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Double clutching while down shifting

On all the other cars I have, I double clutch when down shifting when I'm zorching around. The gears just slide in better.

I find myself doing it when I'm driving my 930. In Vic Elford,s book, he says its a Porsche no no.

But it does make down shifting these gosh darn steel syncros easier.

Am I going to toast this trany?

Should I go back to poke and pray?
Old 07-25-2010, 04:29 PM
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Double clutching won't hurt anything if you match the revs properly. The synchro's will last longer when downshifting because you're spinning up the gearset for them.
If it feels good to you and you like the increased smoothness, do it.

I learned on a BMW 2002 with totally used up synchro's so I've always double clutched on downshifts and also on upshifts when the tranmission oil is really cold and thick and if I want to use the brakes at the same time while downshifting I heel and toe.
Old 07-25-2010, 05:15 PM
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Your clutch many not last as long but you may extend the life of your transmission (before a rebuild). Clutches are cheaper than transmissions, go for it.

Mark
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Old 07-25-2010, 06:07 PM
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OK, thanks!

I heal and toe my other stuff on occasion. I don't think I can heal and toe my 930. The brake peddle is over 2" closer than the gas peddle. My ankle won't bend that way.
Old 07-26-2010, 08:52 AM
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I cut a wood block about 3/4" thick, painted it black, and put two sided tape between it and the gas peddle. Made double clutching very doable.
Old 07-26-2010, 08:56 AM
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Take the wood floor board off and the length of the linkage rod from the brake pedal bellcrank like affair is easily adjustable with kind of like a rod end at the bottom end.

Loosening the lock nut and turning the rod adjusts the brake pedal height, just don't go way too low so that the leverage of the boss it bolts to goes over center and is lost. There is no reason to do that anyway.
Takes about 3 minutes to do at the most.
Old 07-26-2010, 09:05 AM
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You want the brake higher because when you are going fast and braking hard into a turn, it will be significantly lower than just driving around the neighborhood. When it matters, the pedals will be just right for heel-toeing.
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Old 07-26-2010, 10:46 AM
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I just went for a three hour put in the thing. The brake peddle is so much higher, I can get my foot under it when its resting on the gas peddle. with my foot hard on the brake and tilting 45 degrees toward the gas peddle, I got nothing but air.

when the radar detector goes off, I have to do a major leg lift to mash the brake peddle and save my sorry butt,

I'll have to try adjusting that rod.

Thanks!
Old 07-26-2010, 04:17 PM
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There are some really good instances of what a proper downshift shift should sound like in these videos..

Also "heel and toe" the way I do it is my big toe is on the brake.. and I roll my heel around to get the throttle matched...
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Old 07-26-2010, 05:59 PM
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Forgot to add the links

http://www.rudtnersracing.com/Videos/WGI05.wmv

http://www.rudtnersracing.com/Videos/WatkinsGlen05.wmv

http://www.rudtnersracing.com/Videos/LRP.wmv

Notice that none of the shifts upset the balance of the car...
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Old 07-26-2010, 06:05 PM
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OK, I'll bite. Having not read Elford's book I want to ask: why does he say double-clutching is bad in a 911?
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Old 07-26-2010, 09:10 PM
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I have to review my copy -it has been a while since I read it- but I think he said it is "not necessary" because Porsches have synchromesh gearboxes.
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Old 07-26-2010, 11:06 PM
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If I remember correctly, it was Porsche Marketing who decided that all Racing Porsches should use Synchromesh because Synchromesh was a Porsche invention and on all their road cars.

"But, please!" yelled Vic, "I can change gear so much quicker with a crash gearbox." (Or words to that effect.)

Funny though that it was Porsche who invented double-clutch gearboxes in about 1987, and they were about the last of the 'VW Group' to bring it to road cars!
Old 07-27-2010, 03:40 AM
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I remember reading somewhere that Hurley Haywood also said double clutching is not nessesary in synchromesh gearboxes. I have driven with him several times and have only seen him blip on a downshift.

Sloane
Old 07-27-2010, 10:50 AM
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Double clutching and synchromesh do the same thing.
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Old 07-27-2010, 03:39 PM
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Quote:
Originally Posted by Flieger View Post
Double clutching and synchromesh do the same thing.
They achieve the same thing, but do it through different mechanisms.
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Old 07-27-2010, 10:39 PM
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This is turning out to be an interesting thread.

There are two functions with double clutching.

One is the match the gear speeds to alow engagement.

I guess on a proper syncromesh transmission it is not a required function except it dose reduce the wear on the sycncros and engagement teeth.

The second function is to match the flywheel speed to the drive train / rear wheel speed so traction is not upset when the clutch is let out and the rear wheels have to compensate by quickly slowing briefly.

The second function is vital to proper car control to keep the rear from loosing traction under braking into a corner such that is might upset the car.

Thus, if one has a good syncro system it could be that one might be as fast or even faster if one dose not 'double' clutch' and only 'blip' just after the shift.

Last edited by 911st; 07-28-2010 at 07:04 AM..
Old 07-28-2010, 06:55 AM
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Ah, I see. I think we're talking about 3 different scenarios. (And this is for downshifting, not upshifting. Even in a straight cut gear transmission you don't have to double clutch while upshifting.)

The first is double clutching (or double de-clutching, if you're using the Queen's English), as described by Fliegler in post #2.

The second is just single clutching and blipping the throttle once while downshifting, which I suspect Elford is referring to, and allowing the synchros to match engine and transmission speed.

The third is just letting the clutch out (no blip of the throttle) while downshifting.

I guess Quick Vic is right: in a synchro gearbox it's not absolutely necessary to double clutch, as a single clutch (and blip of throttle) will be sufficient. I've done it both ways. But double clutching is a nice skill to have for those few times in your life you'll be in a race car with a straight cut gearbox (or road car with broken/worn synchros). If you're doing it right, the accelerated wear on your clutch should be minimal, though I suppose the throwout bearing may wear quicker. But like they say about any race car driver, you can see the mark on the heel of his left shoe.
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Last edited by Noah930; 07-28-2010 at 07:27 AM..
Old 07-28-2010, 07:23 AM
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Quote:
Originally Posted by Noah930 View Post

...The second is just single clutching and blipping the throttle once while downshifting, which I suspect Elford is referring to, and allowing the synchros to match engine and transmission speed...
The 'synchro' can do the job of the 'first blip' of the throttle when double clutching (engagement teeth speed to the gear speed).

The 'second blip' is to match the clutch disk to the flywheel so traction at the rear wheel is not impacted.
Old 07-28-2010, 07:47 AM
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Quote:
Originally Posted by 911st View Post
The 'synchro' can do the job of the 'first blip' of the throttle when double clutching (engagement teeth speed to the gear speed).

The 'second blip' is to match the clutch disk to the flywheel so traction at the rear wheel is not impacted.
You don't blip the throttle twice while double clutching while shifting up or down so there is never a "second blip" unless you have a problem or screwed up the rev matching the first time and have to do it again while still in nuetral.

Helical or straight cut gearset teeth has nothing top do with it. All the gearsets are always engaged with each other all the time no matter how the teeth are cut but one of the two gears in each gearset is not locked down to the shaft it's on and is freewheeling on it's needle bearings until the synchro slider is moved over by the driver and engaged with the the gearsets selector teeth, locking it to the shaft and engaging that gear.

When double clutching, the first time you press down the clutch pedal is to shift out of gear into nuetral. Then while in neutral you let the clutch out very quickly and blip the throttle at the same time so the engine, clutch, and gearset rpms speed up to the same rpms or a tiny bit higher then they will be when the next gear you're going for is selected.
Then the cluctch is pressed in again and you shift out of nuetral into that gear and let the clutch out and go.

All that is done VERY quickly and the reason a competitive race car driver doesn't double clutch in a car with a synchromesh transmission is it takes a little more time and lap times will be slower because you're not on the gas as much.

The driver will still heel and toe so he matches the rpms while braking into a corner during downshifts but that is so there is no lurch when letting out the clutch in the gear selected at the limits of adhesion entering a corner and downshifting is as smooth as possible.
Old 07-28-2010, 10:41 AM
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