They say a picture tells a thousand words. Not when you’re trying to understand the Wilson preselector gearbox, a thing briefly popular amongst some chiefly British upmarket cars from the 1930s to the 1950s. I’ve looked at images and compared them with written descriptions, but it made my head hurt. Best thing is just to read about and and not think too much about what’s actually happening because therein lies madness.
Here’s the eponymous Walter Gordon Wilson.
A military man as we can see, he was born in Dublin in 1874, the youngest of eight. As far as I can make out he had nothing further to do with Ireland, instead pursuing careers in the British military and later in the engineering trade.
He had a particular fondness for epicyclic gears and used these to sort out the steering difficulties of the first tanks which had relied on the commander shouting out instructions to various crew members on either side of the machine which had wonderful scope for confusion.
“Hard left there, Smith”
“But I said right!”
Post Wilson, the driver could just drive.
Post WW1, Wilson turned his attention to the road and came up with his preselector gearbox which was intended to avoid the difficulties of unsynchronised gears. Like others, Wilson turned to epicyclics because ratios could be changed with clutches and brake bands rather than mechanically. He was concerned with making his gearbox easily serviceable and so he concentrated on brake bands (which went on the outside of the gearsets) rather than clutches (which were buried within).
He used four epicyclics, one each for the lower three gears and another for reverse. Cotal provided four speeds from two epicyclics but reverse was done separately and they didn’t mind using clutches to provide a direct gear from each gearset. Wilson’s aversion to clutches mean he couldn’t use each gearset for anything other than one reduction (or one overdrive) gear.
Let’s start at the start with first gear. It’s simple. The engine drives the sun wheel, a brake is applied to the ring gear, the planet carrier turns with the sun wheel but much slower (Wilson chose a 5:1 ratio).
Second is trickier. The first gear is released and the second gear brake applied. The second gear now provides a low ratio. But it doesn’t directly drive the wheels. Instead it turns the ring gear of the first gearset. This speeds up its output so now we have a gear higher than first.
Third follows the trend. Second gear is released, the third gearset provides a low gear which is used to speed up second which continues to speed up first so we have another higher gear. For reasons that are completely unclear to me it does this differently to second (the third gearset is driven from second, not directly by the engine) but I’m not going there. It just is as it is.
Fourth is a direct gear and despite himself Wilson had to use a clutch this time. It locked everything together so drive passed through the box with no change in speed.
Reverse had its own gearset, except that it didn’t actually reverse the direction of rotation. First gear did that, the reverse epicyclic just lowered the gearing. I’ve looked at the diagram and don’t recommend it. Just accept that it works.
All of the brakes were engaged by stiff springs so Wilson needed a way to easily disengage them. He provided an operating pedal which pushed all the brake bands back against their springs and a toggle arrangement which could selectively disconnect the levers that joined spring to brake thus preventing them being used and allowing just one brake (or none, if you wanted neutral) to be applied. The driver pre selected a gear by using a lever behind the wheel and when the gear was actually needed he (yes, usually he. Different times) pressed the operating pedal. This pushed back whichever brake had been in use and then allowed the next brake as determined by the hand operated lever to be applied.
The driver still needed to get the car moving and there were at least three methods. The operating lever could be used as a clutch (which rapidly wore out the first gear brake), a centrifugal clutch could be used or if smooth but slow progress was important a fluid coupling could look after stopping and starting duties.
The preselector vanished from cars in the 1950s, replaced by synchromesh (at the lower end of the market) and automatic gearboxes (at the higher end).
The story didn’t end there though. The Wilson/fluid coupling combination was popular on city buses and evolved into the Hydracyclic (hydraulically operated) and Pneumocyclic (guess!) semi automatics which dispensed with the operating pedal and instead engaged the gears (now up to five) directly. Some models were even automated. Production continued until the late 80s or early 90s.
You’ll want to know how to drive with a Wilson, won’t you? Ben did. See how it went for him. It’s not obvious from the video, or even to Ben, but the car uses a fluid coupling.