Gearboxes. They’re just sort of there, aren’t they? We take them for granted, which is a bit cheeky of us given just how hard they work. Without a gearbox, your car would just be a shiny tin for your engine. So how exactly do they work? Well, they’re made up of a number of components, which we’ll go through here.
Input shaft
The input shaft is what comes from the engine, into the gearbox, via the clutch and flywheel. It is the raw source of rotational power as generated by the crankshaft within the engine. This feeds into the gearbox and drives the gears that will ultimately drive the wheels, but that power has to make a few more stops first.
Layshaft
By default, the power is routed to the layshaft. This is a shaft affixed with gears that rotate on a constant basis, driven by the input shaft. These gears are fixed to the layshaft, as in they rotate with it, whatever speed the engine is running.
Main shaft
Above the layshaft sits the main shaft. This shaft goes on to deliver the power to the differential, and then ultimately, the wheels. Much like the layshaft, this shaft also has gears affixed to it.
However, they’re mounted on bearings. As such, while they spin constantly thanks to being driven by the layshaft, they don’t actually turn the main shaft, at least not yet anyway.
So if they’re not spinning freely, how do they drive the wheels? Well, that’s where the gear lever comes in. Every time you select a gear, you moved the selector rod, which engages a dog clutch. This moves onto the gears on the main shaft and meshes with them.
It can only do this when the clutch is depressed though, as that disconnects the input from the engine. Hence that horrible crunching/grinding sound if try to do it without the clutch. That’s the sound of spinning metal hitting other spinning metal.
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When you let your foot off the clutch, the dog clutch is meshed with your selected gear. That means the gear on the main shaft is no longer spinning freely on the bearings. Instead, it’s driving the main shaft, then the differential, which in turn drives the wheels.
Much like on a bicycle, the gears on the main shaft differ in size to make for different ratios. This means the power of the engine is being used to its maximum efficiency. Without this, the car would be revving fast, but going slowly.
In older cars, the whole process was a bit more difficult as the meshing of dog clutch and gear wasn’t truly synchronised, but now it is. As you’ve probably guessed, that’s what a synchromesh does. Plus, the teeth on the gears are cut at an angle, rather than straight. This also makes for smoother meshing.
So what about reverse? It works on the same principle of all the forward gears, apart from one minor difference. Between the gear on the layshaft and the gear on the main shaft, there is a smaller gear. This reverses the rotation of the main shaft once the gear is engaged. And there you have it, reverse.