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Double clutching rationale

OK, simple question, maybe.

The deal with double clutching, I think, is you
put the clutch in,
shift into neutral,
let the clutch out
put the clutch in
shift into a new gear
let the clutch out.

My question is why? Once you shift into neutral the first time why let the clutch out? What does that do vs just shifting directly into the new gear?

You’re missing a step. When you’re about to do the second shift, you want the engine (or more precisely the portion of the transmission connected to the engine when the clutch is out) to already be at the speed that it will have once the clutch is fully out, which allows the gears to mesh smoothly. Therefore, after you let the clutch out the first time, you either rev the engine or wait for the engine to slow down so that it’s at this desired speed.

This is necessary only with an unsynchronized transmission, of course.

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why can’t you rev the engine with the clutch still in and the car in neutral? Why do you need to let it out?

When the clutch is pushed, the transmission is completely disconnected from the engine. There is a gear in the transmission and spins the gear you select (1st-2nd etc). That gear needs to rev match the engine. When the clutch is in, this does not happen. You might want to watch a youtube video of a manual transmission in action-hopefully someone has made this.

First, watch this video (I know, the narration is slow and the music is hokey, but it does a pretty good job of explaining what’s going on when you shift)

So, you rev the engine with the clutch still in to match revs for the lower gear you’re about to put it in. Let’s say 2nd just to keep things clear.

This means that your engine (if you do it right) will be turning as fast as it will be turning when in 2nd gear with the clutch out. But! because the clutch is in when you rev, you aren’t getting the gear on the input shaft to spin at the speed it will be spinning once you’re fully in gear.

That’s where the synchro (about 3:47 in the video) comes in. It gets both sides spinning at the speed they need to spin via friction.

Old transmissions didn’t have synchros, which meant drivers had to get both sides of the transmission spinning at the proper speed to engage. By letting the clutch out in neutral and revving the engine, the input shaft spins faster to match what the other side of the transmission is already doing.

If you don’t let the clutch out in neutral, the input shaft doesn’t speed up, and the gears don’t go together nicely without the help of a synchro.

Some people who are especially anal about minimizing wear will double clutch even with a modern transmission to prevent synchro wear. Not really super necessary - those synchros will last a long time - but there’s nothing wrong with doing it either.

Incidentally, double clutching does come in handy when you have a bad synchro, as you’re doing its work for it so that it doesn’t have to.

Modern fully synchronized transmissions don’t really need to be double clutched. Matching RPMs is still useful for downshifting but single clutching will provide that. Some modern sporty cars with electronic drive-by-wire throttles will rev-match FOR you!

The only time I ever tried not even sure if it would be called double clutching, was playing with shifting gears without a clutch. Could be say going to second pushing gently on second revving the rpm and letting off until the shifter dropped into gear. Never had a problem for the life of the trans, think I had to learn this on a unimog I got awarded with to clean snow from alleys.

I used to watch some furious gear shifters pretending to double clutch while the engine was disconnected from the transmission,revving the engine does no good if the clutch is disengaged.I never really understood the concept ,till it occured to me what was taking place in order to rev match the gears,its very useful to be able double clutch the old heavy non synched truck transmissions and some of the older car transmissions that werent synchronized in first . If you know the gear spacing rev wise ,a tach will allow you to downshift perfectly (experience will let you do it by ear ).

Remember in Greece, they had bought old French trains, clickity clak of the wheels on the rails going faster and faster, while the engineer is grinding gears to try and downshift and the cabin filling up with smoke from the overheated brakes, and ladies with a chicken in a plastic grocery bag saying simultaneously Oshicola (this is not good) while praying on their rosaries, I wanted to get off at the next stop we finally made after the gears clicked in, but there were only 3 houses and maybe a half a dozen goats.

Actually, you can do that and I have resorted to doing it in a small pickup truck in order to get home when my clutch pedal would just go to the floor without disengaging the clutch. If you have to stop, do it on a downhill so you can let gravity get the truck rolling fast enough to engage first on an idling engine.
To get out of the gear you are in, just put gentle pressure on the shift lever and slowly lift your foot off the gas, when the engine reaches its zero torque throttle opening, the transmission will let you pull it out of gear just as if you had stepped on the clutch. Then close the throttle more until the engine rpm matches the rpm it would be turning in the next gear and you can put the transmission in gear.

The manual transmission cars I owned had three forward speeds with the shift on the column. First gear was not synchronized. To shift into first while moving, I had to double clutch. I do this in my Rambler when towing a U-HAUL and going up a.grade. When it started to bog down in second gear, I would have to double clutch to get it in low gear. It seems to me that Ford synchronized first gear in its full sized Fords about 1963.

Don’t tell my dad but I used to practice shifting without putting the clutch in just for fun. Generally just 2nd to 3rd and never downshifting. I don’t think I ground that much off the gears.

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You can, but that will only increase the flywheel’s rpm. It won’t spin up the clutch disc, so won’t spin up the input shaft to the transmission, which is what you want.

I learned to double clutch as a teenager driving my dad’s truck, which wasn’t synchronized in 1st gear. And only sort of synchronized in 2nd. Since I learned early on, I just do it all the time even now w/my Corolla. In fact I’d have a difficult time to not do it. I think it does reduce the stress & wear on the clutch parts a little when downshifting at speed. And it makes the downshift feel a little smoother too. In any event, no harm done.

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I learned to drive back in the late 50’s.That was long before i knew anything about synchronized transmissions where you had to doubleclutch. I still do it ot of habit.

I’ll cut to the chase . . .

Is there a reason for your sudden curiosity?

Are the synchros on your car . . . 1995 Corolla, I believe . . . going bad?

It still decreases the lurch when reengaging the clutch on a downshift with a synchronized transmission.

It’s called clutchless shifting. Semi drivers do it a lot.

I often read about ‘heel and toe’ downshifting to match rpms. I know what it is, never did it - anyone here use it routinely?

Just for fun I often click on the screen name symbol ( circle with letter ) to see other topics they have made. This OP actually had a post in 2013 asking the almost identical question . Seems weird.

I have used heel-and-toe downshifting in my road race cars - the ones with weak transmissions. It allows rev matching to reduce driveline shock.

The Super T-10 I used in my last one, I didn’t bother. You couldn’t hurt that box with a hand grenade. The old line about that trans was; “If your arm doesn’t hurt shifting, you aren’t shifting hard enough!”

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