Preselector gearbox

A preselector or self-changing gearbox is a type of manual gearbox (US: transmission) used on a variety of vehicles, most commonly in the 1930s. The defining characteristic of a preselector gearbox is that the manual shift lever is used to "pre-select" the next gear to be used, then a separate control (a foot pedal) is used to engage this in one single operation, without needing to work a manual clutch.

Most pre-selector transmissions avoid a driver-controlled clutch entirely. Some use one solely for starting off.

Preselector gearboxes are not automatic gearboxes, although they may have internal similarities. A fully automatic gearbox is able to select the ratio used, with a preselector gearbox this remains the driver's decision.

There are several radically different mechanical designs of preselector gearbox. The best known is the Wilson design. Some gearboxes, such as the Cotal, shift gear immediately the control is moved, without requiring the separate pedal action. These are termed 'self-changing' gearboxes, but were considered under the same overall heading. In recent years, a similar role is carried out by the increasing number of 'Tiptronic' or 'paddle shift' gearboxes, using manual selection and immediate automated changing.

For the driver, there are two advantages:

In engineering terms, some designs of pre-selector gearbox may offer particular advantages. The Wilson gearbox offers these, although they're also shared by some of the other designs, even though the designs are quite different:

They were common on Daimler cars and commercial vehicles, Maybach, Alvis, Talbot-Lago, Lagonda Rapier and Armstrong Siddeley cars as well as on many London buses. They have also been used in racing cars, such as the 1935 ERA R4D, and hillclimbing cars such as Auto Union "Silver Arrows". Military applications began in 1929 and later included tanks such as the German Tiger I and Tiger II in World War II, through to current tanks such as the Challenger 2.

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