Friday, August 03, 2012

What is Double Clutching?

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. When you shift, devices called "synchronizers" or "synchromesh" 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 do you need to double clutch? It's useful in racing, it's required for non-synchro transmissions, and it's 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 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. Tap the gas to raise engine speed
  5. Push clutch pedal down
  6. Shift into lower gear
  7. Lift clutch pedal up
You just double-clutched!


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