PDA

View Full Version : What is Double-Clutching and how do I do it?


Reedo302
01-16-2006, 02:02 PM
I've heard alot about double-clutching and I don't drive a manual (yet), and I'm wondering what exactly it is and how you do it?

ObD
01-16-2006, 02:45 PM
http://www.standardshift.com/faq.html

Uncle Fester
01-16-2006, 03:04 PM
A double clutch (also called a double declutch) is a driving procedure used for vehicles with an unsynchronized manual transmission. Before the introduction of synchronizers and automatic transmissions, double declutching was the only way to drive an automobile. Due to the difficulty involved in learning the technique, it has largely fallen into disuse. However, drivers of large trucks still use double clutching, as those vehicles are usually equipped with the older, more efficient, and more durable unsynchronized gearboxes.

The purpose of the double clutch is to match the speed of the rotating parts of the gearbox for the gear you wish to select to the speed of the input shaft being driven by the engine. Once the speeds are matched, the gear will engage smoothly. If the speeds are not matched, the gears will "crash" or grate as they come into mesh. A modern synchromesh gearbox accomplishes this automatically.

When shifting up on a double-clutched vehicle, the clutch pedal is pressed and the gearbox shifted into neutral. As the engine idles with no load, the rpms will decrease until they are at a level suitable for shifting into the next gear. The driver then depresses the clutch again and shifts into the next gear. The whole manouver can, with practice, take no more than a fraction of a second, and the result is a very smooth gear change. However, in order to downshift, engine revs must be increased while the gearbox is in neutral. This requires the driver to shift into neutral, apply throttle to bring the revs up to a suitable speed, and then shift into gear. This operation can be very difficult to master, as it requires the driver to gauge the speed of the vehicle accurately and is often conducted as cars in front slow down.

A related technique is called Heel-and-Toe, during which the brake and accelerator pedal are pressed by the right foot while the clutch pedal is pressed by the left foot. Note that Heel-and-Toe can be used with any downshift clutch operation, not just with double-clutching. Though difficult, mastering Heel-and-Toe in conjunction with Double Clutching is essential for high performance driving (e.g., Rallying) where straight-cut gearboxes are often used to minimise the wastage of power.

Penforhire
01-16-2006, 04:14 PM
Rev matching still has value because synchro rings still wear if you shift a lot without matching.

CJsCar
01-16-2006, 04:15 PM
For you kids out there this was the standard of progression up thru the ranks. First you learned to double clutch and when you were good enough to master that you went clutchless except for starts. :ouch: And if you got it wrong everyone knew. Very nice explanation by Fester.

akhbhaat
01-16-2006, 04:59 PM
Double clutching is still a good skill to know and use, as you can really save your synchros (particularly the oft-used second gear synchro) a lot of wear by doing so. I double clutch most of my downshifts just for fun and for the sake of my transmission's longevity.

wwb4
01-16-2006, 05:06 PM
Double clutching is still a good skill to know and use, as you can really save your synchros (particularly the oft-used second gear synchro) a lot of wear by doing so. I double clutch most of my downshifts just for fun and for the sake of my transmission's longevity.
:stupid: ........I too double clutched in my previous manual cars during my downshifts.....I however did it more for the "fun" factor. :D

akhbhaat
01-16-2006, 05:21 PM
One neat little benefit to double-clutching is that you can use it to "feel" the transmission and then place your subsequent engine rev match with a remarkable degree of precision...

Pinecone
01-18-2006, 12:51 AM
A bit of a SEARCH will coe up with multiple threads on BimmerFest on tehs ubject of how as well as discussions of pros and cons.

Moderato
01-18-2006, 07:12 AM
I always rev match all my shifts up or down, if a shift involves a large change in rpms or gears then I will also double clutch. :thumbup:

kyfdx
01-18-2006, 07:44 AM
I always rev match all my shifts up or down, if a shift involves a large change in rpms or gears then I will also double clutch. :thumbup:

I learned to double-clutch when I drove a diesel straight truck.. You have no choice, if you want to downshift... Also, on my '67 BMW.. as it didn't have the greatest synchros..

That said, I rarely do it in any modern passenger car... Jam 'em and cram 'em..

Moderato
01-18-2006, 08:41 PM
That said, I rarely do it in any modern passenger car... Jam 'em and cram 'em..So what you're saying is don't worry about it. :)

Pinecone
01-18-2006, 09:10 PM
Tnaks for the heads up, so nobody who really cares about the tranny will buy a used car from you.

Moderato
01-18-2006, 10:48 PM
Tnaks for the heads up, so nobody who really cares about the tranny will buy a used car from you.Just for the record he's not talking about me. I take care of my cars. :)

DaKine
01-18-2006, 11:28 PM
Double clutching is what they do in the Fast & The Furious. If you want to beat someone in a drag race, you double cluth. It worked for Vin Diesel...

Moderato
01-19-2006, 07:12 AM
Double clutching is what they do in the Fast & The Furious. If you want to beat someone in a drag race, you double cluth. It worked for Vin Diesel...You learn something new everyday! Thanks! :thumbup:

edit - I was being sarcastic!

kyfdx
01-19-2006, 08:07 AM
Double clutching is what they do in the Fast & The Furious. If you want to beat someone in a drag race, you double cluth. It worked for Vin Diesel...

Uh.. if you are drag-racing, you don't use the clutch at all.. Not if you want to win..

Synchros.... They really work.. If you aren't racing (on a track), double-clutching is just something to remind you of your old MG.. If a manual transmission goes bad from not double-clutching on downshifts before 150K miles, then it wasn't right to begin with...

Although, double-clutching will extend your clutch life... But, clutches are wear items.. I don't mind wearing one out, if I enjoyed the driving more..

And... if you are really good at matching revs, you don't need the clutch..

All moot for me, at this time... Our BMW is the wife's car...and it has a slushbox..

ic
01-19-2006, 08:08 AM
I always rev match all my shifts up or down
rev match for shift up? Doesn't the rev drop automatically?

akhbhaat
01-19-2006, 02:20 PM
Uh.. if you are drag-racing, you don't use the clutch at all.. Not if you want to win..

Synchros.... They really work.. If you aren't racing (on a track), double-clutching is just something to remind you of your old MG.. If a manual transmission goes bad from not double-clutching on downshifts before 150K miles, then it wasn't right to begin with...

Although, double-clutching will extend your clutch life... But, clutches are wear items.. I don't mind wearing one out, if I enjoyed the driving more..

And... if you are really good at matching revs, you don't need the clutch..

All moot for me, at this time... Our BMW is the wife's car...and it has a slushbox..Clutchless shifting in a drag race? That's just asking for it. These aren't motorcycles or diesel rigs, you know. One mis-timed shift and you could destroy the synchros or even strip the teeth right off of the dog collar.

Double clutching does nothing more for clutch life than a regular single clutch, rev-matched downshift. It will, however, greatly reduce (or even completely eliminate) the wear on the synchros of whichever gear you're downshifting into. Yeah, modern synchros do work great, but it's still a good idea to take some load off of them during a really aggressive downshift (say 4-2 or 5-2 at 45 or 50 mph). All of this aside, it's a really cool feeling when the shifter slides or pops into gear effortlessly, thanks to a double clutch rev match.

The transmission itself isn't what goes bad from lack of double-clutch downshifting, it's the synchros, and usually only in a particular gear (usually 2nd goes first since that's by far the most commonly used gear for downshifts...and in many transmissions, 1st gear isn't even synchronized to begin with, though it is in the E46).

Moderato
01-19-2006, 07:12 PM
rev match for shift up? Doesn't the rev drop automatically?Yeah, I just ment that I'll time when I release the clutch right when the rpms drop to where they need to be, so I don't shift and then use the clutch to bring the rpms down faster.

mdub
12-01-2006, 01:13 PM
how do you know what rpms "match"?

Fast Bob
12-01-2006, 10:00 PM
how do you know what rpms "match"?

This is done largely by feel....if you`re changing down one or two gears, push the clutch pedal down, blip the throttle quickly to raise the RPMs by about 1000, then shift into the appropriate lower gear "on the down-rev" and smoothly release the clutch while the revs are still elevated. The lower gear will result in a higher engine RPM, and the goal is to "match the revs" in order to smooth the transition. Hope this helps.

Regards,
Bob

BMWpurist
12-02-2006, 02:26 AM
Most truck drivers with unsynchronized trannies don't double clutch,once you know a truck RPM drop between gears...road speed and RPM for each gear the only time you need to use the clutch is to get goin and when coming to a stop.

A good driver who slides gears (changing gears without using the clutch at all) in a big truck will go over 500K without needing a clutch and it's more fuel efficient.

That's how an automated manual tranny works the computer through hydraulically actuated linkages slides gears without the use of the clutch...

BMWpurist
12-02-2006, 02:33 AM
Double clutching is still a good skill to know and use, as you can really save your synchros (particularly the oft-used second gear synchro) a lot of wear by doing so. I double clutch most of my downshifts just for fun and for the sake of my transmission's longevity.


How are you saving your synchros when you double clutch?

Fast Bob
12-02-2006, 08:48 AM
How are you saving your synchros when you double clutch?

Synchros are made of brass (soft) and will definitely wear over time (especially second gear synchro, which takes the most abuse). If you`ve ever performed a good double-clutched downshift, you know that the shifter seems to *glide* effortlessly into the lower gear, "like buttah"....this is because you`re taking all the stress off the synchro.

Regards,
Bob

BMWpurist
12-02-2006, 03:36 PM
Synchros are made of brass (soft) and will definitely wear over time (especially second gear synchro, which takes the most abuse). If you`ve ever performed a good double-clutched downshift, you know that the shifter seems to *glide* effortlessly into the lower gear, "like buttah"....this is because you`re taking all the stress off the synchro.

Regards,
Bob

Even when you double clutch in a car,You're still using your synchros to match gear speed with engine speed for a smooth shift,the only time you won't use your synchros to shift ,is if you can change gears without using the clutch by matching road speed/gear speed and RPM witch is really hard since gas engines drop/gain RPM much quicker than diesel engines.

To give you a good example in one truck if you have 1500RPM at 65 MPH in 12th gear,if you are in 13th gear you get 1400RPM at 65 MPH,now you want back to 12th gear,in order to slide 12th gear in you'll have to raise your RPM back to 1500 and be at 65 MPH or close and shift,you'll get an engagement without using the clutch pedal. You've synchronised the gear speed and engine speed yourself and that's a real manual.

Double clutching is rarely used and primarily used for driver who aren't familiar with a specific setup (rear ends,tranny etc.) to prevent grinding gears in case the driver picks the wrong engine speed/road speed for a specific gear.

jcatral14
12-02-2006, 03:59 PM
I can't wait to drive stick again :D
Just 2 more weeks...

Moderato
12-02-2006, 03:59 PM
Even when you double clutch in a car,You're still using your synchros to match gear speed with engine speed for a smooth shift,the only time you won't use your synchros to shift ,is if you can change gears without using the clutch by matching road speed/gear speed and RPM witch is really hard since gas engines drop/gain RPM much quicker than diesel engines.

When you rev match in neutral with the clutch engaged you are using the engine/throttle and not the syncros to match gear speed with engine speed, so in this case you aren't using the synchros at all.

BMWpurist
12-02-2006, 05:54 PM
When you rev match in neutral with the clutch engaged you are using the engine/throttle and not the syncros to match gear speed with engine speed, so in this case you aren't using the synchros at all.
We're talking about double clutching.

Fast Bob
12-02-2006, 06:12 PM
We're talking about double clutching.

So are WE.... I think you`re the one who`s confused here.... not being a driver of big rigs, I don`t really know *what* they do, but I have a pretty good idea what works and what does`nt on a street car. Double-clutching downshifts WORKS, and takes a lot of strain off your synchros.

Regards,
Bob

BMWpurist
12-02-2006, 08:59 PM
So are WE.... I think you`re the one who`s confused here.... not being a driver of big rigs, I don`t really know *what* they do, but I have a pretty good idea what works and what does`nt on a street car. Double-clutching downshifts WORKS, and takes a lot of strain off your synchros.

Regards,
Bob

Don't believe everything you think!

Properly used a manual tranny will last you a long time even if you don't double clutch,double clutching is for trannies without synchros.


This is like saying use your sunroof when getting out of the car because it takes a lot of strain off the door hinges and it'll make your car door hinges last longer. Has anyone on this forum ever replaced synchros due to not double clutching?

"So are WE" the majority view isn't always right...I'll stop here with this thread.

Good night!

BmW745On19's
12-02-2006, 09:09 PM
I find it hard to blip the throttle with downshifts with the ball of my toes on the brake and heel on the gas, I like to do it heel on brake pedal and ball of foot on gas pedal at sort of an angle. It's just more comfortable.

Anyone else have different techniques they care to share?

CliffyDeuce
12-03-2006, 03:19 AM
I'm no expert by any means, but I find that for my rather large feet, I end up using the ball of my foot on the right side of the brake pedal, and using the right edge of my foot to blip the throttle. The steering wheel hits my leg, so I can't rotate my leg enough to get my heel on the gas with my foot on the brake.

Cliffy

attack eagle
12-08-2006, 04:28 PM
Uh.. if you are drag-racing, you don't use the clutch at all.. Not if you want to win..

Synchros.... They really work.. If you aren't racing (on a track), double-clutching is just something to remind you of your old MG.. If a manual transmission goes bad from not double-clutching on downshifts before 150K miles, then it wasn't right to begin with...

Although, double-clutching will extend your clutch life... But, clutches are wear items.. I don't mind wearing one out, if I enjoyed the driving more..

And... if you are really good at matching revs, you don't need the clutch..



Double clutching wastes time in shifting from one gear to the next. the only time you should consider it is when you are driving a prewar vehicle and you are downshifting to something that will be up in the RPM band.
Hint: the people who make F&F re not car people. For a real car movie that go watch LeMans on DVD. OR gran Prix (see heel toe referenced above)... i prefer Lemans.

Cliffy, I'm the same way.

chun_tak
12-08-2006, 04:59 PM
I used to do double-declutching on my Civic during down-shfit but not any more. I was told that unless you match rev match perfectly, you are causing undue wear of the synchromesh by doing double declutching on a modern gearbox with synchromeshes. You don't need to do double declutching to rev match.

Here is one web site on this topic
How Synchromesh works (http://www.singster.co.uk/mascot%20mar-apr%202003%20(4).html)

A comment from the page above:

If your car has a synchromesh gearbox it is recommended you do not double-de-clutch as this can cause undue wear on the synchro rings.

Fast Bob
12-08-2006, 06:21 PM
If your car has a synchromesh gearbox it is recommended you do not double-de-clutch as this can cause undue wear on the synchro rings.[/I]

Anyone and everyone who has ever performed a properly-executed double-clutched downshift knows that this statement is pure & utter bullsh!t....

Regards,
Bob