How Stuff Works

Fuzz on the marvels of pre-selecting ’Enter the epicyclic, with its easy change and lovely, mellifluous whine’

Epicyclic Gearboxes

Back in the early to mid-Thirties everyone wanted a piece of motoring action. For the over-17s, obtaining a full driving licence was a simple matter of paying a visit to the local post office and handing over a few shillings. It was as easy as that. Then learning to drive could start.

Hardened gear teeth across the land were soon being chipped away by unskilled and unsympathetic ratio changing and cars were kangarooing their way through traffi c.

Vehicle manufacturers saw a justifi ed opening for easy-change gearboxes, to woo nervous car newcomers into driving seats.

Enter the epicyclic, pre-select, Wilson-type gearbox, with its easy change characteristics and lovely, mellifluous whine. Certain car manufacturers, including Daimler, Lanchester, Armstrong Siddeley and Riley, made epicyclic gearboxes a staple of their offerings. Gone was the clutch, replaced by a fluid flywheel and in place of the clutch pedal was the ‘change speed’ pedal. This latter worked in combination with the gear selection lever, usually mounted on a quadrant on the steering column, within easy reach of the steering wheel.

In operation, from a standing start, with the engine running and the parking brake, or footbrake, applied the gear selector is moved from

neutral to first gear position, thus ‘pre-selecting’ the gear. However, not until the change-speed pedal is fully depressed and released will the gear actually engage. Releasing the brake and applying the accelerator sees the car moving smoothly away. As soon as fi rst gear is engaged, the lever could then be moved to second gear position and, at the desired moment, the change speed pedal once more fully depressed and then released to engage second gear and so on.

Simple, quick and perhaps idiotproof one might think, but of course it wasn’t. User error was possible, such as ‘riding’ the change-speed pedal rather like hovering on a clutch, which caused rapid wear of the brake bands within the gearbox. Also, especially on heavier vehicles, failure to fully depress the change speed lever could result in a misselection and see the ‘ brake bar’ in the gearbox pop out of position, sending the change-speed pedal rapidly upwards and into the left shin of the unwitting driver, with often painful consequences.

Later vehicles, especially buses, dispensed with the change-speed pedal, with gearbox operation achieved using electric solenoids (electro-cyclic) or compressed air pistons (pneumo-cyclic), making them semi-automatics. The selector quadrant was also replaced using, in the main, small electric gear selectors or, in the case of a number of Leyland Commercial Vehicles’ offerings, air-operated selectors on a floor-mounted pedestal. These latter selectors were sometimes retrofi tted to other manufacturers’ vehicles, replacing the sometimes-tricky electro-pneumatic valves on electric select/air-operated gearboxes.

In due course, the semi-automatic epicyclic gearbox became outmoded, with gear selection becoming fully automatic, auto gearboxes using epicyclic gear trains as well. Thus, there is a clear lineage from the earliest epicyclic offerings to today’s computer-controlled units.