Double Clutching
Q: What is double clutching?
A: You just watched "The Fast and the Furious" didn't you?
Real answer:
Double clutching is a downshifting technique that promotes smoother transitions and lower transmission wear. It is useful for road racing, prolonging transmission life, and giving you an overall smoother ride.
In normal driving, with modern cars- you don't need to double clutch... ever. When you shift, these neat little devices called "synchronizers" or "synchromesh" (or whatever other name you want to give them) in your transmission help your shifting by matching the rotational speeds between meshing parts. Why do you need to match the speeds between transmission parts when you shift? Simple- they won't go together unless they're all traveling the exact same speed. Your synchros take care of this, so you don't have to worry about matching revs much in normal driving.
So the question now is... why the heck do you need to double clutch? It's useful in racing, it's required for non-synchro transmissions, and it's just a damn cool racing skill to master. Think of your transmission as being separated into two functional halves. One half is connected to your engine, and the other half is connected to your drive axles/wheels. The split between the two halves is right at your gears.
Let's say you're driving down the street in 5th gear. Assume that your gearing is 1:1 all the way though, just for simplicity's sake. Your engine is turning 3000rpm, and so are all the parts in your transmission. You want to downshift to get higher up in your powerband to pass someone, so you mash the clutch pedal, shift to 4th gear, then lift off the clutch pedal. If your 4th gear ratio is twice what your 5th is, your engine is now spinning at 6000rpm (along with the "engine half" of the transmission) while the "driveshaft half" of your transmission is still spinning at 3000rpm. Your car is still moving at the same speed, but you're higher up in your engine's powerband. Now you have more power to pass the person in front of you.
What normally happens when you downshift and don't match revs? You feel the car lurch some while the transmission forces the engine to a higher rev level. The synchros grip against each other to match the gear speeds, the gears mesh, and when your clutch grips- it pushes the engine higher... and you feel the rough transition. To smooth this out, you can raise the rev level of your engine and the "engine half" of the transmission so the synchros have less work to do, and so your transmission isn't pushing the engine around.
How do I double clutch? I never thought you'd ask.
1. Push clutch pedal down
2. Shift to neutral
3. Lift clutch pedal up
4. "Blip" throttle to raise engine speed, "engine half" transmission speed
5. Push clutch pedal down
6. Shift into lower gear
7. Lift clutch pedal up
That's it.
If you did it right, you "blipped" the throttle just the right amount so your engine is at the exact right rev level to match up for the lower gear. Your synchros have less work to do (or none), so your transition to the lower gear is smoother, you effect less wear on the synchros (they don't have to work to match gear speed between the two "halves", you did that), and you effect less wear on the engine as well.
Someone asked me why you have to let the clutch back out before you "blip" the throttle, and here is the reason. The goal of blipping/tapping the throttle is to raise the rotational speed of the "engine half" of your transmission- your engine speed going up is what causes this to happen. Making your engine rev faster is just the way you effect the rise in transmission speed (in the "engine half")... if you could make the transmission rev faster on the "engine half" without revving the engine, that would theoretically work as well. If you rev the engine while the clutch is open (pedal down), then you're just raising the rotational speed of the engine, and not the "engine half" of the transmission. Just opening the clutch (pedal down) and revving the engine to where it needs to be, then shifting and closing the clutch (pedal up) isn't double clutching- but it still helps to reduce wear on your engine and transmission. Why? Your synchros still have to work to match the transmission speeds, but they don't have to work to rev the engine up to where it's supposed to be. On the engine side, your engine isn't being 'forced' to run at a higher rev as soon as you let the clutch out, so its wear is reduced as well.
Why does double clutching help?
1. Smooth transitions are everything in racing. If you're downshifting during a turn at a road course and you're cornering at the limit, a sudden lurch in your car isn't going to be too pretty. Say hello to the curb/barrier or whatever is going to catch you when you spin out. Double clutch properly, and you'll be faster/smoother at the track.
2. If you have a transmission without synchros in it, you must double clutch to downshift, otherwise you'll just be grinding away at your gears while they fail to mesh. You can NOT shift without matching revs in a transmission that doesn't have synchronizers in it.
Double clutching does NOT help during drag racing. You do NOT downshift while drag racing- if you do, you just lost the race. Don't listen to that Fast and Furious crap!
