Double clutching to prevent synchros wear

[kilou]

Hi,

I'm considering to learn and use double clutching to put as less strain as I can on my "old" (260000km) gearbox whose synchronizers start to be weak (especially 3rd gear). Double clutching with rev matching is supposed to relieve strain placed on synchros....if performed properly. Now I have a few question concerning this technique:

- Should I double clutch only when gearing down or when gearing up as well??

- I understand I need to rev match i.e. blip the throttle when gearing down so that both shafts in the gearbox get synchronized. However do I need to do something when gearing up???? No rev matching there?

Thanks,


PS: really I want to double clutch to get as much miles out of my gearbox and synchros, not to play Vin Diesel in Fast and Furious!

 

[rpn453]

I double clutch anytime I'm dropping 2 gears or more, and even for a one gear drop if the revs are up. I enjoy the feel of how much more smoothly it slips into gear and how much easier it is to complete the shift without any jerkiness on clutch engagement.

There's no need for or benefit to double clutching on the upshifts, unless your synchros can't even handle that anymore. At that point, you'd probably be best to do all upshifting without the clutch anyway!

 

[toocrazy2yoo]

Of course, by double-clutching, you're trading decreased synchro wear for increased wear on your throwout bearing, cluth slave, and clutch master cylinder..

 

[chrome]

I double clutch to shift up only from 1st to 2nd, and only when the box is cold in the mornings. Once the gearbox warms up a tad, upshifts are fine. Downshifts are fun, 4th to 3rd to 2nd, double clutching makes the gears slide in very smoothly.

There is no need to blip the gas when you shift up, you are trying to get the input shaft speed to match the next gear, which is lower, so blipping is counter productive. Only blip if you are shifting down.

 

[JHZR2]

In my MT cars, a saab and BMW, and my finacee's car, an acura, Ive found the shift uality to be sufficiently smooth and fast that when going up in gears, I just push in the clutch, move the lever and the revs fall to about the perfect spot naturally, so no double clutching is necessary. Maybe on bagger engines/vehicles it is different, but on my cars it just works.

When I downshift, I always double clutch no matter what. It makes it really smooth to engage. I usually need to anyway, as if Im actually moving and need to downbshift, the engine sped will increase anyway, and so it makes it more pleasant, and otherwise, if I have been coasting in or out of gear, the engine speed is usually significantly lower than before, necessitating the doubleclutch.

It works great, and makes driving a lot better. do it! Just remember, double clutching requires "rev matching" with the clutch pedal out... My fiancee, as excellent an M driver as she is, often forgets to let the clutch back out (in neutral) before setting the RPMs.

JMH

 

[55Test]

I've tried double clutching but it takes me about 3-4 seconds to do the whole manouver, and I just can't do it at all if I'm in a corner slowing down. When I downshift I do try to bring the revs up to where they'd be in the next lower gear though, with the clutch pedal in though.

 

[JHZR2]

yeah, I dont know how top-notch drivers do it... if they even _really_ and _accurately_ do it.

My impression that its biggest application is in trucking, where transmissions dont necessarily even have synchros, and it is a necessity. I really do it for the ease on the driveline and hopefully resultant enhanced longevity. Those things ae important in trucking too, of course... making that perhaps the best analog.

JMH

 

[kilou]

quote:

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Originally posted by toocrazy2yoo:
Of course, by double-clutching, you're trading decreased synchro wear for increased wear on your throwout bearing, cluth slave, and clutch master cylinder..

---

Right but all those things are "easily" changed and you can do that when you change the clutch anyway. As for the synchros, when they're gone it's probably cheaper to get another box than try to change them. I think douvle clutching is a good thing for the box so I'd take that "risk" but I first have to do it properly.

For now I find it hard to do the whole ting quickly. It sounds I have to put alot of gas to have a ""smooth" slip into a lower gear (going from 4th to 3rd sets the rpm about 1000 higher). But the biggest problem is that it's simply impossible to double clutch and rev match while braking. I would probably need to learn "heel and toe" but I seems not possible with my pedal configuration or I should build a pad on the gas pedal to level it with the brake pedal. I was thinking of building a sort of gas pedal button on the gearknob that would allow to manually blip the gas with the hand while braking. Would this be a good thing? (yea I know this starts to become a little crazy!)

 

[mechtech2]

Bending your pedals, or otherwise modifying them is a great idea. We have adjustable seats in all cars, Why not pedals?
The upshifts will naturally synch themselves if your timing is right, as the RPMs drop to what is needed for the next gear.
The double clutch technique is mainly for downshifting.

 

[Palut]

Double clutching is lots of fun. When I started it was very hard and awkward, but with practice it's become easy. Every time I drop 2 or more gears I automatically do it, and it feels awkward not to.

My car has a really, really, really rough 4 to 5 upshift, and I have to double clutch that to avoid getting a HORRIBLE grinding feeling. I read somewhere that a poorly manufactured shift fork or something like that causes the problem. I started double clutching it and it's now my smoothest shift. Even in other people's cars I double clutch the 4-5 upshift, it's become so automatic for me.

Heel-toe downshifting was pretty tricky to get right. But it's become much easier for me. I'm still learning to do it well, but its a lot of fun. Don't give up, before long it will be second nature!

 

[surfstar]

I sometimes double clutch when downshifting more than 1 gear, but I usually blip the throttle with the clutch pushed down anyways and not in neutral. Most of the time for a 1 gear downshift I just shift and blip the throttle before letting out the clutch and there's no problems.
200k miles - original clutch and tranny.

Good synthetic gearbox fluid can help synchro grinding sometimes.

 

[Carbonics]

quote:

---

I've tried double clutching but it takes me about 3-4 seconds to do the whole manouver, and I just can't do it at all if I'm in a corner slowing down.

---

drive a truck, you'll be doing it in less than 1 second. Knowing how to drive a stick helped me breeze through truck driving school, most of the other students didn't even know how to drive a manual - to me, it really doesn't matter how you shift gears as long as you know how - so many people are just too lazy to learn.

 

[Master ACiD]

a friend of mine has a small fleet of mack trucks, fairly modern ones too. mid 90's i believe.
i was riding in one of the trucks one day and noticed the driver ONLY uses the clutch during take off in 1st. for the rest of the gears he just shifts at the right time, no clutch used at all.
curious, i looked in the owners manual and sure enough it said because these trucks have no synchros not to use the clutch.

so whats up with double clutching?

 

[kilou]

quote:

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Originally posted by Drew99GT:
Race car drivers usually do not do a true double clutch; they just blip the gas via their heal as they put the clutch pedal in and make the downshift (which is like a half double clutch because it will speed up the input shaft/ constant mesh gears to about the output shaft speed/dogbone gear - if you do it all simultaneously and really slip the clutch a bit). I just blip the gas a bit to the rpm that the motor will be at in the lower gear when I downshift. No need to truely double clutch; ie shift to neutral, let clutch out, blip the throttle, push clutch back in and downshift. Give me a break! I'm not driving a 1960 semi with an unsynchronized 10 speed! I think I'll agree with toocrazy here...

---

Blipping the gas while the clutch pedal is depressed deserves nothing as far as the tranny is concerned. Your just revving up the engine but not the input shaft because with clutch pedal depressed the input shaft is no more coupled to the engine. The only "benefit" is that you put the engine rpm where they should be in the lower gear but you do not help the synchronization at all. I want to double clutch to lengthen the life of the synchronizers and avoid to stress them too much now that the box is old. There is only one way to double clutch and it involves releasing the clutch while in neutral. Revving up with clutch in only helps the clutch to engage smoother since the engine will be at the correct rpm already but this deserves nothing for synchros.

Remember: synchronized gearboxes do not require double clutching but double clutching reduces wear on synchronized gearboxes and this is what I want to achieve. I don't think this is crazy

 

[XS650]

quote:

---

Originally posted by Master ACiD:
a friend of mine has a small fleet of mack trucks, fairly modern ones too. mid 90's i believe.
i was riding in one of the trucks one day and noticed the driver ONLY uses the clutch during take off in 1st. for the rest of the gears he just shifts at the right time, no clutch used at all.
curious, i looked in the owners manual and sure enough it said because these trucks have no synchros not to use the clutch.

so whats up with double clutching?

---

How do they downshift when a hill slows them down or they need engine braking descending a hill?

ps I realize that's theoretical question in Florida

 

[Drew99GT]

quote:

---

Originally posted by kilou:

quote:

---

Originally posted by Drew99GT:
Race car drivers usually do not do a true double clutch; they just blip the gas via their heal as they put the clutch pedal in and make the downshift (which is like a half double clutch because it will speed up the input shaft/ constant mesh gears to about the output shaft speed/dogbone gear - if you do it all simultaneously and really slip the clutch a bit). I just blip the gas a bit to the rpm that the motor will be at in the lower gear when I downshift. No need to truely double clutch; ie shift to neutral, let clutch out, blip the throttle, push clutch back in and downshift. Give me a break! I'm not driving a 1960 semi with an unsynchronized 10 speed! I think I'll agree with toocrazy here...

---

Blipping the gas while the clutch pedal is depressed deserves nothing as far as the tranny is concerned. Your just revving up the engine but not the input shaft because with clutch pedal depressed the input shaft is no more coupled to the engine. The only "benefit" is that you put the engine rpm where they should be in the lower gear but you do not help the synchronization at all. I want to double clutch to lengthen the life of the synchronizers and avoid to stress them too much now that the box is old. There is only one way to double clutch and it involves releasing the clutch while in neutral. Revving up with clutch in only helps the clutch to engage smoother since the engine will be at the correct rpm already but this deserves nothing for synchros.

Remember: synchronized gearboxes do not require double clutching but double clutching reduces wear on synchronized gearboxes and this is what I want to achieve. I don't think this is crazy

---

Apparently you don't know how to read. IF you slip the clutch enough while doing it, you will put or keep the input shaft spinning.

 

[Carbonics]

master acid - we call it "floating gears", with practice it can be done very easily, I did it in my car a couple times but didn't really like grinding once in a while so I stick to clutch.

for downshifting they just use the brake to get the rpm's down, pull the shifter to neutral and blip the rpm's then drop it in.

 

[Tempest]

quote:

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Originally posted by XS650:
Chances are a person who double clutches isn't going to cluelessly keep his clutch pedal depressed for 2 minutes at a stop light, so his throwout bearing should last longer than average.

---

Apparently I am one of those clueless people.

Would it be better to put a manual tranny in neutral during extended lights/idle? Please enlighten.

 

[TallPaul]

I have heard of people who sit with the clutch pedal depressed at traffic lights. That would be a real pain. I never do that. Besides, if you are distracted and your foot lets up, you will be launched into traffic far more easily than if you let up on the brakes with an automatic.

 

[tdi-rick]

quote:

---

Originally posted by XS650:

quote:

---

Originally posted by Master ACiD:
a friend of mine has a small fleet of mack trucks, fairly modern ones too. mid 90's i believe.
i was riding in one of the trucks one day and noticed the driver ONLY uses the clutch during take off in 1st. for the rest of the gears he just shifts at the right time, no clutch used at all.
curious, i looked in the owners manual and sure enough it said because these trucks have no synchros not to use the clutch.

so whats up with double clutching?

---

How do they downshift when a hill slows them down or they need engine braking descending a hill?

ps I realize that's theoretical question in Florida

---

you just blip the throttle the required amount for the next lower gear. The throttle blip has to start to occur at the moment the lever is starting to move to take the load off the dogs otherwise you won't get the thing (easily) out of gear.

Never used to use the clutch in a dog box when racing, up or down so I could left foot brake like in a kart.
FWIW, we didn't double clutch when using the clutch on a down change, just moved the lever as fast as you can shift on the upchange, and only a tad slower on the down while still heel and toeing to match the revs with road speed. The gear lever only moved through two inches at the most. (Hewland transaxle)

As to double de-clutching wearing out the throw out bearing, clutch fork, slave and master m/c's, etc. I agree in theory, yet my old Land Rover is about to clock 200,000km on the original bits, and I double de-clutch/heel and toe every down change. I've always been hooked on the noise of someone rowing down the gears rev-matching. One of drivings great sounds !