Double clutching is when you press the clutch pedal twice while changing gears: once while you shift into neutral, and a second time while you put the shifter in the new gear. Why would you want to do that with your manual transmission car? Is there any advantage?
You may remember that double clutching was mentioned in the movie, “The Fast and the Furious.” In one scene, Vin Diesel told Paul Walker the reason he just lost a drag race was that he was “granny shifting instead of double clutching like you should.”
The main point to be taken here is that you shouldn't rely on movies to give you accurate information about the real world. Double clutching wouldn’t have helped Walker win his race. In fact, he might have been just a touch slower. Read on to understand why.
What’s the Purpose of Double Clutching?
The purpose of double clutching is to match engine speed to wheel speed when shifting into the new desired gear. When you’re in a low gear, engine RPM is high relative to wheel speed. Inside the transmission, also called the gear box, the output to the differential is spinning slower than the input shaft from the engine. The output shaft runs through a set of toothed gears turning at different speeds. There’s one for each gear. Their speeds are controlled by the gear ratio between them and the gears on the engine side that are driving them. The low output shaft gears spin slower than the high ones.
When you move the gear shifter and put the car into a gear, a collar locks on to the toothed gear and attaches it to the output shaft. The output shaft starts turning at the same speed as that gear.
The collar has dog teeth that lock into the new gear. When you start to shift, the collar and its teeth are turning at the speed of the output shaft, which is also the speed of the old gear. But now the collar is going to lock onto a gear that’s spinning at a different rate. There’s a tendency for the dog teeth to scrape and grind until they’re locked in and the speed of the new gear and output shaft are synchronized.
While conceptually you’re matching engine speed to wheel speed, it’s more accurate to say that what you want to do is match the speed of the new output shaft gear to the speed of the output shaft. The idea behind double clutching is to better match these speeds during the shift. Then the collar can more smoothly lock into the new gear, saving wear and tear on the gears and on the dog teeth of the collar.
Modern Manual Transmission and Synchronization
At one time double clutching was required. If you tried to move the gear lever to a new gear with a single shift, the gears would grind and protest, and the vehicle would go into gear reluctantly if at all. However, in the 1920s, auto manufacturersdeveloped synchronizers, or synchromesh manual transmission. A synchronizer smooths out the shift and eliminates the need for double clutching. There are different techniques for synchronized manual transmission, but all of them modify the input speed and the output speed for that they’re more closely aligned.
How Do I Double Clutch?
Here’s how not to do it: don’t do what Vin Diesel told Paul Walker to do. It appears Diesel wanted Walker to do a bang-bang, one-two, clutch-and-shift, clutch-and-shift from each gear to the next higher one. That does nothing to equalize engine RPM and wheel speed. All it would do is cost Walker a fraction of a second each unnecessary time he clutched and shifted.
In fact, double clutching is more effective during downshifting than shifting gear to a higher gear. That’s because you want to increase engine speed relative to output speed when you downshift. There’s a way to do that: you can “blip” the accelerator during the shift to raise the engine RPM.
When you’re shifting up, on the other hand, you need to reduce engine RPM. The only way to do that is to let the car coast in neutral until the RPM drops. If you do a slow, leisurely shift from fourth gear to fifth gear, the engine speed will drop just slightly, and those dog teeth may have just a little bit easier going locking into the new gear.
If you decide double clutching is something you just have to experience for yourself, here’s how you do it.
For a double clutch downshift:
On the upshift, follow the same steps, only don’t blip the throttle. For smoothest results, pause a moment with the gear shift in neutral to let the engine RPM drop.
As a side note, this process is sometimes called “double declutching.” When you think about it, it’s a more accurate description. The clutch is engaged before and after the shift, so when you work the clutch pedal twice you’re disengaging the clutch twice. You’re declutching twice.
Why Would I Double Clutch These Days?
Some diesel semi-trucks have dual clutch transmission, built to be double-clutched, as do some race cars. There was a time that you might have run into an old rental moving van that needed to be double-clutched. However, for any modern manual transmission vehicle, there’s no need to double clutch.
Double clutching may be just a little easier on your transmission. On the other hand, it requires twice as many disengagements of the clutch. There’s not a difference you’re likely to notice. The best argument in favor of the technique is that, when done properly, it makes shifting more careful and deliberate. That’s never a bad thing.
Here’s one veryspecific situation where is might be good to use part of the double clutching technique. It can happen when you’re slowing to a stop at a red light. Maybe you’ve been shifting down through the gears, or maybe you’re just easing to the stop in neutral. In any case, you’re now in neutral, prepared to stop, but the light turns green while you car is still moving. You need to shift into first gear. In this situation, you might find that the gear stick is reluctant to go into first. That’s because the mismatch between the output speed from the transmission and the engine RPM is significant. To ease the shift, “blip” the throttle while the car is in neutral but before you press the clutch pedal. The depress the pedal and shift into 1st gear. The speeds will be better matched and the car will more easily side into first gear.
So Should I Double Clutch or Shouldn’t I?
It’s your choice. Most drivers will find there’s enough to think about when driving a stick shift without using an old gear change technique that makes very little difference in today’s world.
Whether you ever double clutch or not, it’s good to know what it is and how to do it. As long as you do it smoothly and sensibly, it doesn’t hurt anything. It may save just a little wear and tear on your gearbox. When you do double clutch, there’s a tendency to be more aware of your shifting. Also, it gives you a better feel for what your transmission is doing every time you’re changing gear.