can someone explain "double clutching"

[///BHRpowered]

iv heard it a good number of times, and never really figured out what it is, and what it does for you, and or is it really bad for the car, i.e ricers revving in neutral and putting it into drive at redline, the only cars iv ever driven in stick are a 01 mercedes slk-320 and a test drive in the new mazda 6 s, so im still learning

[caviallo]

I'd explain it myself, but this write up does it better:

The following is a beginner's approach to learning how to double - clutch. This technique allows downshifts from higher speeds without any synchroniser wear. This is VERY important if you ever want to drive really quickly and keep your transmission intact for extended periods!



The Easy Way to Double-Clutch
By Jeff Krause.


Double-clutching is the proper way to downshift at speed without placing excessive wear on the transmission's synchronizers. This allows you to select a much lower gear without the tell-tale lurch you normally get when the clutch is let out after downshifting.

When downshifting my BMW M5 from 4th gear to 2nd at 50 mph, I need to raise the engine speed from 2200 rpm to 5000. To prevent excessive synchroniser wear, the clutch is depressed and the shifter is moved to neutral. The clutch is then released, and the gas is depressed to bring the engine speed up to where it needs to be for the lower gear. The clutch is then depressed again and the shift lever moved into the lower gear. When the clutch is released the second time, the engine is already turning the proper speed . While this sounds complicated, it's easier done than said, and only takes about a second.

To better understand how the process works, a little background on transmission fundamentals will help.

A typical transmisson has two shafts, one connected to the engine through the clutch, and one connected to the rear wheels. There are usually four to six sets of gears on these shafts and they are selected with the gearshift lever inside the car. When changing gears, the clutch is depressed to disconnect the engine from the transmission so there isn't any stress on the moving parts. Since the output shaft is permanently connected to the rear wheels, the only way to match the speed of the two shafts is to use the throttle to adjust engine speed.

Once the engine is turning the right rpm, both shafts will be turning the same speed, and the gear lever will fall into gear WITHOUT using the clutch! (Although most of the time you are shifting too fast to be that accurate)

This article does a good job too, but goes into more detail:
http://www.vtec.net/articles/view-ar...rticle_id=3249

[E36.328i]

wow thats about the best explanation as your gonna get..

[Patrick33]

what is the difference between this and rev matching? why do you have to put it in neutral first (the stick i mean)? couldnt you clutch, shift, railse the revs to where you want and then release?

[m3ltw98]

Double clutching originated a while back when cars didnt have synchros. It just gave the motor time to drop its speed to match with the gearbox. Today since we have sychros, all we have to do is "blip" the throttle. Once mastered, blipping becomes second nature and is very affective while on the track or spirted driving. Later

[///BHRpowered]

hmm, so why not just rev the engine while pushing the clutch in and then as your reving switch gears, wouldn't that reduce the time, sorry if thats a dumb question but im alittle confused

[eurorides]

i can explain double clutching in 1 sentence...its not nessasiry

[dave is cool]

I don't really know much about it either. Instead of letting go of the clutch, then depressing the accelerator to match the revs, then push the clutch in and switch gears, can't you just LEAVE the clutch depressed, switch gear, rev-match by keeping the clutch depressed and accelerator, then let go of the clutch? That seems to make more sense then this process. Anyone want to explain, because I am kinda confused too

[Cannon]

Originally posted by ///BHRpowered
hmm, so why not just rev the engine while pushing the clutch in and then as your reving switch gears, wouldn't that reduce the time, sorry if thats a dumb question but im alittle confused

Even though this topic has been covered a crapload of times, I'll give it a shot.

There are two different techniques being discussed here. First is heel/toe downshifting, the second is double-clutching.

Heel/toe downshifting is exactly what you described in the quote above - you depress the clutch, downshift, and before you release the clutch pedal, you blip the throttle to match the engine speed to the speed of the tranny input shaft. This prevents the jerkiness that would normally be present during a downshift. Normally, this is done during braking, and the heel/toe technique is required in order to blip the throttle while braking at the same time.

Now, double-clutching. The reason that you would double-clutch a downshift is, as stated above, to reduce (or even eliminate) wear/stress on the synchro's.

You have to understand what a synchro does before this will make sense. As above, the tranny can be thought of as an input shaft (the part that is connected to the engine via the clutch) and the output shaft (the part that turns the driveshaft). In between are the various sets of gears (and resulting gear ratios) that you can select via the stickshift.

Every time you upshift or downshift a car, the speed of the input shaft has to change. When you shift from first to second gear, the output shaft speed stays relatively constant, as it is connected to the drive wheels. The input shaft speed has to be slowed down before the gears will engage properly for second gear. The same thing happens when you then shift to third - the input speed again has to be slowed to allow the gears to properly engage. This is how you get a large speed range (0 to top speed) out of a car, even though the motor only has a finite range of available RPM. Just like a 10-speeed bike - as you shift up a gear, the pedals slow down even though the back tire is turning the same RPM.

Now lets look at a downshift. When we go from 3rd to 2nd gear, before the gears will engage, the input shaft must be sped up. Same thing when we downshift from 5th to 4th or 4th to 1st - in all cases the input shaft of the tranny has to be sped up before the gears will engage.

How is the input shaft sped up and slowed down during shifts? The synchros. They are the part of the tranny that 'drag' the input shaft to a higher or lower speed (depending on if it's a downshift or an upshift) during a shift so that the gears will engage.

When you do a double-clutch downshift, you are using the motor - not the synchros - to speed up the input shaft of the tranny - thus saving wear on the synchros. Here's how it typically works:

1. Press the clutch pedal in and select neutral.

2. Release the clutch pedal and blip the throttle (now, since the clutch is released - thus connecting the motor to the input shaft - when the motor revs up, the input shaft spins up too. This is work that the synchros would normally do, but now they don't have to).

3. Press the clutch pedal again and select the desired gear. If the double-clutch was done right, the gearshift will fall into the desired gear so easily it will feel like it's not connected to anything.

4. Release the clutch pedal, and the car is now in the gear you wanted.

I hope this helps

Euroride: It's not absolutely necessary, but it definitely has it's advantages if done correctly.

Chris

[Kos-motate139]

Originally posted by Sean M3
I don't really know much about it either. Instead of letting go of the clutch, then depressing the accelerator to match the revs, then push the clutch in and switch gears, can't you just LEAVE the clutch depressed, switch gear, rev-match by keeping the clutch depressed and accelerator, then let go of the clutch? That seems to make more sense then this process. Anyone want to explain, because I am kinda confused too

You do not need to depress the clutch twice, you are right. You can leave it in, blip (slightly more) and go. In Skip Barber's book, 'Going Faster' they talk about the fact that Danny Sullivan couldn't bring himself to double-clutch, it just threw him all off. Other racers in the book noted it threw them off to not do it. It's just personal preference.

Personally, I fall into the same camp, I don't know why you'd need to press the clutch twice per gear change. You figure in his distinguished career Danny has driven pretty much every kind of race car, synchro, straight-cut, etc, and he never pressed the clutch twice per shift.

[///BHRpowered]

alright i think i get it, bt im gonna have to actually go out and try it a few times at diffrent speeds and see which i personally perfer, i kindi like the jerkyness, gives you more of a sense of power, and that the car can instantly be going 10 times faster

[LeVik]

It's really not necessary, but good to know, especially if you run on track courses often. It's rather simple once you get it down, the explanation above is pretty accurate.

[abakos]

Originally posted by Kos-motate139
You do not need to depress the clutch twice, you are right. You can leave it in, blip (slightly more) and go. In Skip Barber's book, 'Going Faster' they talk about the fact that Danny Sullivan couldn't bring himself to double-clutch, it just threw him all off. Other racers in the book noted it threw them off to not do it. It's just personal preference.

Personally, I fall into the same camp, I don't know why you'd need to press the clutch twice per gear change. You figure in his distinguished career Danny has driven pretty much every kind of race car, synchro, straight-cut, etc, and he never pressed the clutch twice per shift.

Alright, Cannon was explaining how the "Double Clutch" principle works. To apply this technique, you have to press the clutch twice. Nobody ever said you have to use it.

If you don't know what the inside of a manual transmission looks like, you're going to have trouble understanding how double-clutching is beneficial to the life of the synchros.

Kudos to Cannon for an excellent description! The biggest difference is that the input shaft in the tranny has to match the output shaft and the motor speed. When you try a heavy downshift, you have to speed it up with the synchros or by disconnecting it from the output shaft (in neutral) and using the motor (with the clutch engaged). Those are your only two options.

[Whatup1049]

Here's an example of how I would downshift from 4th to 2nd. Push clutch in, shift to second, blip throttle, let off the clutch.

You guys are saying that during the blipping process I should be in neutral with the clutch out? Why does this have to be done? If I'm in gear AND I'm blipping with the clutch down the synchros are not being used at all, so what is the real point?

[///M LIFE©]

YOu could also try a search i am sure there is lots of info on that

[Cannon]

Originally posted by Whatup1049
Here's an example of how I would downshift from 4th to 2nd. Push clutch in, shift to second, blip throttle, let off the clutch.

You guys are saying that during the blipping process I should be in neutral with the clutch out? Why does this have to be done? If I'm in gear AND I'm blipping with the clutch down the synchros are not being used at all, so what is the real point?

The only way you can go from 4th to 2nd without double clutching is to work the synchros pretty hard. You most definitely are using the synchros by doing this - just not when you think you are. In your example, as soon as the gearshift goes through neutral and begins to put pressure on the second gear gate, the synchros are spinning up the input shaft. Once the tranny slides in gear, they have finished doing their job. This happens very quickly, but w/out a double clutch a two gear downshift (ie. skipping a gear) can be especially tough on the synchros. The input shaft has to be spun up that much more. I guarantee you that a 4-2 downshift will be much easier once you've mastered double-clutching.

Look, whether or not to double-clutch an E36 tranny is personal preference. When done properly, it will most definitely reduce wear on the synchro's - and those are usually the first part of a manual transmission to fail. Should you elect not to do this, your tranny is not going to blow up tomorrow (unless you have really bad luck, anyway). Is it worth learning a new technique when it's questionable as to whether or not you'll own the car long enough to benefit from it? Only you can decide that.

I guess what chaps my ass is that you have people adamantly saying that

i can explain double clutching in 1 sentence...its not nessasiry

(nice spelling, btw..) when in reality there is a real, quantitative advantage to doing so.

I'm done here, talk amongst yourselves...

[Kos-motate139]

I don't know everything about a transmission, but wouldn't it be possible to match the input and output shafts just as well without letting the clutch in? It's a bigger blip, perhaps waiting sightly later to do it, but it's the same thing right?

Or is it (from what you're saying) that even if you do match them perfectly without double-using the clutch that the synchros are spinning around like crazy doing nothing, just wearing themselves out for no reason?

I went back and re-read that portion of that book, and it did indeed mention that it could be harder on synchros to do it that way, but Danny Sullivan wasn't known to be hard on trannys throughout his racing career. Interesting...

[E36.328i]

i leraned to double clutch when i bought my car..and now my timings all off if i try to eliminate that..but i dont think i shift very much if at all slower than people i know so....anyways the reason why you:

1. push clutch in
2. put stick in neutral
3. let clutch out
4. rev engine to match speed
5. push clutch in
6. select lower gear

is to let the TRANSMISSION be at the same speed as the a) engine b) the drive shaft

when you rev match by

1) push clutch in
2) rev engine to match speed while selecting gear
3) let up on clutch

the TRANSMISSION is not speeding up you are just making the engine speed match driveshaft speed and its the job of the synchros to bring the transmission shafts up to speed with the rest of the drive train....

[stjobs]

E36.328i, that is a very good explanation. The reason for that is because there is a fundamental difference between having a gear selected but with the clutch pressed and being in neutral. When you are in neutral, the clutch is connected and transferring power, as I understand it; the disengagement and the reason why no power gets to the wheels is because no gear is selected on the transmission, so when you blip in neutral, you are bringing the transmission's input shafts up to speed.

If you rev-match, you are not saving wear on the synchros, since they still have to bring the transmission up to speed? Is that correct?

Anyways, I finally understand why double-clutching is beneficial, and intend to try it out tomorrow on my daily drive to school

stjobs

[Jim O.]

Originally posted by Kos-motate139
I don't know everything about a transmission, but wouldn't it be possible to match the input and output shafts just as well without letting the clutch in? It's a bigger blip, perhaps waiting sightly later to do it, but it's the same thing right?

No, it's not possible. Think about it a bit. The clutch separates the engine from the tranny. Blipping the throttle with the clutch disengaged (pedal down) changes the speed of the engine, but has absolutely no effect on the speed of anything in the tranny. How can it, when the engine is disconnected from the tranny?

Also, you guys don't seem to realize what the synchros actually do, and where they do their work. As Cannon said, the synchros match the speed between the input shaft and the output shaft, so the gears on both shafts can engage each other. This does not happen when you release the clutch - it happens when you move the shift lever out of neutral into gear. That little "catch" that you feel as the shift lever moves across neutral into gear is the feel of the synchros at work. As they work, they wear.

If you want to avoid that wear, you need to double-clutch. So, you dip the clutch and move the shifter to neutral. Release the clutch so the input shaft is coupled to the engine, and blip the throttle. That speeds up the input shaft. Dip the clutch again, and move the shifter into gear. Do it right, and the shifter will slide right into gear, without that little "catch". Then release the clutch to couple the engine to the tranny, and you've completed a double-clutch downshift.

Jim

[Sonic A]

Dumb question but this got me thinking...when you need a new clutch, is it the synchros that are typically worn out or the clutch plate, or what?

Are the syncros part of the transmission or the clutch...the transmission right?

So the double clutch is preventing a "transmision replacment" and not a clutch replacement?

i need to get one of those handly manuals....

[///M LIFE©]

Could someone complile a video or animation or some sort?

[Cannon]

Yes, the synchros are part of the transmission.

The clutch is mounted between the engine and transmission, and it simply disconnects the motor from the input shaft of the tranny. This is how you can sit at a complete stop with the car in gear and the engine running.

As you release the clutch pedal, the motor and tranny are gradually 'connected' together. At first the clutch will slip (this is when you just start to roll - the motor is beginning to 'drag' the tranny with it and thus the car begins to move) and once the pedal is fully released, there will be no more slip between the motor and tranny.

A worn clutch has nothing to do with the synchros. You'll know when the clutch is worn out because you will be able to slip the clutch just by stepping on the throttle - there's no longer enough friction present in the clutch material to prevent it. As the clutch wears more and more, it will be easier and easier to do this. The car will still shift just fine, though.

Worn syncros will be evident because the gears with the worn synchros (every gear has its own synchro on an M3) will not shift smoothly - you might even get a small grind as you shift into that gear. The grind occurs because the syncros cannot effectively match the input/output speeds and the gears are not turning the same speed when they mesh. Crruuunncchh.

In many cars the second gear synchros are the first to wear out. They are usually used the most and must also contend with one of the largest speed differentials in the tranny. In other words, they have to do the most work more of the time.

Chris

[Cannon]

Originally posted by ///M LIFE
Could someone complile a video or animation or some sort?

Now you guys are getting greedy....

A video of what? A double clutch downshift or one on how synchros and clutches work.

Edit - Here's how a clutch works. Very good animation as well.

And here is how a transmission works. Beware that you may have a headache after this. You really need to have one in pieces in front of you to see how it works, and even then it's not always obvious.

[Whatup1049]

AHA! I finally get what you guys have been saying the whole time. For some reason I was thinking the clutch connected the transmission to the drive shaft, AHA it all makes sense now. It like all clicked as I was reading E36.328i post. Very nice post man! I am going to learn to double clutch now!