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      09-13-2014, 10:28 AM   #1
hunginator
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Double clutching downshifts

For the past 6 years of driving manual, I've into the habit of double clutching my downshifts... Is this still required with cars these days? I heard that if you don't double clutch downshifts, it can hurt the synchros overtime.

What I mean by double clutch downshift for those of you who don't understand, the process is like this to downshift to any gear.

1. Clutch-in
2. Neutral
3. Clutch-out
4. Throttle blip
5. Clutch-in
6. Shift into desired gear

Last edited by hunginator; 09-13-2014 at 06:58 PM.
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      09-13-2014, 10:33 AM   #2
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I just heel-toe rev match. I haven't done double clutching in my e92. I don't think you need to?
I just to blip the throttle
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      09-13-2014, 10:38 AM   #3
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Any manual transmission with synchros (that aren't shot) should not require double-clutch down shifting as far as I know. That said, just because you don't need to double-clutch doesn't mean it'll hurt if you do.

Blipping the throttle as you downshift though is exactly what I did when I had cars with manual transmissions and it's just generally good practice to try and match revs as you re-engage the clutch for a smoother shift and less wear on the clutch.
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      09-13-2014, 10:40 AM   #4
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"Granny shifting, not double clutching like you should."

but really, you don't need to double clutch with a synchronized transmission. I see in your steps you left out rev matching, which would be part of double clutching. Are you not rev matching when you do that?

I just rev match with no double clutching or anything
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      09-13-2014, 10:53 AM   #5
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never double-clutched in my E90 and i've driven it 35k miles and never did it in my 95 miata for 20k miles. no problems.
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      09-13-2014, 11:37 AM   #6
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Quote:
Originally Posted by mashimarho View Post
I just heel-toe rev match. I haven't done double clutching in my e92. I don't think you need to?
I just to blip the throttle
+1
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      09-13-2014, 11:39 AM   #7
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Quote:
Originally Posted by MiniXP View Post
"Granny shifting, not double clutching like you should."

but really, you don't need to double clutch with a synchronized transmission. I see in your steps you left out rev matching, which would be part of double clutching. Are you not rev matching when you do that?

I just rev match with no double clutching or anything
+1. Double clutching isn't needed. Revmatching is if you want your clutch to last longer than 20k miles
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      09-13-2014, 11:59 AM   #8
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Quote:
Originally Posted by 10splaya22 View Post
+1. Double clutching isn't needed. Revmatching is if you want your clutch to last longer than 20k miles
20K. LOL
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      09-13-2014, 01:15 PM   #9
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Quote:
Originally Posted by hunginator View Post
For the past 6 years of driving manual, I've into the habit of double clutching my downshifts... Is this still required with cars these days? I heard that if you don't double clutch downshifts, it can hurt the synchros overtime.

What I mean by double clutch downshift for those of you who don't understand, the process is like this to downshift to any gear.

1. Clutch-in
2. Neutral
3. Clutch-out
4. Clutch-in
5. Shift into desired gear
Hi,

Unless you are doing drastic downshifting, such as sixth to second, you don't need to double-declutch in a modern gearbox with synchros. You can, if you find it more satisfying. However, without proper rev-matching, you won't get a smooth shift (i.e. less synchro wear) regardless of whether you single or double declutch.

It can be argued that double-declutching causes more wear elsewhere than single. Consider that double-declutching means you are exercising the throw-out bearing twice as much, which can cause it to fail before the clutch disc itself. If that happens, you are faced with the cost equal to a clutch replacement.

The other area of added wear would be to the crankshaft's thrust bearings. Each time you disengage the clutch, you are applying a force on the crank in the thrust direction. Over time, this adds up. I realize that this is perhaps a more obscure failure mode but it's been known to happen on engines. Not sure how robust the N52/N54 thrust bearings are.
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      09-13-2014, 01:38 PM   #10
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I've driven a few manuals all the way back to an 86 nissan sentra, and I've never double-clutched.
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      09-13-2014, 04:45 PM   #11
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Certainly not required, but it does save some amount of wear and tear of the syncros, at the expense of added wear and tear on the clutch slave cylinder and throwout bearing.

As a matter of habit (I learned to drive on a non-syncro Land Rover) I tend to double-clutch and heel and toe most of my downshifts. Good practice for autocross too. I figure throwout bearings are cheaper than syncros, and it gets replaced when you replace the clutch anyway. But you don't HAVE to do anything fancy, it will all last a really long time generally.
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      09-13-2014, 05:06 PM   #12
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Rev matching is significantly more beneficial to your transmission than double clutсhing. There's no meaningful reason to perform double-clutching today. Note also that under normal driving (not racing on the edge of lateral grip) it is completely unnecessary to hill-and-toe to do a rev match. Rev matching can easily be done by pivoting the right foot from brake pedal to accelerator pedal, blipping it and pivoting the right foot back to brake pedal.

Last edited by AndreyT; 09-13-2014 at 07:13 PM.
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      09-13-2014, 06:09 PM   #13
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I double clutch when I downshift for the most part. Regardless of whether or not it's required, it's just a habit for me at this point. Of course, I always blip the throttle and match revs when downshifting as well.
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      09-13-2014, 06:58 PM   #14
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Whoops, as some of you have mentioned, I left out a step. There's a throttle blip before finally shifting into gear. I'll edit it on the initial post.
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      09-14-2014, 12:10 AM   #15
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Quote:
Originally Posted by norsairius View Post
Any manual transmission with synchros (that aren't shot) should not require double-clutch down shifting as far as I know. That said, just because you don't need to double-clutch doesn't mean it'll hurt if you do.

Blipping the throttle as you downshift though is exactly what I did when I had cars with manual transmissions and it's just generally good practice to try and match revs as you re-engage the clutch for a smoother shift and less wear on the clutch.
Normally, no need to double clutch. Exception: when the synchros just don't want to cooperate, the double clutch pretty much solves anything.
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      09-14-2014, 02:07 AM   #16
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Quote:
Originally Posted by MiniXP View Post
"Granny shifting, not double clutching like you should."
+1 and almost blew the welds on the intake.
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      09-14-2014, 04:47 AM   #17
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now me and the mad scientist gotta rip apart the block and replace the piston rings you fried!
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      09-14-2014, 08:43 AM   #18
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Quote:
Originally Posted by Johnny Boost View Post
+1 and almost blew the welds on the intake.
Gotta keep an eye out for the "DANGER TO MANIFOLD" warning message.
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      09-14-2014, 08:57 AM   #19
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I've got 200,000 on my car. Never changed the trans fluid much less the clutch. I drive it like a scalded ape too double stepping etc. You might actually be hurting it with excess clutch wear so I'd stop the double clutch stuff.
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      09-14-2014, 09:54 AM   #20
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It's not required, but since the first car I ever owned with a manual was an '84 GTI with a trashed 2nd gear synchro, I still do it out of habit. Plus, it sounds cool, and if anyone's following you and hears the throttle blip while your brake lights are still on, they know you know what you're doing
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      09-14-2014, 11:41 AM   #21
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Quote:
Originally Posted by AndreyT View Post
Rev matching is significantly more beneficial to your transmission than double clutсhing. There's no meaningful reason to perform double-clutching today. Note also that under normal driving (not racing on the edge of lateral grip) it is completely unnecessary to hill-and-toe to do a rev match. Rev matching can easily be done by pivoting the right foot from brake pedal to accelerator pedal, blipping it and pivoting the right foot back to brake pedal.
Rev matching will do nothing to save wear on the syncros, only the clutch disk (and only on down-shifts). The syncros still have to match the gearbox shaft speeds. Double-clutching does both - the whole point of it is to change the speed of gearbox innards so that the syncros don't have to do anything. That is the whole reason you have to do it in an unsynchronized gearbox to start with.

While I completely agree that on a modern car it is completely unnecessary, it is a nice skill to have on your driving resume. And it takes practice to maintain that skill. And done correctly, it makes for marvelously smooth progress. Of course these days so few people can drive a manual transmission car at all, never mind driving one properly.
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      09-14-2014, 02:53 PM   #22
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Rev match, heel - toe, like a baws. Double clutching is for grannies. /thread
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