Double clutch?

[mikesride]

Ok here is where I show my true ignorance......what does it mean to "double clutch" and what would be the reason one would want to. Sorry to sound so dumb.

[Flieger]

While decelerating, push the clutch pedal in, shift to neutral, let clutch out, blip the throttle to raise the revs to the higher level needed by the shorter gear, push clutch in, shift to shorter gear, let clutch out. And you can brake while doing this if you heel-toe, both working the gas and modulating the brake with your right foot at the same time. It is only necessary with noon-synchromesh gearboxes. Unless you are very good at the technique, it is faster to just clutch in, shift to the shorter gear, and let the clutch out. The synchros do the work of matching the revs. The idea is to get input shaft, output shaft and the gears spinning at the same speed so the dog clutches engage smoother. I assert that no human can get the revs exactly right every time so there will still be some relative motion and a little chatter unless there are synchros. If it were exactly right and the dog teeth met at the points, it would actually not engage.

[Flieger]

Not to be confused with PorscheDoppelKopplungsGetriebe (PDK gearbox) .

[Craig_D]

With my syncros in need of a major refresh, I heel-toe double clutch down shift every day multiple times. Driving an SC, the throttle response for blipping is so much better now that I have replaced the stock headers/exhaust with SSI and 74 sport muffler, making my double clutching that much more satisfying.

It's a very useful technique and helps extend the life of your synchros. Plus it really impresses a date when done casually and they notice. (If she doesn't take notice, she's probably not worth dating for long.

[cornernfool]

It's what Steve McQueen did alot of in the movie Bullit. Watch and listen to that Mustang.

[Bill Verburg]

Originally Posted by mikesride

Ok here is where I show my true ignorance......what does it mean to "double clutch" and what would be the reason one would want to. Sorry to sound so dumb.

Flieger described double clutch which is used on non syncro gear boxes, do they even still make them these days?

w/ syncro boxes heel and toe allows you to blip the throttle on down shifts, you just have part of the foot on the brake and part on the gas pedal and blip the throttle so that engine revs are matched to wheel revs through the gear box.

what heel & toe does is reduce wear on clutch, gears and tires and the car doesn't get as unsettled as it will when the drive-train does the synchronization. In track use it would be very common to have the rear wheels chirp and the rear get all squirrelly w/o h&t

[Canada Kev]

Originally Posted by Bill Verburg

Flieger described double clutch which is used on non syncro gear boxes, do they even still make them these days?

Only in the large transport trucks. The big Meritor and Eaton transmissions are all non-syncro.

[Jerome74911S]

My first (used) car had no synchromesh. It was a '56 VW, but a European model with no synchro, square cut gears, and mechanical brakes. Really a stripper - the front seats were fastened down with wing-nuts and this was the factory design. Anyway, double clutch all the time, both up and down shifting. You could up shift very slowly without double clutching, but to double clutch was much easier. With constant practice I got quite fast at all shifting this way, but in any case if I wanted a lower gear without terrible crunching the only option was to double clutch.

Right now the transmission is out of my 911 because of busted parts, but they are not broken because of me crunching gears. That old VW provided good practice.

[nineball]

[Quicksilver]

Originally Posted by Bill Verburg

Flieger described double clutch which is used on non syncro gear boxes, do they even still make them these days?

. . .

I have a non syncro box in my Nissan mini truck...

Well uhhh ok, it used to have syncros but the supposedly "rebuilt" box was weak on 3rd from day one and as it swallowed itself I pretty quickly lost all the rest of the syncros.

I was going to get around to replacing it and realized that I have become WAY better at double clutching to the point where I don't have to think about it. I've pretty much decided to leave it the way it is...

[Flieger]

There are racing gearboxes (Hewland?) -"dog boxes"- without synchros. Racers are good enough to match revs and the synchros would just add weight and complexity. They are used more on Formula cars and frequently do not need clutching on upshifts, similar to the Cupcar sequential transaxles.

[911pcars]

Synchromesh is a wear item. The more it's used, the faster it wears, and when it wears to a certain threshold, it no longer works as designed. To reduce wear and increase service life, learn how to double clutch.

If you don't (the PO probably didn't as well), you will have experienced how it is to downshift and found difficulty in doing so. Remove checkbook and begin writing sooner than later.

A blip between shifts with the clutch pedal depressed does nothing to reduce synchro wear.

BTW, non-synchro racing boxes use straight-cut gears which make it easier to mesh w/o gear clashing by double clutching. Passenger car gear sets are designed differently. First of all they're helical-shaped, and the actual gear engagement is accomplished with the synchro mechanism, not by gear-to-gear meshing.

Sherwood

[Bill Verburg]

Originally Posted by 911pcars

....
A blip between shifts with the clutch pedal depressed does nothing to reduce synchro wear.

...
Sherwood

correct, but clutch, gear, cv joint, half shaft, & tire stress is considerably reduced and as I said the main thing on a track car is the cars balance is not upset on downshifts, this is a must do technique for track use

[ToddM]

Originally Posted by 911pcars

A blip between shifts with the clutch pedal depressed does nothing to reduce synchro wear.


Sherwood

Thats right, But you WILL wear our your clutch 2x as fast

[911pcars]

Originally Posted by ToddM

Thats right, But you WILL wear our your clutch 2x as fast

Bill,
That's right. I should have repeated the advantages of the single blip along with the caveat.

Sherwood

[mikesride]

ok so the recomendation for my 82 sc (just a summer toy and not a show piece) would be to practice a true double clutch for each down shift? Higher gears as well as lower gears?

[Bill Verburg]

Originally Posted by ToddM

Thats right, But you WILL wear our your clutch 2x as fast

wrong, the greater the difference in transmission input shaft speed and crank speed the more the clutch will wear when it's dumped, you wouldn't rev the engine to 5k and dump the clutch at a light, you wouldn't power shift to chirp the tires, On upshifts the crank is spinning faser than the trans input. On down shifts the same thing happens w/o a blip, only there the trans input shaft is driven by the wheels and spinning much faster than the crank which idles down when you are off the gas, Upshift or down, the large difference in speeds is what causes the wear & stress, on track it's not unusual to chirp the tires on downshifts, the big thing though is the cars balance is upset by such downshifts, a blip will at the very least reduce the stress levels and reduce the resultant disruption to the cars poise. The blip doesn't even have to be perfect, just close is good enough

I don't know how they do it out west but in NE, if you can't heel&toe(blip) you don't get promoted out of the novice classes, unless you have a tip or PDK

[Laneco]

I know you guys are all awesome drivers, but to truly show your mastery of the process, you must be able to double-clutch while wearing four inch heels like us Pelican chicks do.

Then and only then, have you mastered the technique - LOL!

angela

[911pcars]

Originally Posted by mikesride

ok so the recomendation for my 82 sc (just a summer toy and not a show piece) would be to practice a true double clutch for each down shift? Higher gears as well as lower gears?

As Bill states, the speed differences are greater in the lower transmission gears, and because of this, synchro wear is greater in these lower gears. However, I would suggest double-clutching whenever downshifting in order to practice the proper technique and encourage more muscle-memory rather than focused concentration.

Sherwood

[911pcars]

Originally Posted by Laneco

I know you guys are all awesome drivers, but to truly show your mastery of the process, you must be able to double-clutch while wearing four inch heels like us Pelican chicks do.

Then and only then, have you mastered the technique - LOL!

angela

I agree with Angela. , but we need video validation for archival purposes.

Sherwood