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.