preselector

A preselector is a manual gearbox where you set up your next gear on the shift lever while still in the current gear, then engage it by pressing a pedal (usually the clutch) without needing to operate the lever again. The system separates the act of selecting which gear you want from the act of actually moving into it. Most preselectors used a dog-clutch or synchromesh arrangement to lock the selected gear into the mainshaft once the engagement pedal was pressed.

The preselector appeared in the 1920s and 1930s, most famously in Daimler and Lanchester automobiles, though the principle was later adopted for some trucks and industrial machinery. The Daimler system used a vacuum-servo mechanism to help engage the selected gear, making the shift smoother than a raw dog-clutch. Because you could choose the next gear while cruising, the system promised easier driving in stop-start urban traffic, which was its main selling point.

How it worked and where it failed

A mechanical selector mechanism inside the gearbox sensed which position the external lever occupied and pre-positioned the next gear's synchronizer or dog teeth ready to mesh. When you pushed the engagement pedal, a servo or direct mechanical linkage would snap the selected gear into engagement. The advantage was smooth, clutchless shifts (on some designs), and no need to match engine speed to road speed by hand. The disadvantage was added complexity, higher manufacturing cost, and a delay if you needed to skip gears or suddenly wanted a different ratio than you had pre-selected.

Preselectors fell out of favor after the 1950s as fully synchronized manual gearboxes became standard and cheaper to build. A driver in a modern synchronized box can shift almost as quickly without the extra mechanism. However, the preselector principle survives in some automatic and semi-automatic transmissions, where a hydraulic or electric system performs the pre-selection and engagement without driver input.

The term reflects its purpose: you are selecting the gear in advance (pre-selecting) before the moment you need to engage it. It sits historically between the crash-box (no synchronizers, pure skill required) and the fully automatic transmission (no selection required at all). For industrial equipment and heritage vehicles, understanding preselectors matters because the shift behavior and maintenance differ markedly from modern synchronized manuals.