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double clutching doesn't seem to make a difference

C

cprince

Senior member
I heard that double clutching is supposed to reduce wear on your manual transmission's synchros and make it easier to slide into gear on downshifts, so I practice doing it on my '87 bmw 325e with 225k miles on it. I figure with that many miles, the synchros could use any kind of break you give them. The problem is that I cannot tell the difference in the amount of force I need to downshift between double clutching and not double clutching. To double clutch, I depress the clutch, put in neutral, blip the throttle to about 2k rpm, depress the clutch again, and downshift. Am I doing something wrong?
 
Just because I'm not sure 100% from the way you said it:

Clutch in, gear out to neutral.
Clutch out, rev a bit above desired RPM.
Clutch in, slip into gear with motor at desired RPM.
Clutch out.

Honestly, you shouldn't notice a HUGE difference if your synchros are in relatively good shape, you may feel a bit of notchiness go away but it never had an effect for me so I don't bother. What your doing is getting all of the drivetrain rotating at the same RPM at the same time as you let out the clutch and slide into gear. Or at least, that's the hope.
 
I've never bothered with it. It just takes more time and gives zero benefit IMO. Clutch in, blip throttle while sliding gear lever into lower gear, clutch out. I'm really good at this on my bike and it makes such nice noises when I do this. Hell, sometimes I don't even use the brakes until I'm right up at the light unless I'm really hauling.
 
crazySOB297 said:
What your doing is getting all of the drivetrain rotating at the same RPM at the same time as you let out the clutch and slide into gear. Or at least, that's the hope.
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Yeah...isn't that the purpose of double clutching--to get the input and output shafts rotating nearly the same speed so your synchros don't have to work as hard? I also translate that to mean easier/quicker shifting into the lower gear because you don't have to wait for the synchro to spin the input shaft to the same speed as the output shaft. But, like you, I cannot tell the difference.
 
Properly operating, your synchros work very quickly. I wouldn't expect to notice a difference.

It would be difficult to tell if you're even double clutching properly if your synchros aren't worn.
 
The difference is usually more obvious if you double clutch to shift from 2nd to 1st when going at a reasonable rate of speed (30 mph?).
 
Eli said:
That's a good point.

I'd' start with 15MPH tho. 😱
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Yeah, when I think of shifting from 2nd into first without rev matching/double clutching.......

NNNNNNNNNNNNNNUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
 
Eli said:
That's a good point.

I'd' start with 15MPH tho. 😱
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Haha yeahhh, my car would just about have a heart attack if I wanted to downshift into first at 30mph. 😛

My Subaru is fussy downshifting into first while still rolling (maybe 3mph)...double clutching will make it nice and easy.
 
The only times double clutching ever showed an improvement in shifting for me was on an old Chevy pickup with a 3-on-the-tree shifter and a wearing transmission, and on my '69 Olds 442 W30 with a Muncie rock crusher transmission....that one really did need double clutching when downshifting to do it smoothly.

Other than that, I've never noticed it helping much with other manual cars I've owned, including some very old VW Bugs, a 280ZX, Fiat X1/9, a Fiero, among others.
 
jlee said:
Haha yeahhh, my car would just about have a heart attack if I wanted to downshift into first at 30mph. 😛

My Subaru is fussy downshifting into first while still rolling (maybe 3mph)...double clutching will make it nice and easy.
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ms3 is like that too until I get below 10ish mph. But with fking gas prices, I just stay in gear as long as I can while slowing down/coasting.
 
punjabiplaya said:
ms3 is like that too until I get below 10ish mph. But with fking gas prices, I just stay in gear as long as I can while slowing down/coasting.
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I can get away with staying in 2nd almost all the time, but my garage has a very short (several feet) sloped driveway which is off a hill to begin with, so parking in there is normally a first gear endeavor. 😛
 
the syncho on 2nd gear in my G35 doesn't work that well when it's cold, so I double-clutch only after a cold start
 
soydios said:
the syncho on 2nd gear in my G35 doesn't work that well when it's cold, so I double-clutch only after a cold start
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The 1-2 synchro seems to be a problem on a lot of cars across different manufacturers. I ended up have my entire transmission replaced under warranty when they couldn't fix grinding on the 1-2 shift.
 
Always double-clutch or else you can blow the welds on your intake and fry piston rings, this is proven fact.
 
You're trading synchro wear for clutch wear because of the extra clutch cycle. I would just rev-match rather than double-clutch.
 
Extra clutch wear while double clutching is a non issue. I would be surprised if it caused any wear at all. There isn't a lot of resistance to spinning up free wheeling internal transmission parts.
 
You almost had me? You never had your car. Granny shiftin’, not double clutchin’ like you should. You’re lucky that hundred shot of NOS didn’t blow the welds on your intake. Almost had me? Now me and the mad scientist gotta rip apart the block and replace the piston rings you fried. Ask any racer, any real racer. It don’t matter if you win by an inch or a mile, winning’s winning.
 
Eli said:
Extra clutch wear while double clutching is a non issue. I would be surprised if it caused any wear at all. There isn't a lot of resistance to spinning up free wheeling internal transmission parts.
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I agree that there isn't a lot of resistance to spinning up the transmission parts. However, this is what the synchros are doing and it does cause some wear. The clutch is still matching the different speeds of two rotating assemblies, there will be some, albeit small, amount of wear.
 
Eli said:
Extra clutch wear while double clutching is a non issue. I would be surprised if it caused any wear at all. There isn't a lot of resistance to spinning up free wheeling internal transmission parts.
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I agree, and the clutch is a lot easier to replace than the synchros. I found a great article here:

http://www.car-forums.com/talk/showthread.php?t=3993

Apparently, I was doing it wrong. I have to get the rpm up what it should be if the car were going at that speed in that lower gear. So I have to get the rpm up to more than 2k.
 
JCH13 said:
I agree that there isn't a lot of resistance to spinning up the transmission parts. However, this is what the synchros are doing and it does cause some wear. The clutch is still matching the different speeds of two rotating assemblies, there will be some, albeit small, amount of wear.
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I agree in principle, but.. lol. I mean, the clutch's friction surface is huge, compared to a synchro. I really think the wear would be negligable, not detectable over the life of the clutch. You might get 249,500 miles out of a clutch while double clutching instead of 250,000. 😛
 
i have over 220k miles on my dodge truck, the clutch has been in there for the last 140k or so. i never double clutch or rev match at all. the sync blocks are fine. i dont try to get into 1st over 20mph tho, id have to force it. once it drops below 20mph it pops in without any pressure tho. the transmission has never been replaced or rebuilt and it seems to have much life left in it.
 
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