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?
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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.
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).
You’ve received some decent answers but I wanted to add one thing. None of this will ever be an issue for you on any modern small light weight transmission. Where it is a skill that you must absolutely master is when you try and drive larger vehicles with manual transmissions. Heavy truck transmissions must be shifted properly, including rev matching. If you don’t do this correctly, the transmission will complain loudly and the chance of doing very expensive damage is much increased.
One of the recurring comments is about meshing gears together. This is very misleading as almost all gears are in constant mesh with one another. You don’t move the gears during a shift, you lock the desired gears to the transmission shaft. If you really want to understand what is happening, look for an animation of how a manual transmission, with and without synchronizers, works.
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