What is Double Clutching?
“Granny shifting instead of double clutching like you should”
Words made famous by ‘Dom’ (Vin Diesel) in 2001 film The Fast and the Furious. Maybe the low expectations of the movie led the makers to deem fact checking unnecessary, but this line stands out as a glaring error amongst a host of them relating to motor vehicles and their operation.
Subsequently the Fast & Furious franchise has gone on to become the sixth highest grossing in cinema history taking over $5billion worldwide, mistakes and all. Proving everyone loves fast cars regardless of whether the people driving them know what they’re talking about or not.
The reason this supposed criticism is such a mistake is that double clutching is a technique typically used for shifting down through the gears, and since they had just completed a drag race this was not what ‘Brian’ (the late Paul Walker) should have been doing at all.
‘Granny shifting’ describes the process of methodically sequencing up or down through the gears – as taught by your driving instructor. Other techniques to change gears in certain situations have been devised through the years to speed up or otherwise improve upon the action of changing gears in the quest for more speed.
Double clutching is one such technique, but it is not used to shift up. This is because the process involves matching the engine speed with the gear you want to change into - but as you can only directly affect an engines speed when it isn’t in gear by increasing it (using the accelerator), double clutching isn’t used for accelerating as you would need to drop the engines speed relative to the wheels instead. The only way to do that is to wait for the engine speed to drop.
When looking to change up gears, the process of pressing the clutch and lifting off the accelerator drops your engine speed sufficiently to change into a higher gear and then continue accelerating afterwards. Your transmission speed is almost unchanged; the wheels are still travelling nearly as fast as they were, but by dropping the engine speed you have matched it to the gear you want to engage.
When changing down, the speed of your engine needs to be higher in relation to the speed of the wheels and transmission – which is why if you change down when travelling at speed you will feel the car decelerate noticeably as the engine isn’t rotating fast enough to maintain the current transmission and wheel speed. This is called engine breaking.
Here’s where double clutching comes in. As you look to change down, instead of immediately putting it into the next gear, you instead put it in neutral and release the clutch. This meshes the clutch with the engine instead of the transmission, at which point you tap the accelerator to increase both the engine speed and now enmeshed clutch speed as well.
Now they are rotating at a faster speed than before, you can depress the clutch again and change down into the next gear as normal having matched the engine speed to the gear you want to change into rather than the one you were in.
This all happens very quickly – barely a second with practice – and when done correctly will speed up how fast you can change down gears as you don’t have to wait for the transmission/wheel speed to drop before making the change.
Not that you would notice however - this technique is now largely redundant. Transmissions have advanced to the point where they incorporate the process into their mechanics through the use of synchronisers, which sit between the gears and the clutch and help match their speeds whithout the need of advanced shifting techniques.
We have talked here about how different types of manual transmissions work, including the synchromesh which uses synchronisers, and it provides a better understanding of the processes double clutching affects.
So now you know what it is, you can…not bother with it. Double clutching is now utilised only by certain large vehicles and older cars with transmissions that don’t have synchronisers - but just in case, there's no harm in knowing.