Sunday, 29 May 2016

Pre-selector gearboxes

For a long time I've been fascinated by unconventional car transmissions.  These days there are CVTs and DSGs, and goodness knows what other TLAs.  But there are no pre-selectors, gearboxes where you select the gear you want - before you need it - and bring it into engagement with a pedal.  The theory is easy enough to grasp but, until today, I hadn't had a chance to see one demonstrated.

Daimler was, for decades, associated with the combination of fluid-flywheel and Wilson pre-selector gearbox.  So it was appropriate that my lesson should take place in the comfortable passenger seat of a 1955 Daimler Sportsman.

The first thing I learned was that you absolutely must leave the car in neutral; unlike modern cars there are no safety lock-outs, it will start in gear and creep forward.  You can tell whether the car is in gear or not from the position of the gear-change pedal.  In neutral the pedal comes further from the floor.  Another thing to remember about the Wilson gearbox is that you can select any gear at any time, there's no need to go sequentially through each ratio.  If you try and do anything inappropriate such as selecting reverse whilst going forward there are mechanical lock-outs to protect the car from damage.  In addition the gear change pedal comes further out from the floor and it takes a hefty shove to push it back.

Other things to note about the Sportsman; first gear is very low, designed to get a heavy car going on a steep incline with a full load aboard and trailer hitched up.  When reversing you can set the engine speed using the hand-throttle on the dashboard and control the car using the brake pedal as you would in a conventional automatic.  The hand throttle can also serve as a simple cruise control on long journeys.

Thinking about the pre-selector gearbox, and the Wilson transmission in particular, it was fitted to a variety of cars; from stately Daimlers and Armstrong Siddleys to sporting Rileys and ERA Grand Prix cars, or the contrasting extremes of glamour as found between busses and the coachbuilt Talbot-Lago.

I suppose it was inevitable that the automatic gearbox effectively supplanted the pre-selector, but sometimes it is nice to revisit the old ways of doing things.

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