The Honda DCT Gearbox - Dual-Clutch Transmission Explained


Unless you're stripped a conventional gearbox down, and had the opportunity to spend a few hours studying the parts, gearboxes are harder to understand than a book on quantum physics written in a obscure language. So let's start small.

Imagine two gears (a gear pair). Providing the teeth are meshed, turning one will turn the other. No problems so far. Now, if there are 10-teeth on both gears it stands to reason that turning one a full revolution also turns the other a full revolution. But if one gear has five-teeth and the other has ten, something useful happens.

Turn the ten-tooth gear one full turn the five-tooth gear has to do two revolutions. Equally turning the five-tooth gear one revolution with turn the ten-tooth gear half a turn. This is gearing – and means we can vary the speed (and torque) with which pairs of gears rotate.

Now picture our two gears on two shafts. Turn one shaft (forcing the gear to turn) and we force the other shaft to turn too. Depending on the gear ratio (driven teeth divided by driving teeth) the other shaft will spin faster, slower or at the same rate as the one we're turning. But motorcycles have six speeds and each one is a different ratio – which causes a problem. If the ratios are different it means as we turn the shaft, one set of gears is going to try and turn the other shaft at one speed while the others gears try to spin it at another.
The solution is to only have one gear from each pair attached to a shaft (it doesn't matter which one), that way each pair of gears is free to spin at whatever ratio (speed) it wants to. The problem now is that because one gear in each pair is essentially free-wheeling, when we turn one shaft the other doesn't spin. All the gears will rotate, but not the shaft. To make this happen we need a way to lock each free-wheeling gear onto its shaft (one at a time). Once that is done it'll force the shaft to spin at the same time and we have a working gearbox.

This is what happens when you move a gear lever. The force from your foot is used to rotate the selector drum, which controls what gear is locked to a shaft. Because the selector drum produces a fixed sequence of movement it's impossible to have two gear engaged at the same time – and if none of the gears are locked to a shaft you're in neutral.

The other bit to mention is the clutch. In a gearbox, the function of the clutch is to make or break the gearbox's connection to the crankshaft. When the clutch is disengaged, you can be in gear but the motorcycle won't move. As you engage the clutch and the friction plates are pressed together toque is transferred from the clutch basket (the other bit) to the hub (the middle bit). In turn this is connected to the input shaft (aka main shaft) which makes it turn.

DCT (Dual Clutch Transmission) gearboxes

The advantage of DCT gearboxes is they can have two gears selected simultaneously – but how do they do it? The answer is to introduce a third shaft in the gearbox and a second clutch.

In a DCT gearbox there's only one clutch basket meshed to the crankshaft like in a normal motorcycle, but two hubs that spin independently of each other – not like a normal motorcycle. The other hub (the one you'd see if you took the clutch cover off), is attached to what appears to be a normal input shaft, but the second hub is attached to a hollow input shaft that fits over the top of the first one – covering half of it up. We now have an inner and outer input shaft – and depending on the state of the clutches, one, both or neither can be forced to turn with the crankshaft.

You start the engine and put it in gear. For this to happen one clutch (the outer in fact), disengages and the selector drum turns to lock-in first gear. As the clutch engages you ride forwards. While this is happening, both clutches are engaged and all the gears in the gearbox are spinning (meaning there is virtually no additional drag compared to a conventional gearbox).

At some point however the system thinks to itself 'I need to get second gear ready'. At this point the second clutch disengages and the selector drum turns a bit more – but because the selector drum is different to a normal drum it doesn't disengage first gear before it selects second. At this point the only thing stopping the gearbox from breaking is the fact that one of the clutches is slipping . When the motorcycle decides that it's time to change gear, it disengages the first clutch then engages the second.

At this point you ride forwards in second gear and the system again rotates the selector drum a bit more to disengage first gear. However, it doesn't have to engage third gear at this point, and as long as the inner main shaft is in neutral, both clutches can be engaged to reduce drag.

We weren't able to obtain the exact details of the sequencing algorithm that decides when and how to select each gear. But the fact there's a neutral holding position built in between each gear on the selector drum and the fact Honda claim an increase in fuel economy means that in all but manual mode, the motorcycle probably only engages the next gear moments before its needed. And in manual mode it may not select the gear until it receives an input from the user (rider) – although that's our guess rather than fact.Tag: DCT Dual-Clutch Clutch Gearbox
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