Double-clutching Question

So, I’m going to be buying a manual transmission car in the next couple weeks, and I have a question about double-clutching.

As I understand it, if you’re downshifting from a higher gear (to accelerate quickly/pass someone/whatever), it’s easier on the synchros in the transmission if you double-clutch instead of shifting normally. The idea is to get the engine RPMs to match the speed of the wheels before shifting, so the synchros don’t have to bother.

The process as I understand it is as follows:

  1. Clutch in, shift into neutral, clutch out.
  2. Rev the engine to where it needs to be for the speed you’re going and the gear you’re shifting into.
  3. Clutch in, shift into the lower gear, clutch out.

What I don’t understand is why you need to take the clutch out before revving the engine. And also, why you can’t simply rev the engine after you’ve shifted into the lower gear but before you’ve let the clutch out?

According to the manual for my '96 Corolla, double-clutching is “unnecessary in modern vehicles and can cause premature wear to the transmission” (or words to that effect).

When the clutch is pushed in the engine is not connected to the transmission so when you rev it up you are not changing the speed of the transmission. The syncros do their thing when you push the lever into gear, so reving the engine after that would not do anything for them. I would recommend leaving the double cluthing to the truck drivers, they have to because their are no syncros on a truck.

This is basically correct. Unless you have some ancient abused POS, the technique is entirely useless. You also have a high chance of fucking something up while learning to do it.

It really isnt even worth a thought on a new, modern car. As long as youre not constantly dropping the clutch (or riding it), you will have no trouble.

Now, if you want to talk about heel-toe and whatnot…

The last vehicle that I had to double clutch was a Vietnam era deuce and a half. It is also helpful in some older trucks especially if certain gears are difficult to shift into. Usually revving the engine will allow you to shift without grinding.

OTOH I don’t use the clutch much on my '95 Saturn. When the RPMs are about 2,000 it will slide into gear with nary a squeak, squeel, or gnashing of teeth.

The whole point is to get the rotating speed of certain internal transmission parts synchronized. You rev the engine because that’s the only way you have to affect the speed of the transmission input shaft – you’re not revving because you simply want to raise the engine speed. The transmission has to be in neutral, clutch pedal up (clutch engaged) in order to achieve the goal of changing the input shaft speed relative to the output shaft speed. And since you’re trying to ease the stress of the shift itself, you have to do this BEFORE shifting into the next (lower) gear – otherwise why bother?

I find it hard to see how properly done double clutching can accelerate tranny wear. I can see how it would accelerate clutch wear, and I would agree that it’s of minimal value in normal driving. However, it’s an extremely handy skill to have for dealing with certain types of clutch problems (failure to release) or nursing along an already worn synchronizer assembly.

I was under the impression that double-clutching was only necessary on cars without synchronized transmissions.

Not true?

True. From the OP: “…it’s easier on the synchros in the transmission if you double-clutch instead of shifting normally.” No one here is saying it’s necessary, and several have specifically said it’s not.

My guess is that the Toyota recommendation against double clutching is based on the assumption that the average driver would screw it up so badly as to damage the transmission.

Actually, you should do just that. That’s rev matching, and it’s the second half of the double clutcing process. It minimizes clutch wear by syncronizing the clutch disc and the flywheel. Double clutching may be unnecessary, but rev matching is just good stick-driving habit.

Rev matching happens pretty naturally during an upshift, since the engine speed falls off a little bit during the shift with the foot off the accelerator. This generally gets the engine speed pretty close to that of the transmissions input shaft.

To do this for a downshift, you have to actively try by “blipping” the throttle. That’s the key to rev matching. If done right, the release of the clutch become silky smooth, without the sharp braking effect that’s felt as the cluch speeds the engine up.

Rev matching isn’t necessary in a modern car, any more than double clutching is. But it certainly doesn’t hurt. It’s pretty easy to master, it’s hard to screw things up, and it has definite benefits like minimizing strain and wear on drivetrain components. I’ve gone 180,000 miles on a factory original clutch, and I’m fairly confident it has a lot to do with habitual rev matching.

When you can shift without the clutch, you have arrived…

Ahem. Some of us call them “classics”.

(The '66 MGB I’m having restored does not have synchro between 2nd and 1st. That didn’t come out until 1967.)

Heh, sorry about that. My old car is not quite that old, and is fully syncho. Should have said “ancient OR abused”. Have fun with the MG (though I am a Fiat guy myself…)

I was thinking more of my friends early 80’s beater subaru with nearly 250k on it. It had synchos at one point…

Come to think of it, my '46 Willys doesn’t have synchro between 2nd and 1st either.

Fiats are :cool:, but I’ve always liked the Alfas.