Double Clutch Question

[lespaul]

Ok, I have read about how it is wise to double clutch when downshifting -- and have tried it. It is smooth (sounds cool too). However, I find I can get nearly/exactly? the same effect by simply single clutching and blipping the accelerator before letting the clutch out. Single clutching seems faster and easier than double clutching.

Maybe I am missing something. Can an expert double clutcher explain the benefits of fully double clutching as opposed to blipping to raise revs while single clutching?

Also related -- I have never tried heel/toe shifting, but the way my pedals are set, I don't see how anyone could possibly do it in my 911. The brake pedal sits way higher than the accelerator - so unless you have a size 20 shoe, I don't see how you can keep your heel on the accelerator and still brake.

You racers out there -- please educate me.

Thanks

Brad

[Noah930]

Double-clutching harkens back to when cars had non-synchromesh gearboxes. Now, with synchromesh gears, it's possible to single-clutch like you mention and not crunch gears. Quite a few racecars (like a Formula Ford I once drove) still have non-synchromesh gearboxes. Personally, I double-clutch out of habit. I don't think it's any slower than single-clutching, as it's not my feet, but rather the travel of the shift lever that seems to be the limiting factor for me.

[Noah930]

I also find it difficult to heel-toe in a 911. The brake pedal is too "high." At least it is, for me, on the street. Mind you, I've been heel-toeing in my other street cars for about 12 years, so it's not just "me." But on the track, or for heavy duty braking situations, the Porsche brake pedal depresses far down enough that it's the proper level for blipping the throttle with the side of my foot. Remember, that's the way a proper racecar is set up. The brake pedal should never depress further down than the accelerator (and account for brake fade and pedal mushiness in track situations), as that would lead towards an unsafe situation in which you could stomp on the brakes and accidentally partially depress the accelerator at the same time (if the brake pedal were to sink "lower" than the throttle).

To partially help/compromise, for the street, I've drilled and bolted on a second (plastic) gas pedal on top of the stocker.

[lespaul]

Noah,

Great responsive answers. Thanks!

Brad

[Jack Olsen]

Very few racers double-declutch synchromesh transmissions.

Those that do, however, are often devoted to the practice. I'll leave them in peace if they'll do the same for me.

Heel-and-toe downshifting is good for both your transaxle and your lap times. Some people claim they do it on in street driving, although I can't imagine coming into corners that hot on most public roads.

I've been heel-and-toeing so long that when a student asked me how to do it, I had to look at video of myself to see what I was doing. I actually use the ball of my foot (brake) and my heel (gas), which is a little bit unusual. The Thunderhill video in my link shows some of it, although the YouTube image is muddy and small. I don't heel and toe when I'm driving in normal driving. It'd be hard to get the brake that far down without possibly collecting whoever's unlucky enough to be behind me.

[Noah930]

Originally posted by Jack Olsen

Heel-and-toe downshifting is good for both your transaxle and your lap times. Some people claim they do it on in street driving, although I can't imagine coming into corners that hot on most public roads.

Agreed. Again, that's mostly a result of proper pedal placement (like in Porsches). The brake pedal never sinks low down enough (in a street situation) to allow for proper heel-toeing. But in the daily drivers I've had (VW, Nissan), they're not really track-appropriate; the brake pedal will easily depress down to, or lower than, the adjacent throttle. So it's simple to roll the right foot over to blip the throttle on downshifts, even at street speeds.

[911pcars]

As for the mechanics, the object of double-clutching is to synchronize the speed of the gear to be selected via the input shaft and the rotational speed of the engine.

With the clutch depressed (disengaged), there is no connection between the engine and gearbox, thus the input shaft and clutch disc (splined onto the input shaft) are freewheeling - no amount of throttle blipping will affect the gearbox. With the clutch disengaged, the speed of the clutch disc could be zero rpm while the engine is at 4000 or whatever.

In order to attain gear synchronization speed, the gearbox must be in neutral with the clutch disengaged (pedal up). At this point, use the throttle to increase the input shaft speed to what it would be in the selected gear. With the gear and input shaft at the correct synch-speed, declutch and shift and re-engage to complete the double-clutch. Faster done than described.

Synchros are consumable items (they wear out). They wear the most when the gear matching speeds are vastly different. You can extend their life by helping to attain the proper sync-speed using the proper double-clutch technique.

http://youtube.com/watch?v=0j-3xIZK-Bk
http://youtube.com/watch?v=0j-3xIZK-Bk
http://youtube.com/watch?v=jAvOOLYPmMM&mode=related&search=

Sherwood

[rusnak]

If you modify your throttle pedal rod (lenghten) or get an adjustable gas pedal, then you'll be able to heel and toe.

There is a great explanation of heel and toe shifting with photos in Bob Bondurant's book on high performance driving. There is also an explanation with less clarity, but some great photos anyway, in Vic Elford's performance driving handbook. There is much emphasis in Bondurant's book on SMOOTH shifting, and not being merely fast. It's a great read.

[911pcars]

This one is good just to show the fast footwork of Walter Rohl driving a WRC Audi and the spectators next to the road.

http://youtube.com/watch?v=_HjXXUyQhPE

[barney911rs]

Sherwood nailed it on the head. That is the reason for doing it, and it will do nothing but help even in a synchro gearbox. I was never able to get myself comfortable doing a double. Doing a proper and well executed single clutch will work just fine on our synchro gear boxes.

I've seen guys attach a block of wood to their gas pedal to make it easier for them. One of these days in the not too distant future I want to get one of these Rennline Gas Pedals

Between this and my adjustable Tilton brake pedal, I won't have any excuses.

[rusnak]

barney, the Rennline pedal is well made, very adjustable, and has a nice return spring, which makes it possible to blip the throttle and get a precise rpm response with your heel.

But you have to apply red Locktite to the mounting screws that thread into the floor. Those darn screws have a tendency to come out at the most inopportune moments.

[ZOA NOM]

The only comfortable double-clutch shift for me is from 4th to 3rd, and that's only on the street. I don't find that I downshift to 2nd very much on the street. The 4-3 downshift is really very smooth with a double clutch, and the gearshift seems to just fall into gear with the right rev. At the track, I heel-toe, and there just doesn't seem to be time to double-clutch.

[Doug&Julie]

My SC has worn sychros, so I have to double clutch on the upshift...but only from 1st to 2nd and 2nd to 3rd. My mechanic tells me double clutching will help the life of my tranny until I can afford to get it rebuilt. (If the car doesn't sell first ) Once the synchros are replaced, I won't need to.

For the record, I can heel-n-toe downshift in my Boxster during street driving just fine. In fact, it's one of my favorite things about the Boxster...it's easy and butta smooth. Whether I'm "hard" on the brakes or "soft" on the brakes results in slightly different techniques (it's much smoother when I'm "hard" on the brakes). I can also heel-n-toe downshift in the SC, but only with "hard" braking.

Like Jack, I use the ball of my foot / big toe on the brake, then rotate the heel over to the throttle to blip it.

[911pcars]

Double clutching on the upshift isn't usually necessary since engine speed normally drops as you shift upwards, but whatever works in your case.

Zoanas writes:
"I don't find that I downshift to 2nd very much on the street."

That's unusual. The majority of downshifts on a street car are from 3rd to 2nd, and coincidentally why most of the synchro wear is at 2nd gear.

Not downshifting into 2nd means taking corners in either 1st or 3rd. For me, that would mean unnecessary revs while in 1st or lugging the engine if in 3rd.

Sherwood

[ZOA NOM]

[i]Zoanas writes:
"I don't find that I downshift to 2nd very much on the street."

That's unusual. The majority of downshifts on a street car are from 3rd to 2nd, and coincidentally why most of the synchro wear is at 2nd gear.

Not downshifting into 2nd means taking corners in either 1st or 3rd. For me, that would mean unnecessary revs while in 1st or lugging the engine if in 3rd.

Sherwood [/B]

Actually, I take the turns fast enough in 3rd that it isn't lugging. It's the nature of my commute. I'm lucky enough to have windy backroad drive that allows a spirited drive to and from work. Also, I work shiftwork, so I'm commuting at all hours of the day and night.

BTW, my daily driver just turned 200k with no major work, and no tranny issues.

[911pcars]

Zoanas,
I'd like to drive where you drive. If you want to exercise your second gear synchro, drive where I drive, So. Cal.

Sherwood

[ZOA NOM]

Originally posted by 911pcars
Zoanas,
I'd like to drive where you drive. If you want to exercise your second gear synchro, drive where I drive, So. Cal.

Sherwood

Yeah, I consider myself very lucky, especially in the Bay Area.

[livi]

On an instinctive level, I believe working up a good feel for engine and road speed versus correct gear and a smooth operating of the gear lever and clutch is as important for longevity as double clutching. I do both at times, but I 'feel' I can change gears with the same smoothness without double clutching. I understand the 'internal' concept of DC, I just think a smooth, well negotiated single clutch procedure is probably not significantly more harming the internals of the box.

[crosby50]

Not one comment on that Walter Rohl video?

[Doug&Julie]

Originally posted by 911pcars
Double clutching on the upshift isn't usually necessary since engine speed normally drops as you shift upwards, but whatever works in your case.

For me, it's really more about timing the shift; i.e. slowing down my shifting. Not that I'm a speed shifter, but if I double clutch, I never crunch. If I just...pause...sometimes I still shift "too fast" and get a little crunch.