Double clutching. A term you may have heard bandied about amongst car enthusiasts, but what does it actually mean? Well double clutching, or double declutching as it’s sometimes called, is a technique where the clutch is depressed twice between gearshifts.
It’s only possible to do in manual cars and has nothing to do with a double-clutch automatic gearbox. We discuss why someone would do such a strange thing and what the benefits of double clutching are.
Double clutching meaning
In a manual car you depress the clutch before each gear change. The clutch is essentially a pair of discs/plates , one of which is linked to the engine, and one of which is linked to the gearbox (which is ultimately linked to the wheels). You need to disconnect the engine from the gearbox when selecting a new gear, and the clutch facilitates this.
Double clutching is when you depress the clutch once as you take the car out of gear, then release and depress it a second time before slotting the gear lever into the next gear.
Double clutch gearboxes, meanwhile, are a type of automatic transmission that use a pair of clutches to shift between gears seamlessly. You cannot double clutch in a vehicle with an automatic transmission as there is no clutch to depress. Some modern manual cars offer a rev matching feature which matches the transmission and engine speeds, smoothing out shifts. Double clutching attempts to match the engine, clutch and gearbox rotational speeds between each shift.
How to double clutch
The best way to explain the process is breaking it down step-by-step. Keep in mind that the key components here are the engine, clutch and transmission.
Let’s assume you are driving along in third gear at a steady speed and want to change down a gear.
1. Depress the clutch
The clutch and transmission will remain in sync, but the engine will be rotating independently.
2. Move the gear lever from third into neutral
Now the clutch, gearbox and engine are all rotating independently of each other.
3. Let the clutch out
This will allow the engine and clutch to rotate at the same speed. The transmission will now be rotating faster.
4. Blip the throttle to raise the revs slightly (referred to as rev matching)
This will bring the rotational speed of the engine and clutch in line with the transmission.
5. Depress the clutch a second time, engage second gear then release the clutch
This will allow for a seamless downshift if done right.
Double declutching as you shift up through the gears is a similar process, although instead of blipping the accelerator, you would lower the engine revs slightly to match the lower rotational speed of the transmission.
It’s a process that requires a bit of practice to perfect, but is it really necessary?
Why would I need to double clutch?
Double clutching can help you carry out smooth gear changes at any engine speed. It can also reduce the wear and tear on the drivetrain components.
To understand why double clutching is a thing, we need to look at how the clutch, gearbox and engine interact. In a conventional petrol or diesel car, the engine transmits the power it produces to the clutch which in turn goes through the gearbox to the driven wheels.
Each of these systems can operate at different rotational speeds. The clutch helps match the engine and gearbox rotational speeds. When the car is in a high gear the gearbox output shaft will spin fast relative to the engine, and vice versa in lower gears.
In modern cars, manual transmissions have synchromesh gears, which comprises a set of small gears that match the speed of the gearbox output shaft to the engine speed. Before the advent of the synchromesh gearbox, double clutching was essential to change gears smoothly.
It is a good idea to double clutch in older cars as synchromesh gears tend to wear out with age and high mileage. The second gear synchromesh tends to see the most use in regular driving and if you hear a grinding or crunching noise when shifting into second, the gearbox may need attention.
So, while it isn’t necessary to do it these days, it can have certain benefits.
Do I need to double clutch with a modern manual gearbox?
It is not strictly necessary to double clutch when driving a modern car with a synchromesh manual gearbox. It can still be beneficial, though, as you can reduce the wear and tear on the transmission components by regularly double declutching. With some practice you can also execute smoother gear changes than just slamming the gear lever through to the next gear and leaving the synchromesh to sort it all out.
Is double clutching actually useful?
Aside from smoother shifts and reduced wear, double clutching can be helpful in daily driving. Rev matching is a component of double declutching which requires you to blip the accelerator pedal when down shifting so that the lower gear engages smoothly. This is especially useful when engaging first gear while the car is still in motion.
Attempting to do this without matching the engine revs to the transmission will cause the car to jerk and can damage the gearbox if done regularly.
While this is not something you regularly experience on your daily commute, matching the gearbox and engine speeds when slowing down through the gears from high speeds will prevent the driven wheels from locking up. A useful skill to have on a track day, as well as when attempting to come to an emergency stop on a slippery stretch of motorway.
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