upshurc

With synchronizers in modern transmissions, does double clutching (mainly in downshifting) do anything significant? I've been reading about it lately.
I'm accustomed to just matching rpms as I downshift, but not double clutching.
I guess quick heel-toe downshifts don't double clutch, do they? That just involves rpm matching like I mention I do, I think.
THanks for any advice and/or instruction.

Bearcat94

Southern Boy should be along any minute.



If not, search posts by him. He is the KING of the manual shift.

gauravp123

ooooh, southernboy, where art thou southernboy?????

yoooohoooooo

tidus888

i don't think you need to do the OLD SCHOOL double clutch.... but more of a blip before you disengage the clutch...

double clutch(e.g from 4 to 3) = 4th -> clutch -> natural -> clutch -> 3rd

tidus888

heel-toe also!!!

6spMike

Just matching revs makes the shift smooth.
Double-clutching too makes it easier on the synchros.
In modern cars, it's probably the difference in the synchros lasting 500k miles instead of 300k miles, though.

stillhere153

iTrader: (1)

why double clutch with a stock clutch?

01tl4tl

Clutches wear out with use and are not cheap
Brakes are for slowing the car down
The throttle is to make exit of corner

I hate cars

Double clutching is for non-syncro transmissions and is far different than matching revs. It involves pushing the clutch in, putting the trans in neutral, and pushing the clutch again and putting it into gear. It sort of brings everything up to speed in the trans and is totally unnecessary. I had to do it in a rental dump truck one time. In the TL, you're just putting unnecessary wear on the master and slave cylinders along with about everything in the clutch mechanism.

SouthernBoy

Double clutching, which is rev-match of the engine AND transmission components is not only a desirable method, it will significantly prolong the life of, primarily, synchronizers, as well as other components in your tranny.

Most of you probably already know that our transmissions are "constant mesh". In this type of manual transmission, when you change gears you are not really shifting any gears at all. What you are doing is sliding collars with dog teeth into gear hubs. In order for this to take place without grinding, synchronizers were introduced. And they are not modern by any stretch. Synchronized manual transmissions have been around since 1929.

Anyway, to double clutch, you do this;

o Depress the clutch fully and begin your downshift.
o As you pass through the neutral gate, let the clutch out some while at the same time, blipping the throttle to raise engine speed beyond where it will be at the completion of the shift.
o Fully depress the clutch again and complete the shift.
o Let the clutch back out.

If you do this correctly, your shift will be seemless and the transition so smooth, a passenger would never notice the change of gears. Now the "why" of it.

If you just rev-match and don't double clutch, you are only doing part of the job. You ARE raising engine speed by doing this so that your drive wheels do not have to bring the engine back up to speed via the clutch engagement. However, you are NOT spinning up your transmission gears in preparation for the downshift, so the synchronizers (which are actually like little clutches) have to do this for you. Since they are made of a softer material (generally brass), the wear factor is greater when gear speeds have to be brought into "synch" by them. When you double clutch, you are nearly eliminating the stress of their job.

Read this segment of an article I found.


Dog clutch
The gear selector does not engage or disengage the actual gear teeth which are permanently meshed. Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs through its hub. The shaft then spins together with that gear. The output shaft's speed relative to the countershaft is determined by the ratio of the two gears: the one permanently attached to the countershaft, and that gear's mate which is now locked to the output shaft.

Locking the output shaft with a gear is achieved by means of a dog clutch selector. The dog clutch is a sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth that fit into slots (splines) on the shaft, forcing it to rotate with that shaft. However, the splines allow the selector to move back and forth on the shaft, which happens when it is pushed by a selector fork that is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the selector. The selector is typically symmetric: it slides between two gears and has a synchromesh and teeth on each side in order to lock either gear to the shaft.


[edit] Synchromesh
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh, where before the teeth can engage, a cone clutch is engaged which brings the selector and gear to the same speed. Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker (or "baulk") ring. When synchronization occurs, friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.

The synchronizer[1] has to change the momentum of the entire input shaft and clutch disk. Additionally, it can be abused by exposure to the momentum and power of the engine itself, which is what happens when attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force it into gear without using the clutch, the synchronizer will make up for any discrepancy in RPM. The success in engaging the gear without clutching can deceive the driver into thinking that the RPM of the layshaft and transmission were actually exactly matched. Nevertheless, approximate "rev-matching" *with clutching* can decrease the general delta between layshaft and transmission and decrease synchro wear.

SouthernBoy

If you blip the throttle before you disengage the clutch, your car will jerk since your drive train would still be locked.

SouthernBoy

You should never double clutch (or rev-match) on upshifts. I say this because you said, "mainly downshifting" which infers "sometimes upshifting".

"Quick heel-toe" shifting is generally done in aggressive driving, as on a road course. And yes, the experience drivers who race do double clutch when heel-and-toeing.

SouthernBoy

Whether or not the clutch is a stock unit makes absolutely no difference since double clutching affects the transmission. The clutch is only a device to lock or unlock the engine from the rest of the drive train.

SouthernBoy

I disagree with this. Yes, if your transmission has no synchronizers (or extremely wore ones), you must double clutch in order to effect a shift.

Is it unnecessary? Absolutely not and for the reasons I've given. Yes the clutch control mechanisms (the pedal and hydraulic system) will be used more, but this is not going to cause any problems. And if the hydraulic system does manage to have a slightly shorter life, it is less expensive to replace than tearing down a front drive tranny for overhaul.

stillhere153

iTrader: (1)

Experienced drivers used performance after market clutches... so again I post a further question... what is the need to double clutch using a stock clutch specifically on a TL 3rd Gen...

94eg!

If you are unaware, the DBW (drive-by-wire) will automatically rev-match a down-shift to the next lower gear when you double clutch. The are three requirements though:

1) you have to blip the throttle past 3000rpm
2) you have to have your clutch completely out when you blip the throttle
3) you have to be going to the next lower gear (ie: 5th to 4th, or 6th to 5th not 5th to 3rd)

It's a little slower than a "do-it-yourself" rev-matched double-clutch since you have to wait for the rpms to come back down and stabilize, but it's quite accurate and very smooth every time. Since I learned of this, I do it everyday on my way home. There is a large grade on the freeway just before my exit. When I'm cruising in 6th I tap the clutch & drop to neutral. Now with the clutch out, I blip the throttle past 3k, and wait for the tach to stabilize. Now tap the clutch again, and drop into 5th. Smooooooth!!!

This feature totally blew my friends minds when I showed them. I love this car!

94eg!

Double clutching helps prolong the life of the synchros when down-shifting. Double clutching actually has nothing to do with prolonging the life of the clutch itself. Thats what rev-matching is for...

Heel-toe is just a process of using the brakes while rev-matching. You can heel-toe with or without double-clutching.

SouthernBoy

For the reasons I, and some others on this post, have already given.

stillhere153

iTrader: (1)

mmh I guess I am stubborn going back to my gsr tranny days on my b18

94eg!

I Heel-toe, double-clutch & rev-match my Type-R swapped Civic on every down-shift. The lightweight flywheel makes super easy...

mishar

There is no point in double clutching TL. Yes, it will save some synchronizers life, but they would last forever anyways if you don’t force them. I definitely do not see the point in forcing them on downshifting, by all means not as alternative to slow double clutching. Synchronizers task is generally the easy one, especially on a 6 speed short shift gearboxes. They only have to accelerate or decelerate main shaft and not for much.

RPM matching is a part of smooth driving and it saves some clutch life. I don’t think that TL is doing it by itself.

evoman

Southernboy, i think i get what you mean.

So for Upshifting

1. Fully press clutch
2. Shift from lower gear into neutral gate then higher gear
3. release clutch

For Downshifting

1. Fully depress clutch
2. Shift from higher gear to neutral
3. Release clutch halfway momentarily then depress fully again
4. Shift from neutral to lower gear, blip throttle
5. Release clutch

Is this correct?

3rdGenHybrid

double clutching...lol....unless you have a dog gear tranny.....i dont see the point of it..."pro longing the life of your synchros.." ummm...sure...i guess...

007TL-S

We Semi-truck driver's double clutch also, only durning rapid shifting ! ..so the synchro's don't get damaged........I only use the clutch twice , from a stand still start from the stoplight & when the pressure doesn't want to release!........... I float gear's 99% of the time, even on the down shift !......I drive 10-speed!

I hate cars

Still totally unnecessary in the TL. Not to mention it makes for a slower shift. You might get an extra 5 miles in 100,000.

T Ho

I tend to agree here. In particular, the close ratios do help the trans lead an easier life.

It's a nice theory, but the reality is that a trans that's not brutalized will typically outlast the car. I've taken two manual cars to nearly 200k, and I have an old Camaro with a 6spd manual that's been treated "harshly" for 50k miles (often a 1/4 mile at a time, and I row that stick for all it's worth ), and I've never had any synchro problems.

So, yes the theory is there. In practice, I don't think it makes a significant lifetime difference. You boys who still have your 6MT 3Gs in 2022 let me know.

94eg!

Do you think that a single-clutched down-shift to the next lower gear (going from 3rd to 2nd) puts more stress on a synchro than skip-shifting (going from 2nd to 4th)? I would personally think so considering the main-shafts tendency to quickly decelerate as the shifter passes through neutral and the clutch is pressed in. Keep in mind Acura specifically says not to skip shift their manual transmission because it does ruin the synchro and void the warranty (no matter how close the gear ratios).

But nobody said you have to double clutch. It is what it is...

No. For a downshift, you want to do it like this:

1. Press in the clutch while dropping shifter to neutral
2. Let out the clutch and blip the throttle to match the rpms of the desired lower gear
3. Press in the clutch while putting the shifter into desired gear
4. Let out the clutch

The idea is to blip the throttle (rev-match) while the clutch is out. That is what accelerates the transmissions main-shaft to match speed with the next lower gear reducing or eliminating the work done by the synchronizer. If it's done quickly enough, you should be letting the clutch out for the second time just at the right rpm for a smooooth down-shift...

mishar

Skip up shifting doesn’t put any stress to synchronizers, unless you want to be very fast. As a matter of fact it is good way to save some fuel and cut emissions. Corvette does that by default if you don’t push it higher in first.

Skip downshifting does put synchronizers to extra work, unless you are braking strongly so the car deceleration is close to that of main shaft.

I wonder how it would work with voiding that warranty. Do they expect that somebody would come and say – Look guys, I was skip shifting all the time and now my synchronizers are broken.

94eg!

Not if you double-clutch...

black label

The point of the double clutch is to rev match (properly) for smoother down shifts. By rev matching you will avoid "compression braking" as the sycros mesh.

If you are downshifting to move into a higher RPM range (downshifting for power or accelleration) you should double clutch.

If you are downshifting for compression braking, you shouldn't double clutch.

It has nothing to do with whether you are using a stock clutch or aftermarket, it's all about the gear ratios relationship with engine RPM, vehicle speed, and what the driver is trying to accomplish with the downshift.

mishar

You are absolutely right, but only if you match revs exactly.

On the other hand it is more work for me and since it doesn’t create any benefit I hate it.

mishar

Well, if we want to be peaky, if you are downshifting one gear it is faster not to double clutch. You should only rev match, or go higher if you want to accelerate immediately. If you are skipping some gears than it is faster and less stressful to double clutch. If you know how to do it properly.

By the way, in order to check your double clutching skills you need car with broken synchronizers. Otherwise you will newer know. It may be just for show.

SouthernBoy

No so.. if you know what you're doing. The clear indication that the double clutched downshift was correct is when the clutch finally comes out, the transition is smooth and not noticed.

SouthernBoy

Gentlemen;

In plain English, these are the facts. When rev-matching and especially when coupled with a double clutched downshift, you will;

o Increase the life of a host of components from the clutch assembly to the transmission.
o Vastly improve your expertise regarding the operation of a manual transmission.
o Gain and maintain better control over your car as you improve your technique.
o Know that you are among a select few who know how to properly operate a manual transmission.

Do you have to do this? Is it necessary?

No to both of the above. It is, afterall, your car and your wallet and your driving. So you have every right to do as you see fit, whether it is the correct way or it isn't.

Frankly from my perspective, I have spent a long time doing this and perfecting it. I would imagine more years than a good many of you have been alive. And I know it is the right way, not only for me, but it is just that. I could care less what others do as long as I do it right.

So those of you who argue that none of this is necessary or in the more extreme case, that is is incorrect, I would suggest you do some research and be willing to learn a bit more. This is by no means an insult or a flame but more along the lines of a friendly suggestion. As I wrote in some text I posted a bit ago, there are a number of ways to operate a manual transmission, but there is only one correct way to do it. I certainly didn't invent it.. I just learned it at a young age.

Best of luck to those of you who wish to improve your skills.

94eg!

Well, you know for sure you've rev-matched properly when the clutch comes out smooth. But you REALLY know you've double-clutched properly when the shifter practically falls into gear by iteslf (can't feel the synchros stall whatsoever)...

That's not exactly what compression-braking is. Compression-braking is just a more technical term for engine braking. What your refering to as compression braking is actually just slipping the clutch. You should NEVER use the clutch as a brake as it puts tons of wear on the friction surfaces. And you should also NEVER do this during an emergency stop because the sudden extra braking torque on the drive axle can:

a- confuse the ABS computer making the stop take longer (on ABS cars).
b- lock up the front wheels causing under-steer and possibly stalling the engine (on non ABS cars).

If you want safe compression braking you should rev-match a down-shift (while double-clutching of course), and let the rpm of the motor do it's thing. If you have an emergency stop, you should put the clutch & brake in at the same time as hard as you can (if ABS equipped). After all, you don't want ABS shock traveling up through the drivetrain do you?

SouthernBoy

Hey, it sounds like you and I are from the same school! Glad to see your input.

Your response to my comments about a smooth transition (first paragraph) is good. Most of the time when I downshift, it almost feels as though the shifter is being "taken" from me and the shifting forks are inserting the dog teeth on their own. A little "snick" and in it goes like it was taken from you.

The comments about serverly damaged/worn synchronizers being a mark of a good downshift when grinding does not occur is also right. My first car had a cable-operated clutch and it broke once on a double date. I had to drive back home, about 15 miles, with no clutch, so I was really doing some serious rev-matching to line up the dog teeth with that car. I was 18 at the time and fully understood the concept of rev-matching and double clutching. Had to or I would have had to abandon that car in the country.

black label

That's not exactly what compression-braking is. Compression-braking is just a more technical term for engine braking. What your refering to as compression braking is actually just slipping the clutch. You should NEVER use the clutch as a brake as it puts tons of wear on the friction surfaces. And you should also NEVER do this during an emergency stop

While I agree that this is more stressful to the clutch, I have 153000 miles on my clutch and I downshift for decelleration purposes an a daily basis (and redline the engine, it hasn't been an easy 153k ). In terms of an emergency stop, clutch to the floor and brake as hard as I can (hoping the ABS doesn't kick in).

94eg!

For fun in my Civic I used try to zero-clutch shift as far as I could go without coming to a complete stop (made it from work to home once). Of course this was before the expensive motor/trans swap. It certainly adds a level of intensity to street driving, and isn't too difficult when you have the relatively light flywheel that my Civic CX had. Later on my brother had to use this technique when his clutch hydraulic system failed him in his Z. My clowning around showed him it was possible, and he managed to adapt on the fly.

And just last year the hydraulic clutch system in my wifes 03 Element completely failed due to a bad master cylinder swap done by the dealer (blew out all the fluid). I had to limp the truck all the way to stealership 10 miles away. I can say that on a car with a heavy flywheel (slow free revving), it is NOT advisable to do this. I think I ended up destroying the 3rd gear synchro to get it there. Fortunately it was all due to their bad install so they had to foot the bill on the rebuild.

Moral: Don't zero clutch your TL unless you absolutely have to, as these cars have quite a heavy flywheel.

I hate cars

LOL. Your main and rod bearings love super low rpms... I had people looking at me funny in the GN one time when the TCC solenoid in the trans went bad. I was running around with an open downpipe so it was loud as hell. The torque convertor clutch would stick on once it was engaged. Everytime I came to a stop it would kill the motor and it was obvious to anyone in a 100' radius. Bet no one on here has killed an auto before.

I do agree that rev matching is essential and I have never not rev matched. It hurts me to drive with other people that instead of rev matching on a downshift, they slooowly let out the clutch to make it smooth. I think black label has confused rev matching with double clutching.

I was on the way to pick up the future fiancee for the first time ever in a friend's BMW. On the way third gear would grind no matter how slow I made the shift. I panicked since I was nervous already. Double clutching solved the problem but due to my nervousness and lack of coordination I ended up just skipping third when I picked her up because I knew I would screw up eventually.

6spMike

I have to disagree here, unless your habit is to press the clutch and brake at the same time for every stop. The reason is the time it takes your brain to think "ooh - emergency stop - push the clutch too" will delay your reaction time, so it will take farther to stop from the point you noticed you had to stop.
I think shorter stopping distance trumps any supposed stress on the trans and engine. I've locked up the brakes on non-abs manual cars, which obviously stops the engine pretty quickly, with no problems.
I can't believe that ABS pulses through the drivetrain would cause damage; the whole purpose of ABS is to not lock up the wheels (drivetrain).
Bottom Line - You don't have to push in the clutch in an emergency stop. Just stomp the brake.