Double-clutching

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TofuStoreDrift

TofuStoreDrift

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Is it necessary? I was watching a video on YouTube yesterday when I came across a guy showing people how to double clutch when downshifting. In the video he talked about how it is easier on the transmission if you use this technique. Which brings up my question. In a normal car, does a clutch moving twice per shift actually do anything, beside burn up the clutch? I mean as we all know, the clutch is the link between the trans and the flywheel, and pushing the pedal twice won't move the the clutch to any magical land where the trans is safe. In a good working car, it doesn't matter how many times the clutch pedal is press, the clutch disc will alway make the same movement. I'm I missing something here? Sorry if my wording is a little hard to follow, not all of us are gifted like JohnBM01:sly:.
 
Hello, TSD. If you do a quick Advanced Search for "double clutch", limited to thread titles on the Cars In General board, you should find that this topic has at least one sizeable thread on it already.
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But, in a nutshell, double-clutching is not typically required in a modern synchromesh transmission. However, rev-matched downshifting (heel and toe) is of benefit in saving the driveline from excessive shock and wear, and is an important skill for driving smoothly and quickly.
 
Thanks, I didn't use Cars in General board, and my search led to countless g25 questions. So off I go to search again. Thanks
 
Basically what Duke said. I can't tell a bit of difference if I double clutch in my car, feels just like using heel and toe. The only place for it in a modern vehicle is with a semi truck, but from what I've heard of those you don't even use the clutch after you get moving, you just pull it out of gear, match the revs for the next, and drop it into the next one. I've managed to do that a couple of times in my Civic, but you've got to have the RPM exactly where it should be or you'll get a nasty grind.
 
It saves wear on the synchronizers.

The synchronizers are friction devices which match the speed of the gear (locked to its speed by the drivetrain since modern trannies are constant-mesh) to the speed of the shift ring (being carried over by the shift fork - when engaged with the gear, actually attaches it to the output shaft.) Since you're downshifting, the input shaft has to be sped up before the gear can engage, and it's the synchronizer's job to do this. If you look at it, that's a lot of steel for a 2-inch diameter cone-shaped friction surface to impart angular momentum onto.

In a normal single-clutch downshift it's up to the synchronizer to bring the shift ring up to the speed of the gear so it can engage without crunching. In a gearbox with weak synchronizers you feel this as a delay in the shifter motion; it feels like you have to force the shifter into gear. That delay is the wait for the mass of the input shaft to match the required speed of the gear being selected.

When you double-clutch, you clutch, select neutral, release the clutch, blip the throttle, clutch, select next gear down, release the clutch and drive. Sounds like a lot, but it's actually not even a second's worth of time.

The blip in neutral imparts rotation on the input shaft of the transmission. It's basically a rev-up in neutral. Now it's easier for the synchronizers to match speed on the upcoming engagement with the lower gear. The input shaft has already been sped up, the synchronizer has less work to do, and will last longer.

As for clutch wear, depressing the clutch pedal produces almost zero clutch wear. It's so close to zero as to be completely irrelevant. Clutch wear comes from slipping the clutch, like drag starts or trying to get unstuck from sand. Maybe towing too large a load on an uphill start. What might show additional wear is the throwout bearing. Usually that's the part that has failed when someone says their clutch has gone out, anyway. Throwout bearing wear comes from resting your foot on the clutch, or coasting on ("riding") the clutch.
 
Sorry, I feel like a noob. I know its against normal policy, but if any knows the sizable thread Duke is referring to, can you please point me in the right direction. I searched and searched, but nothing I find quite deals with this exact subject. I know that it isn't all that important, but I'd like to see other peoples views. Thanks
 
Apparently the new Nissan 370Z has a special gearbox/ clutch that does the double-clutching for you. Read about it in this month's Top Gear magazine.
 
It blips the throttle to rev-match, which is not the same as double clutching. I rev-match, but never double clutch. No point on a stock car with synchronizers.
 
wfooshee
It saves wear on the synchronizers.

The synchronizers are friction devices which match the speed of the gear (locked to its speed by the drivetrain since modern trannies are constant-mesh) to the speed of the shift ring (being carried over by the shift fork - when engaged with the gear, actually attaches it to the output shaft.) Since you're downshifting, the input shaft has to be sped up before the gear can engage, and it's the synchronizer's job to do this. If you look at it, that's a lot of steel for a 2-inch diameter cone-shaped friction surface to impart angular momentum onto.

In a normal single-clutch downshift it's up to the synchronizer to bring the shift ring up to the speed of the gear so it can engage without crunching. In a gearbox with weak synchronizers you feel this as a delay in the shifter motion; it feels like you have to force the shifter into gear. That delay is the wait for the mass of the input shaft to match the required speed of the gear being selected.

When you double-clutch, you clutch, select neutral, release the clutch, blip the throttle, clutch, select next gear down, release the clutch and drive. Sounds like a lot, but it's actually not even a second's worth of time.

The blip in neutral imparts rotation on the input shaft of the transmission. It's basically a rev-up in neutral. Now it's easier for the synchronizers to match speed on the upcoming engagement with the lower gear. The input shaft has already been sped up, the synchronizer has less work to do, and will last longer.

As for clutch wear, depressing the clutch pedal produces almost zero clutch wear. It's so close to zero as to be completely irrelevant. Clutch wear comes from slipping the clutch, like drag starts or trying to get unstuck from sand. Maybe towing too large a load on an uphill start. What might show additional wear is the throwout bearing. Usually that's the part that has failed when someone says their clutch has gone out, anyway. Throwout bearing wear comes from resting your foot on the clutch, or coasting on ("riding") the clutch.
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As far as I can tell, the only benefit you describe comes from rev-matching alone - not double-clutching.
 
Rev matching saves clutch wear because you're not using the clutch to bring the engine up to the lower gear's speed. It does nothing for synchronizer wear. The synchro still has to bring the input shaft up to the lower gear's speed if there's no blip in neutral. Doing that blip in neutral uses the clutch surface to speed up the input shaft, which for the clutch is effectively zero load.

Conceptually, the blip in neutral is a rev-match for the input shaft, allowing smoother engagement of the synchronizer as the shift is completed. Another clutch, another rev for engine matching, snick into gear and release the clutch.

Is your tranny gonna come apart if you don't double-clutch? No.

But it will last longer. As someone who keeps cars for 15 or 20 years and a couple hundred thousand miles, that's important to me. If the synchros are the next guy's problem, then who cares?
 
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TofuStoreDrift
Sorry, I feel like a noob.
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No, don't feel like a noob, and no reason at all to apologize. I just did the very search I recommended to you, and for some reason, there is a thread that I'm remembering that is not turning up. So it's not your fault at all.

The one that was linked is a decent one, but there is an even longer one that I can recall. I'll see if I can find it. Sorry about leading you astray.
 
I was having trouble too finding it Duke, that's why I linked him that one because it seemed the longest without being locked for not searching. :cool:

And why should you apologise, you're an admin! :p
 
wfooshee
Rev matching saves clutch wear because you're not using the clutch to bring the engine up to the lower gear's speed. It does nothing for synchronizer wear. The synchro still has to bring the input shaft up to the lower gear's speed if there's no blip in neutral. Doing that blip in neutral uses the clutch surface to speed up the input shaft, which for the clutch is effectively zero load.
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I don't see why this is specific to downshifts - in which case you should be doing this regardless of which direction you're shifting to save your synchros. If that's true, and you're not double-clutching on EVERY shift, then you're acting quite arbitrarily.
 
It's specific to downshifts because the next gear down needs the input shaft to speed up, because engine speed is higher in the lower gear. When upshifting, the input shaft will be slower than the previous gear, and the coast-down during the shift comes out pretty close to right. The synchronizer has much less work to do on an upshift than a downshift.

And like I said before, on a car whose expected life before resale is 4 or 5 years and maybe 60,000 miles, it's a "who cares?" issue. In my case, I just sold my '95 Probe GT, purchased new, still running the original clutch and throwout bearing at 178,000 miles and 14 years. It was running poorly, with several of the vacuum solenoids that control its multi-mode intake manifold having broken over the years, but the buyer has a gen-I Taurus SHO motor he plans to drop into it. The paint was kinda shot, too, although everything else on the car still worked as new; windows, locks, seat, cold air. Them Mazda boys make a good car.
 
wfooshee
It's specific to downshifts because the next gear down needs the input shaft to speed up, because engine speed is higher in the lower gear. When upshifting, the input shaft will be slower than the previous gear, and the coast-down during the shift comes out pretty close to right. The synchronizer has much less work to do on an upshift than a downshift.
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There's enough friction on the shaft to slow it down several thousands RPMs in a fraction of a second?

If you answer yes, then I have to wonder how much effort it really takes for the synchros to adjust the speed. I have no doubt that double-clutching would be necessary for a car with dead syncrhos. But synchros can die for reasons other than a lack of double-clutching.

If you answer no, then I'm still wondering why you're not double-clutching on ALL shifts. If the RPM difference is 3k in the negative direction vs. 3k in the positive direction, that's the same amount of work.
 
wfooshee
A few hundred, not several thousand, rev difference.
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I'll interpret this as meaning that the input shaft is not spinning in a 1:1 ratio with engine RPMs. I don't pretend to know whether that's the case (I would think it would be velocity-dependent). Regardless, I'm surprised that the shaft, which you've described as difficult for the synchros to accelerate, coasts down enough during the short period of time in which a shift takes place as to "come out pretty close to right".

If there is substantial friction slowing the input shaft even while the clutch is depressed, then I can see why downshifting would be harder than upshifting for the synchros. That's just very counter intuitive for me.
 
Danoff
There's enough friction on the shaft to slow it down several thousands RPMs in a fraction of a second?

If you answer yes, then I have to wonder how much effort it really takes for the synchros to adjust the speed. I have no doubt that double-clutching would be necessary for a car with dead syncrhos. But synchros can die for reasons other than a lack of double-clutching.

If you answer no, then I'm still wondering why you're not double-clutching on ALL shifts. If the RPM difference is 3k in the negative direction vs. 3k in the positive direction, that's the same amount of work.
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EDIT: I had to remind myself of what synchros actually do.

The power comes from the trans input shaft, transfers to a layshaft which is on a plane next to the input shaft, goes up to the gears which are in constant mesh with complimentary layshaft gears and are free-floating on the trans output shaft, and then through the output shaft/driveshaft, which is spinning in concert with the differential and wheels.

Synchronizers are connected to and spinning with the output shaft. What they do is connect the output shaft to the gears, layshaft, and input shaft. The load the synchros have to carry is the weight of the gears, layshaft, and input shaft.

When you upshift the synchros must slow the next gear down to the speed of the output shaft, unless you're shifting into overdrive in which case it must speed up the next gear. When you downshift the synchros must speed up the lower-numbered gear to match the output shaft (also the reason you rev-match), unless you're downshifting out of overdrive in which case the synchros must slow down the lower-numbered gear to match the output shaft.

So the synchros are always doing something. Because the gears are free-floating, the entire input shaft/layshaft/gear assembly will eventually come to a stop (barring friction) when the clutch is disengaged and the transmission is put in neutral. So I suppose the opposite of a double-clutch downshift would be a hold-the-clutch-and-throw-it-in-neutral-and-wait upshift. Good luck timing that though because the trans output shaft is also slowing down with road friction. That's why you hear old dump trucks grinding upshifts all the time, because it's genuinely difficult to upshift a non-synchro'd transmission.

You can't save all the wear, but you can save about half of it by double-clutching downshifts.

transmission-simple.gif
 
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Keef
You can't save all the wear, but you can save about half of it by double-clutching downshifts.
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Why not double-clutch all of the shifts? I can't think of anything that makes it easier to double-clutch a downshift. In fact, it seems to me that it would be easier to double-clutch an upshift because you're not likely to be braking while upshifting.

I was quite confident that the synchros were working on upshifts as well as down (I can tell when I upshift too fast).
 
I don't think I've ever tried double-clutching an upshift. So let's see, it's...clutch-down, neutral, clutch-out, don't rev (to slow the trans input down with the engine), clutch-down, shift, clutch out.

I'm going to go try this. I'll report my results in a bit.

EDIT: So, that was terribly complicated.

At the speeds I was driving, double-clutching downshifts only worked from between 3rd, 2nd, and 1st. I tried doing it from 5th to 4th, and then I remembered that's actually the opposite of what you should do downshifting from 5th to 4th.

It's really hard to get the hang of double-clutching upshifts, and I never really did. I randomly got a couple of them right, meaning the stick went in gear really easily, but during the second clutching the revs dropped quicker than my foot and hand could move, so when I let the clutch out the revs were too low and it jolted. If the revs wouldn't drop so far it might work.

I'll give it a "plausible".
 
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Just wondering,

When you say blip the throttle in netural you mean go to red RPM's? Or can you just let the rev rise slightly before pressing the clutch in again and selecting the lower gear.

Doesnt this waste petrol? (at the cost of saving your clutch).

How would one go about double clutching when changing up?

Sorry if this sounds dumb but I dont know much about double clutching.

Thanks
 
The word "blip" in our sense means to quickly tap the gas pedal a bit to raise the rpms a thousand or so.

When downshifting, yeah I suppose it does use gas that you wouldn't use otherwise. If you simply dragged the clutch to downshift you would slow down, and the engine wouldn't use any fuel since it doesn't (isn't supposed to) use any fuel at zero throttle above idle.

I tried to find out how to double-clutch when upshifting in my last post. It seems that this should work: "clutch-down, neutral, clutch-out, don't rev (to slow the trans input down with the engine), clutch-down, shift, clutch out", but I couldn't come to any good conclusions. It's either really hard or really awkward to do it on an upshift.
 
I counted the number of upshifts and downshifts on my way to work this morning. I upshifted 40 times, but only downshifted 3 times. Slowing to a stop and then starting in 1st gear was not considered a downshift.

That's a solid 13:1 ratio. Which means that if upshifting causes 13 times less wear on your transmission than downshifting, double clutching is equally valid on both.

I tried double-clutching going from 1st-2nd. It seemed to help a bit. I usually get a single ping of disagreement when going from 1st-2nd in a cold car. That went away with double-clutching.

I'd lay odds that most of my 40 upshifts were from 1st to 2nd. So my guess would be that double-clutching only that shift would do more good than double-clutching every single other shift.
 
For clutch wear, you don't need to double clutch, you just need to heel-and-toe. Don't skip gears. Clutch, blip and shift, release clutch. It's called heel-and-toe because you're usually braking during the downshift, so you operate both the brake and the blip with the same foot. It's usually actually the ball of your foot on the brake, and the outside of your foot, sometimes towards the heel (in a pigeon-toed position) on the gas. Depends on your pedal layout. The best cars for this have the brake and gas closely side by side, with the brake pedal just a tad higher than the gas.

The blip during the shift brings the engine speed up to match the transmission speed, so you don't drag the engine up to speed by slipping the clutch. Just downshifting and releasing the clutch with no throttle blip is very hard on the clutch surface and will wear it out quickly. Almost as bad as drag race starts.

Adding the double-clutch does nothing for clutch wear, it reduces synchronizer wear. It's a different process with a different purpose, although it still requires simultaneous work on the brake and gas (heel-and-toe.) You don't need to double-clutch as a daily driving method unless you want to keep your transmission for 15 or 20 years, and you drive pretty hard.

For Danoff: Normal daily city-based driving does not need a lot of downshifts; you're correct in your assessment of the pattern. Most of the time when you're slowing down you're approaching a stop sign or traffic light. Just get neutral and coast or brake to your stop. Spirited driving in the twisties requires proper selection of ratios, and braking for a curve won't involve a complete stop. You have to be able to downshift correctly to get the best performance, to be easiest on the equipment, and just as important, to keep your passengers the most comfortable. Lurching grabby shifts make people carsick. If you run Solo II or some other club's autocrossing events, your times will suffer if you can't downshift properly, and your equipment will suffer even more.
 
wfooshee
For Danoff: Normal daily city-based driving does not need a lot of downshifts; you're correct in your assessment of the pattern. Most of the time when you're slowing down you're approaching a stop sign or traffic light. Just get neutral and coast or brake to your stop. Spirited driving in the twisties requires proper selection of ratios, and braking for a curve won't involve a complete stop. You have to be able to downshift correctly to get the best performance, to be easiest on the equipment, and just as important, to keep your passengers the most comfortable. Lurching grabby shifts make people carsick. If you run Solo II or some other club's autocrossing events, your times will suffer if you can't downshift properly, and your equipment will suffer even more.
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This thread (and others like it) have already changed my driving habits a bit. I used to put the clutch in, coast all the way from 80mph to zero, and put the car in first without ever hitting neutral. I don't do that anymore. I find myself using neutral a lot more already. I have you guys to thank for that.

I've also bit the bullet and started heel-toeing. Mostly so that I don't spend time after a turn shifting gears (the way I used to) and to avoid the lurch you talk about.

My spirited driving sessions are few and far between, but in general I think double-clutching is not for me. It doesn't help with car balance, or lurching, or speed, it's just to save the synchros - and I'm pretty confident I won't be wearing mine out. The one I'm most concerned about almost never gets downshifted into anyway.
 
For what it's worth, Danoff, I never double-clutch (in either direction), though I do heel-and-toe my downshifts, and I only really downshift if I am not coming to a full stop. I get looooong life out of my manual transmissions and clutches. In fact the last time I actually put a clutch in a car was only because I had the engine out anyway and it was silly not to.
 

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