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

The gearbox have an input shaft. This is what the clutch is connected to and it also have a number of gears on it inside the gear box. So it have quite a bit of momentum. When you press the clutch and put the gearbox in neutral this input shaft is not connected to anything and will spin freely. Since it was spinning when you put the gearbox in neutral it will continue to spin at that speed as it slows down over time. Double clutching is not about matching the speed of the input shaft to the engine but rather to match the speed of the input gear to the output gear. After you put the gearbox in neutral you release the clutch and rev up the engine to speed up the input shaft. Then you can press the clutch and the gears will now mash nicely as they spin at the same speed.

Most modern gearboxes have a set of synchronization rings in front of each gear. This kind of acts like a clutch. When you put the gear leaver into the gate you bring the gears close enough that the synchronization rings touch and spin the input shaft up to speed. You can then mesh the gears without any grinding. You may still have issues today if you try to change gears too fast as the synchronization rings do not have time to spin the input shaft up to speed. But this is quite old technology which have been perfected today so you rarely have any issues. On a 30 year old worn out gearbox however things are a bit different. And the synchronization rings might not even work properly in all gears so you have to double clutch.