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).
Explanation of the automotive overdrive system, detailing its function and advantages for performance, economy, and engine life.
| Identifier | ExFiles\Box 125\2\ scan0116 | |
| Date | 1st May 1937 guessed | |
| THE OVERDRIVE EXPLAINED HOW CHRYSLER ENGINEERS SOLVED A MAJOR PROBLEM AND ACHIEVED AN ENTIRELY NEW STANDARD OF PERFORMANCE. WHAT IT MEANS TO YOU IN PLEASURE, ECONOMY OF OPERATION AND LONG ENGINE LIFE THE history of automobile transmission development has of necessity been one of compromises. The adaptation of the internal combustion engine to the self-propelled vehicle from the beginning required the use of an intermediate speed-reducing and torque-increasing mechanism. This function in the automobile is performed by the combination of the rear axle gearing and the transmission unit. The latter, however, has always been considered a necessary evil because of the requirements for starting from rest or heavy pulling at low speed. However, since it required manual operation on the part of the driver, it has been the aim of car designers to cover the greatest possible range of speed and load conditions by means of the fixed rear axle reduction alone. This limitation has led to a compromise which has a number of undesirable aspects. In order to satisfy the demand both for rapid acceleration at low speeds and high maximum car speeds without resort to gear shifting and without excessive engine size, the maximum operating engine speed has been increased from year to year, until finally maximum engine speeds well over 4,000 revolutions per minute have become common practice. This not only reduces engine life but results in excessive fuel and oil consumption at high road speed. In the past, only the unusual driver habitually drove at sustained high speed. However, the tremendous improvement which has been accomplished in comfort and safety in the modern automobile and the development of good highways has raised the average cruising speed to the point where sustained high engine speed is a normal condition of operation. This situation has had to be met by the provision of a greater range of transmission speeds from the engine to the driving wheels. This problem has been a chronic one with automotive engineers; to-day it has become acute. Many attempts have been . 5 . drum, the weights are able to insert themselves in the closely fitting notches. The mechanism is now locked in the overdrive. The “cushion hub” previously mentioned softens the shock of the shift so that you have to listen hard to hear it. The path of power in overdrive is indicated by the broken arrow in Illustration [D]. Looking at the centrifugal weight pictures, it will be noted that a spring-backed lock ball helps to hold the weights in either their inner or outer position. This feature prevents “wandering” of the weights at some critical speed, but it also causes the overdrive to cut in at 45 m.{Mr Moon / Mr Moore} p. h.{Arthur M. Hanbury - Head Complaints} and cut out at 37. When the weights are at their inner position, both the lock ball and the main coil spring are resisting centrifugal force, requiring, in this case, a speed of 40 m.{Mr Moon / Mr Moore} p. h.{Arthur M. Hanbury - Head Complaints} to overcome both. But when the weights are in their outer position, the spring must overcome both the lock ball and centrifugal force, which means that centrifugal force must drop far below its former value before the spring is able to overbalance both this force and the resistance of the lock ball. Both free-wheel and overdrive are locked out by sliding the mainshaft to the right. ADVANTAGES IN ENGINE LIFE The tremendous advantage of the overdrive in conserving engine life is readily apparent from the fact that it reduces engine speed 30 per cent. for a given car speed. This means that at sixty-five miles an hour the engine is running at a speed corresponding to forty-five miles an hour in conventional direct drive. Furthermore, the overdrive automatically limits the maximum possible engine speed. For example a car which in conventional drive has a top speed of eighty-five miles . 8 . IN CONVENTIONAL DRIVE SPEEDOMETER TACHOMETER IN OVERDRIVE | ||