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Jason C
01-05-2005, 10:15 PM
I posted this on another forum, and I thought the curmudgeons here might be interested. I'm pretty sure everything is correct, but if someone finds a mistake feel free to correct the following:



Before understanding what double-clutching (and rev-matching, and clutchless shifting, etc etc etc...) actually is, it really helps to understand how a transmission works.

The three pertinent shafts in a manual transmission are the input shaft, the counterspeed shaft, and the output shaft. The counterspeed shaft is also known as the countergear or countercluster shaft, and it is a one piece unit. There is a fourth shaft, the reverse idler, but it is not that important for the above discussion on double-clutching.

In any constant-mesh manual transmission...

-The input shaft is the one that's receiving power from the engine. It is usually pointing towards the engine. The clutch friction disk rotates with the splined end of the input shaft. The input shaft ends with the clutch gear inside the metal transmission housing, which is always engaged and rotating with an opposite gear on the counterspeed shaft.

-The counterspeed shaft is a metal rod with gears always fixed to (rotating with) it. The counterspeed shaft is always meshed with a) the clutch gear rotating with the input shaft b) the speed gears on the output shaft. Hence the name "constant-mesh transmission."

-The output shaft send the power out of the transmission to the rest of the vehicles drivetrain. It usually points to the rear of the vehicle in a RWD car. The driven speed gears are on the output shaft, but do not rotate with the output shaft when the transmission is in neutral. Oftentimes in a transmission (not a transaxle) the input and output shaft are on the same axis, and often appear to be one piece. In those cases, there is an input-to-output pilot bearing which is in the clutch gear and allows the input and output shaft to rotate freely of each other.

At a stop, with the clutch engaged (pedal up) and the gear selector in neutral, the input shaft is receiving power from the friction disk/flywheel and is spinning. The power is being transmitted from the clutch gear to the counterspeed shaft. All the gears on the counterspeed shaft are also spinning, and thus the speed gears on the output shaft (which are CONSTANTLY MESHED with the counterspeed gears) are also spinning. The power flow stops there, however - as the speed gears ride on bearings on the output shaft, and thus are not locked with the output shaft and spin freely whenever the engine is running and said gear is in neutral. The power does not go to the output shaft, so it is not spinning. This is why the engine does not stall at a stop while engaged in neutral.


When a driver is in first gear and shift to second, here's what happens...

-The driver lifts off the gas and pushes in the clutch pedal, then slides the gear lever from 1st to 2nd.

-As he lifts and completely disengages the clutch, the transmission is no longer receiving any power from the engine. It is now freewheeling, except the output shaft which is always rotating with the rest of the drivetrain.

-Sliding the gear lever from 1st to 2nd actuates whatever shift linkages is used by the vehicle and the movement of the selector ends up being transmitted through the shift rods and to the shift forks.

-The shift fork(s) move with the shift rod(s) and are always grasping the synchronizer sleeves. So now the action of the driver slides the synchronizer sleeve away from the 1st speed gear and towards the 2nd speed gear.

-As the synchronizer sleeve moves away from the 1st speed gear (driven by the corresponding counterspeed gear), the grooves on the sleeve moves away from the splines on the 1st speed gear. The sleeve is locked to the synchronizer hub, which is locked to the output shaft. So in other words, the sleeve is always locked to and rotating with the output shaft. Now that the sleeve has moved away from the 1st speed gear, they are no longer locked together. So 1st gear is freewheeling independently of the output shaft on its roller bearings. The driver is no longer in 1st gear.

-The driver moved the selector past 1st and into 2nd. So that means now the sleeve has moved past the neutral position and is heading towards the 2nd speed gear on the output shaft.

-As the 1st-2nd sleeve moves past the neutral position, it presses on a synchronizer blocking ring, usually made out of brass. The blocking ring has sharp groves on the inner surface and is made so the inner surface fits onto a corresponding raised cone area on the speed gear(s). The blocking ring is sandwiched between the sleeve and the speed gear.

-The blocking ring is being moved by the sleeve and the sharp grooves press into the coned mating surface on the 2nd speed gear. These sharp grooves cut away at the oil film (transmission fluid) on said mating surface. As the oil is being forced away, synchronization is happening. The 2nd speed gear, which is encountering frictional force from the blocking ring, is matching speeds with the rotating sleeve. At the same time, the rotating sleeve is changing speeds to match the rotational speed of the 2nd speed gear.

-Once synchronization has finished, the 1st-2nd sleeve slides over and past the dog teeth (indents) on the blocking ring and onto similar teethes on the 2nd speed gear. Once the sleeve has locked itself with the 2nd speed gear, both rotate as one. We are now past synchronization and engagement. The input shaft is locked and rotating with the output shaft, and the power is going through the 2nd speed gear.

-The driver lets the clutch pedal out. Now the transmission/driveline is using the friction disk to engage with the flywheel on the engines crank.

-As soon as the friction disk stops slipping, the drivetrain is now locked together, and the driver happily motors along in second gear! :mrgreen:



So what happens when double-clutching? Well, it helps if you understand what's going on when someone matches revs first.

When a driver is in a higher gear and shift to a lower gear while rev-matching, here's what is going on...

(Simplified, assuming basic knowledge of above)

-While the selector is out of the higher gear and moving past the neutral position, the driver blips the throttle.

-The throttle input increases the engine RPM's to whatever speed the engine would be turning at while in the lower gear at the same speed.

-The driver slides the selector into the lower gear. The synchros do their work, and the speed of the output shaft is matched with the speed of the speed gear. Now the drivetrain all the way to the friction disk is rotating in the lower gear and rotating with the tires - which are at the same road speed they were rotating at when the driver started to match revs.

-Now the driver releases (engages) the clutch pedal. Since the driver bothered to rev-match, the flywheel is now rotating at the same speed as the friction disk, and they engage together effortlessly.

-The driver has successfully matched revs! :mrgreen:

Note in the above, the driver relied on the synchros to match the speed gear rotation to the output shaft speed.



So NOW what about double-clutching? The driver takes the synchronizer blocking rings out of the equation.

When a driver is in a higher gear and shift to a lower gear while double-clutching, here's what happens...

(Simplified further, assuming basic knowledge of above)

-Instead of the driver moving the gear lever right to the lower gear, he moves it to the neutral position. Then he completely releases (engages) the clutch pedal, if only for a moment.

-Remember the above? As the transmission is in neutral and the clutch is engaged, the powerflow stops at the speed gears. So now the vehicle is no longer sending power to the output shaft.

-The driver blips the throttle and if he doesn't completely understand double-clutching, he'll probably think that he's matching the [engine RPM] to [gear RPM at the current road speed]. What he's actually doing is matching the [RPM of the engine/transmission up to the speed gears] to the [rotational speed of the output shaft].

-As the driver blips, the power is being sent all the way through the constantly meshed unit and stopping at the speed gears. So now the speed gear rotation is matched with the output shaft (and the hub, the sleeve, driveline, wheel/tire rotating at road speed, etc).

-Now that the speed gear and the sleeve are rotating at the same speed, why do we need a blocking ring? Answer: We do not, they're already synchronized.

-The driver, with the clutch pedal still all the way up (engaged), finished matching revs and now pushes the clutch pedal down, disengages the friction disk, and locks the already-synchronized sleeve and speed gear together.

-The driver lets the clutch pedal out. The entire drivetrain engages together with nary a shudder to be had, if he was any good at double-clutching.

-The driver has successfully double-clutched! :mrgreen:

I hope that helped. If you need further help, start by referencing a diagram of a transmission powerflow while reading the above.

JST
01-05-2005, 10:40 PM
Here is a good set of pictures/animations to go along with the above text.

http://auto.howstuffworks.com/transmission.htm

Jason C
01-05-2005, 11:01 PM
Heh, how cartoony. Real blocking rings are never that fat. :roll: ;)

I already caught one (really minor) mistake in my description. I'm not going to edit it out though, I'm going to see if anyone can find it. :twisted:

ADDED: Howstuffworks referred to the countercluster as the "layshaft." WTF? :?

blee
01-05-2005, 11:59 PM
Heh, how cartoony. Real blocking rings are never that fat. :roll: ;)

I already caught one (really minor) mistake in my description. I'm not going to edit it out though, I'm going to see if anyone can find it. :twisted:

ADDED: Howstuffworks referred to the countercluster as the "layshaft." WTF? :?

"Layshaft" is a common term used to describe transmission parts. You often hear of "twin layshaft" manual trannys. What exactly a layshaft is, however, is not within my realm of knowledge.

Jason C
01-06-2005, 12:03 AM
Heh, how cartoony. Real blocking rings are never that fat. :roll: ;)

I already caught one (really minor) mistake in my description. I'm not going to edit it out though, I'm going to see if anyone can find it. :twisted:

ADDED: Howstuffworks referred to the countercluster as the "layshaft." WTF? :?

"Layshaft" is a common term used to describe transmission parts. You often hear of "twin layshaft" manual trannys. What exactly a layshaft is, however, is not within my realm of knowledge.

:dunno: I've never seen the term "layshaft" in any text/technical papers I've read, nor have I hears experienced techs refer to a transmission component as a "layshaft." If it's some sort of generic term, I can see why - we generally prefer more precise names for everything.

blee
01-06-2005, 08:36 AM
http://www.epinions.com/auto-review-6902-1618F526-3A351631-prod4


The previous technique avoids excessive clutch wear by matching the speed of the drive shaft to engine speed. HOWEVER, the layshaft is still spinning at a slow speed when the lower gear engages. The sequence of speed matching with gas bliping while the clutch is disengaged is as follows:

1 engine turning at a slow speed, higher gear is engaged, layshaft turns at a slow speed (to match the higher gear)

2 clutch is disengaged, engine is now independent of the transmission

3 gear shifted to neutral, the layshaft is now independent of the driveshaft

4 blip gas, engine speed is higher (to match the need of the lower gear), layshaft is still turning at a slower speed to match the previously engaged higher gear

(etc. etc. etc....the list goes on)


Sounds like layshaft is used to describe "the shaft between the one that goes into the transmission and the one that goes out of it."

Jason C
01-06-2005, 11:07 AM
Again, I can only :dunno: as I can't find that term anywhere in my general automotive book, which is pretty basic compared to the stuff I'll be seeing in about a year.

Autarch
01-06-2005, 11:22 AM
http://static.howstuffworks.com/gif/transmission-5speed-gears.gif

Isn't the first gear location icorrect in this pic? Shouldn't the first sprocket actually be second gear and the second one over would really be first gear due to the movement direction of the shifter?

blee
01-06-2005, 11:25 AM
Yeah, it looks backwards to me as well. But see? The layshaft is indeed "the thingy between the engine thingy and the output thingy." :lol:

Jason C
01-06-2005, 11:26 AM
Eh, going from that pic, you would appear to be correct.

I'll check the Toyota 5-speeds when I get into the shop later today and see if that holds up. Can't quite remember at the moment. :P


But see? The layshaft is indeed "the thingy between the engine thingy and the output thingy." :lol:

Shaddup blee, don't take howstuffworks as the final source for all things automotive. :mad:

blee
01-06-2005, 02:18 PM
But see? The layshaft is indeed "the thingy between the engine thingy and the output thingy." :lol:

Shaddup blee, don't take howstuffworks as the final source for all things automotive. :mad:

Hey, I don't know what they're teaching you over there....but I don't know squat and I still know what a layshaft is. :D

Jason C
01-06-2005, 02:28 PM
But see? The layshaft is indeed "the thingy between the engine thingy and the output thingy." :lol:

Shaddup blee, don't take howstuffworks as the final source for all things automotive. :mad:

Hey, I don't know what they're teaching you over there....but I don't know squat and I still know what a layshaft is. :D

If I ever find the term "layshaft" somewhere in the BMW DIS, you'll be the first one to know. :speechle: :tongue:

ADDED: I recently had a competency/parts ID test in transmissions. I'm pretty sure that pointing to "the thingy between the engine thingy and the output thingy" and referring to it as a "layshaft" would have cost me points. So :flipoff:

;)

Jason C
01-06-2005, 09:49 PM
Just had an interesting conversation:

Captain: So can I ask you a transmission question?
Instructor: Yea go ahead.
Captain: Is the countergear, counterspeed, or countercluster in your average transmission ever referred to as a *layshaft*?
Instructor: Errh, no, not really... :?
Captain: Because I saw the term used like that a few times recently and wanted to know if that was a more common way of referring to it.
Instructor: No, it's usually just called a counter cluster shaft.
Captain: Hmm, I wonder why it was being used like that then. :P
Instructor: Layshaft... haven't heard that one in a while!

So not only is it improper, but it's also archaic.

How old were you again, blee? :tongue:

Btw, I also saw the reference "countercluster" on Pelican Parts. There's your final proof! 'Cause like we know all too well, if you can't trust the knowledge of P-Car enthusiasts, who can we trust? :lol:

Pinecone
01-10-2005, 09:18 AM
One error in your description of double clutching, is you are actually matching the speed of the engine AND the input/countercluster (layshaft) to the speed of appropirate to match the next lower gear selection to teh output shaft speed.

When downshifting the output shaft speed remains somewhat the same (if you are heavily braking you are of course reducing the output shaft speed as you reduce the speed of the vehicle).

So at any given output shaft speed there are specific engine RPMs that match each of the various gears to the proper input/output shaft relationship. By doulbe clutching you are choosing the appropraite input speed to match everything to the current output shaft speed for the gear you are selecting. This saves wear and tear on the synchros (which are wear items) and teaches you how to drive the tranny when the synchros are gone. :)

Also during synchronzing, since the mass of the car is more than the mass of the transmission roating mass, the synchro matches the speed of the clutch-input shaft-counter cluster (layshaft)- driven gear to the output shaft speed.

Also the synchros I have seen match cone to cone, and the grooves are what finally lock the driven gear to the output shaft.

Jason C
01-10-2005, 02:40 PM
One error in your description of double clutching, is you are actually matching the speed of the engine AND the input/countercluster (layshaft) to the speed of appropirate to match the next lower gear selection to teh output shaft speed.

Huh? :?

Didn't I say the same thing? Matching the speed of the powertrain all the way up to the speed gears to the rotation of the output? They roll as one unit when the transmission is engaged in neutral, don't they? I didn't intend to describe what the driver should or should not do. I just described what's actually happening inside the mechanical guts of the car.

No, there should be a more obvious error (that no one has commented on as of yet :P ).

Pinecone
01-10-2005, 06:24 PM
Duh, it didn't read that way to me the first time.

Pinecone
01-10-2005, 06:27 PM
Duh, it didn't read that way to me the first time.

You mean the part about the rotating sleeve of the shifting part matching speed with the second gear speed gear?

That's what I meant in the part about the relative masses.

Jason C
01-10-2005, 08:43 PM
Duh, it didn't read that way to me the first time.

You mean the part about the rotating sleeve of the shifting part matching speed with the second gear speed gear?

That's what I meant in the part about the relative masses.

:?: :?: :?:

So is that a "yes, there's a mistake in your original post" or "no, I read it wrong"?

:scratch:

Pinecone
01-12-2005, 08:03 PM
The Duh was I read it wrong. Even on teh second reading is wasn't totally clear.

The second part is what I feel is a mistake in your description