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.
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.