The Preselector Gearbox

 

Drivers of modern cars with manual transmissions need little skill to change gears smoothly and silently. This is because of a wonderful device called synchromesh. This equalises the speed of the components as each gear is selected. Prior to the development of synchromesh in the 1930s, drivers of most cars had to contend with so called crash gearboxes. Drivers had to display considerable skill when changing gear if embarrassing grating noises were to be avoided, particularly when changing down, for example, from top to second. This required double-declutching, a process in which the clutch was momentarily released as the gearlever passed through neutral. The engine speed was increased at this point, in an effort to bring the soon to be meshed gears to the appropriate speed.

 In 1928 Armstrong Siddeley introduced the preselector gearbox. As its name implied it allowed the driver to select a gear in advance. Engaging the gear involved nothing more than depressing and then releasing the change pedal. This pedal was, in appearance, the same as the clutch pedal used with a conventional manual gearbox.

 The photograph  to the right featured in an Armstrong Siddeley advertisement depicts the change lever on the centre of the steering hub in what appears to be a 1928 20hp.

 Little skill was required and changes were almost instant! For this reason preselector gearboxes were commonly found in sports cars and race cars. 

The preselector gearbox may be viewed as a forerunner of the modern automatic gearbox factory fitted to the vast majority of passenger cars. Both transmissions rely on what are referred to as planetary gear sets and clutches to transmit drive from the engine to the differential and hence to the wheels.

 In 1952 the new Sapphire was released. For this model the simple mechanical lever was replaced with a so-called electric arm containing a miniature gearlever and gear pattern that replicates that of a conventional 4-speed manual floor shift.( see photos aboive and to the right) The gear lever is moved by fingertip. A photo of the electric arm appears below. In this system electric solenoids rather than a mechanical linkage operated the gearbox.

 This system was known as  “preselectric” reflecting the combination of electric actuation and a preselector gearbox. The last Armstrong Siddeley so fitted was the Mk 2 Sapphire .

 It is ironic that today virtually all modern automatic transmissions are fitted with an override to allow the driver to change up or down in a way not dissimilar to the Armstrong Siddeley developed Preselector………….

 For more detailed and technical info visit http://www.v-twin.org.uk/riley/psg/ASpsg.html