Abstract
Overdrives are by no means new, having been extensively used in the United States of America, and to a lesser extent in continental Europe, for many years. Their widespread use in Great Britain is comparatively recent although the American type was fitted to a small number of British cars before the 1939–45 war.
In America the Borg-Warner unit appears to hold the field, whilst on the Continent, probably the most popular type is the extra pair of overspeed gears fitted into the syncromesh gearbox. British use of the device has consisted mainly of the Laycock-de Normanville unit, although there are other types, such as the Daimler method of changing one of the trains in the Wilson gearbox from a reduction gear to a step-up gear, for example, turning a four-speed into a three-speed and overdrive gearbox. The Handa overdrive is also a British invention using dog engagement for selection, necessitating the use of the clutch pedal, and operated by manifold depression. Another mechanism producing a similar effect is the Eaton two-speed axle for commercial vehicles. This latter is not an overdrive in the strict sense; it should technically be described as ‘underdrive’, being a means of varying the gear reduction between the propeller shaft and the rear axle half-shafts. ‘Overdrive’ is a means of driving the propeller shaft faster than the engine, and while this is perhaps not the most suitable term it is now universally accepted.
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