Double-(de)clutching revisited

[Dervorin]

Inspired by this (http://boards.straightdope.com/sdmb/showthread.php?t=380889) thread over in IMHO, I've been doing some reading on double-clutching. Now everywhere, it says the secret to this esoteric art while downshifting is to release the clutch in neutral before matching revs. My question is simply: why not just match revs with the clutch fully depressed, and then release the clutch? That's what I do all the time - and if I'm doing something wrong, I'm willing to learn. I've driven a manual all my life, and everyone I know matches revs with the clutch depressed; are we all wrecking our transmissions? I'm not sure if my car has a synchromesh box or not.

[TheLoadedDog]

Well, in theory, you're perfectly right. Match revs correctly, and not only do you do away with the need to double shuffle, but you do away with the need for a clutch at all.

The clutch is there for two reasons:
1. For moving off from a standing start
2. (the important one for our purposes) for shifting all other gears.

The second job would be done away with if everybody matched revs perfectly every time, but the fact is most people couldn't drive a greasy pole up... well, let's just say the average motorist is far from perfect, so the clutch is necessary. Racing drivers, and even some regular motorists who know their car very well, shift without the clutch being used at all.

Double clutching in a "crash" box as found in ancient cars and in trucks, or single "granny shifting" in a synchro box do the same thing - they allow for imperfectly matched revs. If your car was built in the last 100000000 years, it would have synchro. You are doubtless doing it no harm at all with your current driving style, and can carry on with it.

[Rick]

As TheLoadedDog mentioned cars now have snycromesh transmisssions so that the transmission matches the speed of the gears for you.
It was not that long ago that many cars, and most large trucks did not have syncromesh transmissions and the driver had to manually match the revs or risk damage to the transmission. (Old truckers's comment hearing someone not double clutch "Could I have a pound of medium grind?)
For a non-syncro trans or one where the syncros are worn out what you described in the OP is SOP.*
For a modern trans in good shape, the way you drive is fine.

*You can also double clutch if you are trying to drive a car with a bad clutch. NOT recomonded unless you know what you are doing.

[Anachronism]

When you push in the clutch you (effectivly) separate the engine and transmission, in a vehicle without syncros (IE a big truck) you rev up the engine to match the speed of the transmission with the road speed for the new gear. If you rev up the engine with the cluch depressed you are not changeing the speed of the transmission at all.

[Gary T]

Compare 2nd gear to 3rd gear at a given speed. The rear wheels and transmission output shaft main their rotational speed (it corresponds with the road speed, which for this illustration is fixed). But in 2nd gear, the engine speed AND the transmission input shaft speed are higher than in 3rd gear. (The gearing inside the transmission makes this possible, and that's the whole point of having different gears.) When you downshift from 3rd to 2nd, in order to achieve the desired smoothness you must match BOTH the engine speed and the tranny input shaft speed to the gears connected to the tranny output shaft. With the clutch disengageged (pedal down), you can only control engine speed. With the clutch engaged (pedal up) and the tranny in neutral, the engine and tranny input shaft are connected to each other. Now you can control, and therefore match, both their speeds to the relevant gears in the tranny.

By not doing this in a non-synchromesh tranny, you grind gears and can wear them rapidly. By not doing this in a synchromesh tranny, you make the synchro (baulking) rings do their job and wear them slowly. There's no compelling reason to double-clutch with a synchronized transmission, but doing it can prolong the life of the synchro rings and the corresponding dog teeth on the gears.

[Dervorin]

Hah. That makes perfect sense. Thanks, all. I think I'll keep my driving style the way it is, since double-clutching seems to be more trouble than it's worth.

[KlondikeGeoff]

Hah. That makes perfect sense. Thanks, all. I think I'll keep my driving style the way it is, since double-clutching seems to be more trouble than it's worth.

Wise move. However, once you learn double-clutching (as I did in the army with 2.5-ton trucks), it becomes as automatic as regular shifting. Today, how many people can even drive an manual transmission car at all?

[Necros]

Of course, the only real reason to double-clutch a modern synchromesh transmission is the eliminate the unsettling of the cars that happens when you let the synchros do their work. If you are "shocking" the drivetrain at high speed through a corner, when all of the friction available to your tires is doing a combination of braking, turning, and accellerating, the sudden shock can cause you to lose control of your car. This is, to say the least, undesirable in a racing situation. :)

That said, I double-clutch every downshift on the street, because it's an easy place to practice. If you relied on your seat time on the race track to perfect your technique, it would take a lot longer.

[TheLoadedDog]

There's no compelling reason to double-clutch with a synchronized transmission, but doing it can prolong the life of the synchro rings and the corresponding dog teeth on the gears.
That makes perfect sense, but why did the manual that came with my Corolla state something along the lines of "double clutching is unnecessary, and can cause damage to the transmission"? I would rather believe your version, because I could never fathom a reason for Toyota's one.

Anyway, it had a nice little tranny, and I never did double clutch it. The only vehicle I've double-clutched in recent times would be my friend's Nissan ute (pick-up) because it had shithouse synchro in third, and third would always grind. Strangely, he doesn't seem to care, and happily grinds it every time, but it makes my hair stand on end, so when I drive the thing, I do the ol' double shuffle. When third eventually dies, I don't want to be the one responsible.

[Gary T]

...why did the manual that came with my Corolla state something along the lines of "double clutching is unnecessary, and can cause damage to the transmission"?.
My guess would be that poorly done double clutching can be harmful, and they don't trust everyone to do it properly.

[Dervorin]

Wise move. However, once you learn double-clutching (as I did in the army with 2.5-ton trucks), it becomes as automatic as regular shifting. Today, how many people can even drive an manual transmission car at all?
As far as I've seen, in England and definitely in India, there are more cars that have a manual transmission than an automatic. In India, in fact, an automatic is a rare creature. My guess is that it would be true through most of the developing world as well - manuals are cheaper and slightly more fuel-efficient, when driven semi-decently. I wonder if it's mostly a North American thing, this rarity of manuals.

[TheLoadedDog]

They're increasingly rare here in Australia.

I think that anywhere with big tank-like cars and relatively cheap fuel will have a lot of automatics. Even if you don't mind paying the extra fuel in the UK, your car will likely be small, so you'll like that extra grunt from a manual.

Australia now has that "pass your test in an automatic, only drive an automatic" rule, so I guess that might entrench the things here. PErsonally, why anybody would want to spend an extra A$2000 for a transmission that is not only less fuel efficient, but less fun to drive, is beyond me.

[Dervorin]

Personally, why anybody would want to spend an extra A$2000 for a transmission that is not only less fuel efficient, but less fun to drive, is beyond me.
Convenience. At least, that's the only reason I can think of. It's much easier to learn to drive an automatic, and from anecdote, once you've learned on an automatic, it's not easy getting into a manual. Driving a manual requires a lot more co-ordination and dexterity, and for some people driving isn't so much about fun as just getting from A to B. I wouldn't... but I can see why some people would.

[StuffLikeThatThere]

Today, how many people can even drive an manual transmission car at all?

Old thread. (http://boards.straightdope.com/sdmb/showthread.php?t=211974)

The condensed version is that, in the US, the ability to drive a manual transmission seems much more common in the older population, and in the rural population.

Kids these days.

[Baffle]

Automatic transmissions are great for eating and driving, or talking on the cell phone and driving -- in other words, doing stupid stuff that's probably going to take your mind off the road anyway.

When I bought my first car (only a month ago) I made sure to get a manual, for the fuel efficiency. It was only afterward that I realized how much more fun it is to drive.

Of course, among young people that I know, very few can drive manual (I only have two friends who can) and generally when somebody sees the shifter I get a "Wow, you can do that?!" look. Kids these days.

[audiobottle]

I tried double-clutching, but could never do it quickly enough. From what I gather, it's supposed to be pretty darn quick considering people do it when racing, but how much faster can it be? Assuming I have this right, you push in the clutch, change to neutral, let out the clutch, match revs, push in the clutch, switch to lower gear, and clutch out again. Can you double-clutchers out there do it as quickly as just shifting gears but leaving the clutch down and rev matching that way?

[Johnny L.A.]

Can you double-clutchers out there do it as quickly as just shifting gears but leaving the clutch down and rev matching that way?
Pretty much. I hadn't had to double-clutch for many, many years until I got the Triumph. It came back quickly.

You have the sequence down, but you have to practice to get the revs just right and to do it quickly. If you downshift at too high a speed, it will take a lot of revs. The trick is learning how many for any given speed. Once you have it, it becomes second nature. A lot of people simply push in the clutch and come to a stop before shifting into first.

Downshifting with a modern transmission is much easier. But I like the sound and the feel of double clutching in a classic car.

[Rysto]

PErsonally, why anybody would want to spend an extra A$2000 for a transmission that is not only less fuel efficient, but less fun to drive, is beyond me.
Manuals become a lot less fun to drive in bumper-to-bumper traffic.

[pulykamell]

Manuals become a lot less fun to drive in bumper-to-bumper traffic.

People (almost always automatic drivers) tell me that, but I don't agree. Manuals at least give me something to do during bumper-to-bumper traffic and keep me otherwise occupied.

The only thing that occasionally gets to me about manuals is trying to talk on the cell phone without a hands free device (which is illegal in Chicago, anyway). That's a pain.

[Arnold Winkelried]

I agree with you pulykamell. I drive a manual, and it can be difficult sometimes when I'm on the freeway holding my burger and trying to dip the french fries in ketchup, while my wife is trying to give me a grocery list on the phone and the newspaper (open to the comics page and propped up on the steering wheel) flaps up because of the current from my air vents and obscures the windshield. Then some rude person has the gall to honk at me!

[TheLoadedDog]

People (almost always automatic drivers) tell me that, but I don't agree. Manuals at least give me something to do during bumper-to-bumper traffic and keep me otherwise occupied.
I tend to agree too. 95% of my driving is city driving, and I still prefer manuals. The stop-start thing I find easier in a manual because you can often just leave the revs constant, and use the clutch to control your speed, when the traffic is going stop - walking pace - stop - walking pace - stop. In an automatic, you're having to shift from the accelerator to the brake and back constantly.

I used to drive an old beater automatic, and about thirty seconds from my front door, I'd come into a long, long line of traffic that was approaching a roundabout (rotary) at the top of a hill. The thing with roundabouts, as opposed to traffic lights or stop signs, is that the queue tends not to stop and start so much, but will move at a consistent crawl. This was really, really awful in my old car in winter, because the engine was cold, and at that crawl, and uphill, it kept wanting to conk out, but I couldn't rev it any because it was an auto, and I'd run up the arse of the guy in front if I did. I couldn't jam it into neutral and rev it, because of the hill and the crawling pace - I'd be rolling back within a second or two. So I had to sit at home and warm the thing up forever. In a manual, I could have bought some time with the clutch when I heard the motor about to die.