Double Clutching?

[Blokey]

What is "double clutching"? I've heard the term but don't know what it means. What's the advantage to doing it? I understand this is done in high performance driving (mods move this thread if it's not appropriate here).

In the movie "Bullitt" where Steve McQueen does a million gear shifts during the chase, I hear the engine syncopating during gear changes instead of one engine speed to another; is this double clutching? It sounds cool but is his car accelerating any faster doing it?

Thanks for any replies.

 

[san123chez]

Double clutching was mandatory back before synchros. The technique is (for example, shifting from 4th to 3rd gear) 1) push in clutch 2) go from 3rd to neutral 3) let ouy clutch and rev the engine to match the rpm's it'll be doing when in the lower gear 4)again push in clutch 5)put in third gear and let out clutch. This all has to happen pretty quick, and takes practice.

The idea is to match the speeds of the new, lower gear, but today synchonizers do this for you, so double clutching isn't a big deal. I don't race, so maybe someone else can explain if/why racers do it.

 

[XS650]

Most of the engine sounds in the movie were from a Ford GT40 with a non-syncro tranmission.

With non-syncro tranmissions you need to double clutch when downshifting. The doen't explain the double clutching on the upshifts. Maybe they wee able to shift it a bit faster that way instead of waiting for the engine to slow down during the upshift.

If you double clutch properly, the syncros in your trnsmission will never wear out.

 

[JohnBrowning]

Blokey, You might also hear someone tell you to give the throttle a "blip" when down shifting. If you ever drive a car with a dog box you will get good at this! You seldom see synchroless transmissions anymore outside of raceing or older heavy equipment!

Often you can blip the throttle and slip it right into gear with out useing the clutch at all!This is especialy true with helical cut gearing!

 

[Rebel Rouser]

In the non-synchronized transmissions I'm familiar with, primarily heavy trucks, you either double clutched, or didn't use the clutch at all when shifting. On upshifts it wasn't necessary to increase engine rpm to match the next higher gear, you actually had to allow engine rpm to slow a bit to engage the gear.

Downshifting required moving the lever to the neutral position, then accelerating the engine a bit to match the rpm of the next lowest gear. With practice, it was much easier and faster, not using the clutch during this procedure, compared to using the double clutch method.

 

[MAJA]

When downshifting, as soon as the clutch plate is disengaged from the flywheel, it is slowing down. To downshift, the input shaft/clutch plate on a non synchro trans must be sped up to match the speed of the next downshifted gear, hence the need to double clutch to reingage the clutch and speed it up. The same can be done with modern synchro transmissions, without using the clutch. One can down shift by moving into neutral, rev the engine while trying to shift into the next lower gear, and engage that gear, all without using the clutch. One can also "powershift" on the upshifts the same way, without using the clutch. Just shift into neutral, and when the revs slow (match), the next gear can be engaged. Correct me if I am wrong, but this technique is the easiest on the synchros, in the trans.

Older race car drivers "double clutching" had to "heel and toe" during a race when braking and shifting, by using the toe of the right foot on the brake, and the heel of the right foot to blip the gas to match revs to complete the shift. The left foot was working the clutch. Some interesting clutch arangements in some race cars had the gas in the middle, with the clutch on the left and brake on the right to ease the heel and toe technique. But the idea didnt catch on, as during an emergency manuever, the pedals got confused too easily with disastrous effects as we can imagine. I am not sure how much double clutching is done in todays race cars today.

 

[rpn453]

I double-clutch on my '04 Mazda3 when downshifting two gears, usually 5th to 3rd on highway for a pass or 4th to 2nd in the city if I need to get ahead quickly. I even use it to drop one gear to second or first sometimes when the revs are high. It goes into gear a lot more easily, it's better for the synchros, and I find it easier to do the power transition smoothly.
It's a very quick sequence and doesn't add much time to the downshift.

quote:

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Originally posted by MAJA:
One can also "powershift" on the upshifts the same way, without using the clutch. Just shift into neutral, and when the revs slow (match), the next gear can be engaged. Correct me if I am wrong, but this technique is the easiest on the synchros, in the trans.

---

I'm not sure, but I would think that this would actually be very hard on the synchros. If you don't have it perfectly rev-matched, the synchros would be trying to accelerate/decelerate the mass of the engine and flywheel instead of just the input shaft of the tranny.

[ March 29, 2005, 02:43 PM: Message edited by: rpn453 ]

 

[Crashbox]

I double-clutch my downshifts almost every time in my '95 Civic. I want to do my part to help the synchros last as long as the rest of the car (probably) will, at least 300,000 miles

It takes some getting used to but it becomes second-nature over time.

 

[Crashbox]

Oh yes, just one more item: a non-synchronized transmission is also known as a "crashbox"

 

[groomernut]

Double clutching simply means pushing the clutch pedal twice on every shift. To properly DC you need your hand on the shifter and your foot on the clutch pedal should almost move in unison. You basically clutch out of gear and clutch into gear with a short pause in neutral to get your engine speed to the proper rpm. While in neutral you need to realease the clutch to bring the tranny input shaft and clutch disks back down to engine rpm to properly match speed for the next gear. The bigger trucks need DCing because of the weight of all the components are bigger and the syncros would wear out quickly if you shifted it like your car.Many old timers swear by shifting without the clutch. The problem is if you don't time it perfectly and there is still torque on the input shaft , the shifting forks and the sliding collers/synchros all wear prematurely . when you clutch, you release the torque of the crankshaft and only the inertia of the clutch disks and the input shaft are applying a little torque. The old trucks used to get overhauled at 500,000 and they would throw in a new clutch and go thru the tranny at the same time. Nowadays they get a million miles on a motor and expect the tranny to go that long too. Proper clutching and shifting will make it last that long. Shifting improperly will shorten its life considerably re:500,000. I know that some over the roaders will still get a million, but this is all in general.I have seen gravel hauling dump trucks wear em out in less than 200,000. sorry if I got long winded and confused ya.

 

[doubleclutch]

I could not have explained it any better! Well said!

 

[Blokey]

Great replies everyone! Thanks, I've learned a lot!

 

[labman]

I never double clutch. I am on the second clutch in my truck and the first set of syncros. Perhaps if I double clutched, I could be on my third clutch.

 

[theguru]

I have used DC in the past and is great with practise. If you get it wrong though you will more than likely shorten the gearboxes life.

In frequent gearchange situations with a heavy clutch it is a bit tiring on the left leg. Probably why many don't use the clutch.

Even when not DC I still blip the throttle to match engine and gear speeds. I think it is better on the clutch.

 

[tdi-rick]

Race Drivers still need to 'heel and toe' to match the engine revs to wheel driven speed on down changes to eliminate the driven wheels snatching (compression braking) and thus reducing the effectiveness of the brakes. (and screwing up you corner entry)

Dog engagement gearboxes such as Hewland's and Staff's dont even need to be double clutched, as the engagement woindow on the dogs is relatively wide, and the ratios nice and close.
They can also be shifted unbelievably quickly, and sympathetically without the clutch.
The only time I used to use the clutch on a dog box was starting and stopping.

FWIW, I always heel and toe and double clutch on the downchange in my road vehicles.
Force of habit (helps the synchros no end) feels better (you don't feel the synchro clutch hubs engage, if the revs are right. The proverbial 'knife through butter' change)

And it sounds cool to hear someone rowing down the gears rev matching each down change, even if it is a trundly Land Rover.

 

[Gurkha]

I have a 69 Nissan Patrol based 4WD made by the ministry of defence, instead of the original inline 6 gas engine, they put a high torque 4 cylinder HINO diesel, the result is the puny 3 speed needs not only careful double clutching but I have to make sure to clutch before the revs drop or the drive-train gets a massive shock, I learnt it the hard way when I lost two trannies thinking the sync 2nd and 3rd never needed double clutching.