Top Gear’s guide to driving well: how to double declutch
Everything you wanted to know about driving but were too afraid to ask, vol 5
So, hands up: who knows what a synchroniser is? OK, put them back down; turns out we’re not allowed to use your phone or computer camera to see if you have your hands up. Something about ‘privacy’ and the ‘unholy violation’ of it... we dunno. Sounds like a bunch of legalese from where we’re sitting.
Anywho, while clever clogs will know what synchronisers are and how they relate to cogs, let’s get this out of the way now for everyone else: they’re the reason you don’t already know how to double declutch. Because, as the name suggests, synchronisers match the speed of the gear you’ve selected to the speed of the output shaft.
This means you can put the clutch in, nonchalantly slip from third to fourth gear and let the clutch out, while the synchros in the gearbox bring everything up to speed and ensure you don’t crunch and wear gears unnecessarily. Nifty, no?
You might think that, when you shift back down from fourth to third and blip the throttle, you’re matching the speeds of everything in your drivetrain, but that’s not quite the case – you’re matching the speed of the engine and the gearbox input shaft. It’s still a much better idea than shifting into a lower gear without matching them, but you’ve not... well, synchronised every part of the driveline.
But that’s okay! You don’t need to, thanks to synchronisers doing the heavy lifting for you. But you’re the one reading this article, so we may as well illustrate the most mechanically sympathetic gearshift you’ll ever do.Advertisement - Page continues below
So what’s this Double Dragon thing and how do I do it?
In the main, double declutching really is what is says on the tin – for one gear change, you push the clutch in twice. Once to move out of gear and into neutral, and again to move into your chosen gear. But this alone won’t get your engine, clutch, gearbox and driveshaft as in sync as a boy band.
Double declutching includes rev matching, but it’s a step beyond blipping for downshifts while leaving the clutch depressed for the entirety of the gear change. This is the process of manually doing the job that gearbox synchros were invented for.
Let’s use an example. Say you want to shift from fourth to third; you’ll come off the power, dip the clutch, move the gearstick to neutral and let the clutch back up. Then you’ll blip the throttle so that revs match road speed, as per heel-and-toeing, then dip the clutch again, select third gear and let the clutch out. In a modern car with synchros, it’s unlikely that the operation will feel any different or smoother than just a quick downshift blip, but you’ll have made the synchronisers’ jobs much easier.
Wait a minute... how does this work?
If it helps, think about the engine, gearbox and driveshaft as distinct units that can be linked or unlinked. It probably should help, because that’s exactly how they work. For instance, you can have the engine idling, be in neutral and have the clutch disengaged (i.e. pedal in). Then, each part is operating independently of the others.
But what if you were rolling along at 10mph? Well, your wheels would be spinning the diff, which in turn would be turning the driveshaft. But your gearbox and engine aren’t linked to this speed in any way.
Now imagine re-engaging the clutch by releasing the pedal. You’re still in neutral, so the drive shaft isn’t interacting with the gears. What you have done, though, is link the engine speed to the speed of the gearbox input shaft by engaging the clutch. You’re not in gear, so there’s still no connection between engine and drive shaft, so it’d have no effect on your rolling speed.
Let’s try another hypothetical scenario: slotting the gearstick into second gear and rolling along at 10mph with the clutch in. Now, your gearbox will be spinning along in second at a rotational speed that matches your 10mph road speed. Yes, you’re in gear, even though you’ll roll along just about as freely as you would if the gearstick was in neutral, because the engine and drivetrain are separated by your foot on the clutch pedal (and a generous bit of hydraulic pressure) holding the clutch plates apart.
Then, if you let the clutch out, you’d link the operation of the engine with that of the gearbox input shaft (via the clutch) and the driveshaft (via the gearbox, in second gear).Advertisement - Page continues below
What if I try to shift down a few gears at once?
Remember how we said the job of a synchroniser was to match the speed of the gear you’ve selected with the speed of the output shaft? Well, another part of their job is ensuring that you can’t select that gear until they’ve had a chance to match those speeds.
So if you’re doing 60mph in fifth and try to shift into second, chances are that you’ll struggle. By trying to shift back down to multiple gears at speed, you’re asking the synchroniser to bring the lower gear up to somewhere near its maximum operating speed. And that, at the very least, is going to take longer than it does for you to move the gearstick. If you force the gearstick in search of the gear, you can sometimes overcome the synchro and grind the gear as it fails to engage. Otherwise, you’ll be locked out until either the gear speeds up enough, or your road speed – and the rotation of the output shaft – lowers enough for the gear to mesh.
But let’s say you need the gear – perhaps you’re going down a hill and your brakes have failed. You can (if panic doesn’t set in entirely) use double declutching to your advantage here. Shift to neutral and let the clutch out, rev the engine up, dip the clutch again and shift to a lower gear. Second might still be a stretch (and could over-rev your engine), but it’s better a blown engine than a broken back. At least, that was what we told the insurers when we handed back a Mustang in seven separate bits.
Because the clutch is engaged, increasing engine speed also increases the speed of the input shaft of the gearbox. This means the synchronisers have less to do to match the speed of the input shaft and output shaft, so you’ll actually be able to select and mesh the gear you need. By then quickly dipping the clutch, selecting a gear, then releasing the clutch, you’ll have matched the speed of each component and can then use compression braking (or complete breaking) of the engine to help slow you down.
With that said, the much better answer is to make sure your brakes work before the hill. Just food for thought.