | 4.7 | 810 22 | 12 | 2 | 1903 | 80 x 120 | 4 | 15.9 | 2409 | 1.05 | 24.52 | 120 | 8 | 3.0 | 810 23 | 12-16 | 4 | 1912 | 80 x 150 | 4 | 15.9 | 3012 | 1.20 | 46.88 | 130 | 19 1/2 | — | 815 24 | — | 4 | 1909 | 106 x 170 | 4 | 27.8 | 6001 | 1.71 | 51.43 | 175 | 41 | 3.4 | 815 25 | 12 | 4 | 1912 | 75 x 130 | 4 | 13.96 | 2298 | 1.26 | 43.48 | 195 | 28 1/2 | 4.7 | 810 30 | 15 | 4 | 1910 | 80 x 120 | 4 | 15.9 | 2409 | 1.38 | 37.66 | 160 | 22 | — | — 32 | 15.9 | 2 | 1912 | 80 x 120 | 4 | 15.9 | 2409 | 1.35 | 43.27 | 115 | 18 | — | 810 33 | 15 | 4 | 1905 | 90 x 120 | 4 | 20.1 | 3052 | 1.37 | 30.3 | 100 | 11 | — | — * Extracts from the official report issued by the R.A.C. last week. † Closed body. | ||