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Discussion Starter #1
( This is a re-post, and a continuation of the "Brake and Gas pedal" thread. Someone asked why I double clutch.....)

I double clutch ( when downshifting) in order to bring BOTH the engine speed, AND the internal speeds of componants in the transmission into syncronization with each other. This saves wear on the syncromesh rings, and puts less strain on other componants of the drivetrain as a whole.

Double clutching, as I am presenting it here, involves mindfully stopping briefly as you pass through neutral, letting out the clutch and giving the gas a buzz, in order to spin the gear cluster now connected to the clutch up to the same speed as the gear cluster that is connected to the rear wheels. The clutch is then rapidly dissenguaged, and moved on into the intended gear.

Quite literally, one depresses and releases the clutch twice during the downshifting process, rather than once.

This spares the syncro rings the task of doing the same job (matching gear speeds in the transmission) , AND....not coincidentally, allows for a rapid downshift that does not strain the rest of the drivetrain, because the engine as well has been brought to a speed that matches what the road wheels would demand, given the particular gear and road speed.

Granted, when downshifitng like this, one does not get that sudden surge of engine drag, that some people use to help themselves slow down at stoplights. To my mind, that sort of usage really does eat clutches, and strains other drivetrain componants as well.

At the same time, when double clutching while downshifting in (or approaching) a corner, you also don't get that burst of engine drag eather. A burst of drag, and a resultant rapid shift of the mass of the car that can really upset the smoothness of the cornering process.

[ Some folks used to pull this move off with their toe on the brakes, and their heel on the gas ( hence the term "heel and toe" ) but in my experience, most people now simply roll the ball of their foot over to the gas, while using the other side of the ball of their foot on the brake. ]

My 510 and 240 both have shifters that are a bit more directly connected to the transmission than the 300, and when moving from the first clutching action on into the next gear, a proper double clutch will often cause the stick to feel like its connected to......absolutely nothing at all. No feel of the transmission moving through the syncros......no anything at all. Because of this.....you can often pull off a downshift smoother AND faster than if you had let the syncros do the work.

And when you can pull this off in the midst of a corner, while using the brakes at the same time......

Well, that's what I call the joy of "Sporty Driving".

"Old School" perhaps......but at 57, I got a right to that.

Now....if they had only put the gas and brake pedals a little closer to each other on the 300...

Bob

59 Morris Traveler....wish I still had it.
59 Rambler American 2 door wagon....wish I still had it too!
69 510 wagon......rolled
72 510 wagon .....daily driver
72 240Z.....sold
71 240Z....getting ready to sell it.
94 300ZX Slicktop NA....My new car
 

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mr. johnson,
i have to say... i am only 21 and i know exactly what you're talking about, i was taught how to drive a stick by some what you might call "old school" drivers. i think it brings more excitement into your daily driving experiences. its difficult to heel-toe on the 300 since my legs are so long my knees hit the under side of the dash and would be very nice to have a tiny bit wider brake pedal as you were saying. but at least this design gives me room to blend up some new driving technique while trying to get the desired effect =)



Post Edited (Jan 20, 7:23pm)
 

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I do the same thing without the 2nd depression of the clutch. As I'm downshifting, I tap the gas, and the RPM's shoot up a little. I've been doing it for a while now, so I'm pretty good at getting them close.
Why hit the clutch a 2nd time? Nothing else is gonna move that isn't already moving.....
 

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because were talking about downshifting not accelerating. Main reason I do some of this is to stop "violent" acceleration when downshifting. It makes for a smoother corner instead of brekaing the tires loose.
 

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I think Iii've got downshifting with a single clutch down pretty well under normal driving. I can blip the throttle and shift quick enough to wear there is no "jerk" or violent lurching. It just stays at the RPM I just blipped it to. . .if that makes sense. Under hard cornering and racing conditions I doubt I'd be as accurate, but it works for the DD.
 

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Haul'n I think that's called "floating" IIRC. That's what I've always done
 

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floating the gears is shifting and not using the clutch at all.i wouldnt reccommend doing this unless its an emergency(like losing the slave cylinder in the middle of nowhere).
 

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well i'm still using the clutch, so I don't thinks thats an issue. press the clutch in, shift gears while simultaneously blipping the throttle, then let clutch out while rpms are matched to tranny.
 

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Double clutching is what you do while shifting to a higher gear (not needed for street cars). It allows the engine rev's enough time to drop in order to match the upcoming gear. This allows for a smother shift between gears (ask a truck driver).

What we are talking about here is usually called rev matching which is done while down shifting (and I strongly recommend that we all follow this rule). Do not use you gearbox to slow your car, that's what your brakes are designed to do; brake pads are a lot cheaper than synchro's. A quick blip of the throttle while down shifting keeps the gearbox internals happy (you do not need to press the clutch twice, just blip the gas pedal while changing gears before letting up off the clutch).



Post Edited (Jan 21, 3:39pm)
 

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Discussion Starter #13
Aces full of 10s wrote :

"...Why hit the clutch a 2nd time? Nothing else is gonna move that isn't already moving....."

Darn, I wish my "lingo" on transmission innards was better, or I could do a better job of explaining all of this.

So bide with me here....

Basically there are two shafts with sets of gear clusters in a transmission ( not counting reverse) . One has the ability to sit there connected to nothing, but **only** when the chutch is depressed. Otherwise it is always connected to the engine. Even when you are in neutral, it's spinning.

The other shaft is always hooked up to the rear wheels, so it's always spinning too.

When you downshift and blip the throttle while you have the clutch depressed ( "single clutching" if you will ) you ARE bringing the engine speed up to match that of the shaft that is connected to the rear wheels. And that's good.

But since you have the clutch depressed, the other shaft ( called the input shaft?) is actaully coasting down in speed, rather than accelerating in speed, as it needs to do. And that's why you have syncromesh rings : through mecanical friction, they bring the speed of that shaft up to what it needs to be.

The cost of all this is slow but sure wear on the brass ( are they still brass ? ) syncro rings and cones, and in time, they wear out, and begin to have a harder time doing their job. That's when you start to hear that crunch, if you try and go down too fast.

Also, there is a bit of a lag time ( some friction that you can often actually feel) in the downshift, as you run through the meshing process.

By quickly stopping in neutral, letting out the clutch, and bliping the throttle, you speed that shaft up, rather than letting it slow down, and because you have now brought the speed of the shaft closer to the speed of the shaft that is connected to the rear wheels, the syncros have MUCH less of a job to do. In fact, they often have no job to do at all, and hence will not wear nearly as much.

Anyhow, that's how it was explaned to me in my youth. And I believe it to be true.

When shifting up, one would actually pause, and let the revs decrease, to9 match speeds, rather than blipping..

Bob


swampman wrote:

> yes im saying i accelerate the engine when downshifting.. aka i
> give gas to the engine to increase the revs to keep in chyme
> with the box
>
> but clutching twice i dont know why
>
 

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birdboy is right about the double clutching, what it does, and why you do it. bliping the throttle while downshifting is revmatching and prevents the sudden drag that would occur otherwise. try bliping the throttle on a vehicle without synchros and you will grind your gears. Without synchros, downshifting like this is impossible without double clutching. with all this said, i dont understand why you double clutch your Z since it does have synchros. The only purpose of the synchros is to allow downshifting without having to double clutch, it has nothing to do with upshifting or anything else. So if you do wear the synchros out by using them, then you could start double clutching as it would be necessary without repair of the synchros. just my thought.
 

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Discussion Starter #15
Mandreso75


The truth of the matter is that most of all....I do it for the joy of doing it, and out of a fond memory for my youth, when I read everything I could about sports cars, and learned the technique on my very first car (1966).

Sure it saves the ( downshifting) syncros, but.......it's a fun (and legitimate) driving skill as well.

"Sporty Driving" and.... all that.

:- )

Bob
 
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