6th September 2019

Pre-Selector Gearboxes

You may think your paddle shifts on the steering wheel represent the latest in 21st century technology but, in fact, a remarkably similar system has been around since the 1930s. In a world without computers or electronics, the driver had to work out when to change gear and which gear to select but, in essence, the pre-selector gearbox behaved much like today’s dual clutch transmissions.

The controls for a pre-selector gearbox comprised of a selector lever on the steering column, used to choose the next gear, and a foot pedal to activate the shift into the next gear, replacing the conventional clutch pedal. After starting the engine in neutral, moving away from rest and accelerating up to speed required the driver to position the selector lever for the next gear then operate the foot change pedal to activate the shift. Sequential shifting was not necessary. You simply drove along with the selector set to whichever gear you thought you’d need next, rather than the one you were in; hence the term pre-selector.

This requires a degree of anticipation. Is there a steep hill ahead? If so, maybe a downshift will be needed. Will the traffic lights turn from red to green as you approach? Perhaps the shift to first can be cancelled. You can change your mind with a pre-selector and choose a different gear instead; nothing happens until the foot pedal is depressed and released.

So what was the attraction of a pre-selector gearbox? Before the widespread development of torque converter automatic transmissions, manual gearboxes without synchromesh were common. These required considerable skill to operate; downshifting on steep hills was particularly challenging. The pre-selector made the driver’s life easier, and was a stepping stone to the fully automatic transmissions that would follow.

Typical pre-selector transmissions incorporated either a rudimentary centrifugal clutch or fluid flywheel (an early torque converter) to enable pulling away from rest. Combined with removing the skill requirement from gear shifting, this made the car more relaxing to drive and pre-selector gearboxes were most common on luxury cars, such as Daimler and Armstrong Siddeley.

Pre-selector gearboxes often used epicyclic gears which are easily controlled by braking different elements within the assembly to change their rotating speed. With the technology of the day, braking devices were more robust and durable than clutches and easier to maintain. Epicyclic gear arrangements can also handle high torque within a small package.

The durability and compactness appealed to racers, who also gained a benefit from eliminating manual gear changes because the pre-selector could change gear much faster. The most famous application was probably on the fearsome Auto Union ‘Silver Arrows’ but ERA and Bugatti also used such a system for their race and road cars.

The next time you roll up to some traffic lights in your dual clutch car just as they change to green and you press on the throttle, only to feel the uncertainty as the transmission shuffles down to first then back to second, you might think of your counterpart 80 years ago. Would they have anticipated the shift better?

Does your classic car have a pre-selector? If so, tell us what you have and what you think about it in the comments below.