What’s the difference between double clutching and single clutching, if in single clutching you raise the engine speed while the clutch is engaged?

Question

I keep reading articles about double clutching and they all say the same thing. You double clutch so you can rev match.

When I down-shift, I press the clutch, and while I’m moving the shifter to the next gear, I rev the engine. Then when I release the clutch the engine is spinning at a higher rate and the transition into the lower gear is smooth.

So why is that not the same as double-clutching and rev matching? Is there an extra benefit to revving while the clutch is disengaged? That’s what I can’t find. If the articles say “double clutch so you can rev-match”, but you don’t need to double clutch to rev-match, then why do you need to double-clutch?

In: Other

Answer 1

With double clutching, you lift the clutch while you’re in neutral. What that does is connect the lay-shaft of the gearbox to the engine. None of the gearbox output gears are in mesh because the gearbox isn’t in gear, but in that configuration, neutral with the clutch up, changing the rev of the engine now changes the speed of the lay-shaft and the various ratio gears of the gearbox which is what was required on old pre-synchronizer gearboxes in order to get the next gear to slot in.

If you hold in the clutch, while the gearbox is in neutral the lay-shaft and all the constantly meshed ratio gears will just coast to a stop, and it’ll only get spun up to match the road speed again when you move the shifter into whichever gear you’re going for by the synchronizer.

In other words, you’re not achieving a whole lot.

You can get smoother shifts, doing what you’re describing but only because you’re matching both sides of the clutch (or in other words gearbox input shaft to the engine) after the gearbox has shifted it’s nothing much to do with the gearbox gears.

Blipping the throttle going down means the clutch isn’t now expected to pull the engine to the right speed but it’s nothing to do with the internals of the gearbox, that’s still being dealt with by the synchronizers.

True double-clutching means changing the speed of the gearbox, using the engine, and that requires the clutch to be up in neutral.

You must’ve come to a quick stop, and gone into reverse too quickly and got the crunch. That’s an example of the sort of mismatch true double-clutching is trying to prevent, and that happens because the gearbox internals haven’t yet coasted to a stop (which is required for reverse to engage as it doesn’t have a synchronizer).