How do DCTs work?
12-02-2018, 08:58 PM
#41
Race Director
Thread Starter
I don't know everything about what they will use or do with the C8. I've done quite a bit of driving and coaching (autocross, track time) in VW/Audi cars with their DSG.
The driver interface, from the driver's point of view, it looks like any other automatic. You've got a shift lever with the normal "PRND" markings, and a gate to a "+/-" section. If you move it to the manual mode (slide it over so it moves in the "+/-" section, the paddles on the column are also activated. It seems the same as any other paddle shift automatic.
Operationally, and I'm talking about what's happening inside the transmission, it's much more like a manual transmission with a few key differences. On the Audi/VW system, there is no torque converter. There are just the 2 clutches. The biggest difference is that where there's one pinion/output shaft in a conventional manual gearbox, the DSG (their name for their DCT) has two shafts. One shaft has the odd numbered gears (1,3,5[,7]), and the other shaft has the even numbered gears (2,4,7[,8]). Each shaft has a clutch that is controlled by the computer. The gears on these shafts are very similar to gears in a manual transmission, with dog rings that slide to engage the gears. I'm not sure if there are synchronizers or not, but the operational movements inside are very similar to a manual transmission, with movement controlled by solenoids and hydraulics. The VW/Audi system in the transverse mounted gearbox has a pinion gear on each of the shafts (two pinion gears on the same ring gear). That simplifies things a bit and reduces some of the parts count and gear interfaces. If the ME C8 has the diff in the same case with the gears, they will probably have a similar arrangement.
In normal operation, the computer will select the "next" gear on the opposite shaft, and when the time comes to shift, the current clutch will disengage as the other clutch engages, transferring power nearly instantly from one shaft to the other. As long as you're doing what the computer expects, it all works good. But if you were mildly accelerating in 3rd gear (with it in "manual shift mode"), the computer is going to have 4th gear all engaged and ready to go, waiting on your to tap the "+" paddle or tap the stick in the "+" direction. Lets's say you're approaching a curve in the road, though, so instead, you left foot brake, taking your foot off the throttle pedal at the same time, and hit the "-" paddle (or tap the stick in the "-" direction), the shift might take longer than you expect, and perhaps longer than a modern automatic. First, the thing will have to move the gear forks/sliders around to select 2nd gear on the even shaft, probably having to do some "rev matching" and monkey motion with the electronically controlled throttle and the clutch on the even shaft to get things synced for the shift, before it finally swaps clutches and fully engages 2nd gear.
In short, it's awesome when you do exactly what it expects, and it's a bit weird and quirky when you don't. For autocrossing or track use, if you happen to hit max revs in a gear, the Audi/VW system automatically upshifts (whether you ask for it with the paddle or shifter or not). We had one autocross course last year with some very tight maneuvers out of the start, and the thing shifted to second right at the first point where you had to brake in the really tight stuff. It was doubly annoying. You're trying to slow down as it's dumping high revs onto 2nd gear for you, which means you're fighting the engine with the brakes. On top of that, you're now going through a corner that you want to be in first at the exit, and the thing is in second, so you have to do a downshift with the paddles (or shifter) after slowing enough that it will allow you to go back to first. There can be similar issues under braking near the end of any straight on a track or autocross course. If you go from full throttle to hard braking, at best you're going to face a longer downshift time. It may even ignore your request if you tap the paddle when you're moving too fast for your target gear. You may be able to tune around some of this, depending on the implementation.
To answer your other questions. If you put it in "D", it works like an automatic. When you stop, the thing is smart enough to disengage the clutches for you. When you press the gas, it will engage the clutches slowly and take off. The one difference from a conventional automatic is that it won't "creep" forward if you take your foot off the brake. You have to at least touch the throttle a little bit to make it move.
If you use the manual shift mode, you can go up one with the "+" and down one with the "-". You can use the paddle or the shifter on the VW/Audi, your choice. If you want to shift multiple gears, push it multiple times. If you ask for something impossible (try dropping to 1st at 70mph, for example), the VW/Audi system just ignores you. In "competition" modes, I hope Chevy queues up the requests, so if you request 3rd for example as you're braking for a corner you plan to exit in 3rd, it should get you to 3rd when it's appropriate for your speed, even if you asked for it when it wasn't possible. I wish the VW/Audi did that.
The driver interface, from the driver's point of view, it looks like any other automatic. You've got a shift lever with the normal "PRND" markings, and a gate to a "+/-" section. If you move it to the manual mode (slide it over so it moves in the "+/-" section, the paddles on the column are also activated. It seems the same as any other paddle shift automatic.
Operationally, and I'm talking about what's happening inside the transmission, it's much more like a manual transmission with a few key differences. On the Audi/VW system, there is no torque converter. There are just the 2 clutches. The biggest difference is that where there's one pinion/output shaft in a conventional manual gearbox, the DSG (their name for their DCT) has two shafts. One shaft has the odd numbered gears (1,3,5[,7]), and the other shaft has the even numbered gears (2,4,7[,8]). Each shaft has a clutch that is controlled by the computer. The gears on these shafts are very similar to gears in a manual transmission, with dog rings that slide to engage the gears. I'm not sure if there are synchronizers or not, but the operational movements inside are very similar to a manual transmission, with movement controlled by solenoids and hydraulics. The VW/Audi system in the transverse mounted gearbox has a pinion gear on each of the shafts (two pinion gears on the same ring gear). That simplifies things a bit and reduces some of the parts count and gear interfaces. If the ME C8 has the diff in the same case with the gears, they will probably have a similar arrangement.
In normal operation, the computer will select the "next" gear on the opposite shaft, and when the time comes to shift, the current clutch will disengage as the other clutch engages, transferring power nearly instantly from one shaft to the other. As long as you're doing what the computer expects, it all works good. But if you were mildly accelerating in 3rd gear (with it in "manual shift mode"), the computer is going to have 4th gear all engaged and ready to go, waiting on your to tap the "+" paddle or tap the stick in the "+" direction. Lets's say you're approaching a curve in the road, though, so instead, you left foot brake, taking your foot off the throttle pedal at the same time, and hit the "-" paddle (or tap the stick in the "-" direction), the shift might take longer than you expect, and perhaps longer than a modern automatic. First, the thing will have to move the gear forks/sliders around to select 2nd gear on the even shaft, probably having to do some "rev matching" and monkey motion with the electronically controlled throttle and the clutch on the even shaft to get things synced for the shift, before it finally swaps clutches and fully engages 2nd gear.
In short, it's awesome when you do exactly what it expects, and it's a bit weird and quirky when you don't. For autocrossing or track use, if you happen to hit max revs in a gear, the Audi/VW system automatically upshifts (whether you ask for it with the paddle or shifter or not). We had one autocross course last year with some very tight maneuvers out of the start, and the thing shifted to second right at the first point where you had to brake in the really tight stuff. It was doubly annoying. You're trying to slow down as it's dumping high revs onto 2nd gear for you, which means you're fighting the engine with the brakes. On top of that, you're now going through a corner that you want to be in first at the exit, and the thing is in second, so you have to do a downshift with the paddles (or shifter) after slowing enough that it will allow you to go back to first. There can be similar issues under braking near the end of any straight on a track or autocross course. If you go from full throttle to hard braking, at best you're going to face a longer downshift time. It may even ignore your request if you tap the paddle when you're moving too fast for your target gear. You may be able to tune around some of this, depending on the implementation.
To answer your other questions. If you put it in "D", it works like an automatic. When you stop, the thing is smart enough to disengage the clutches for you. When you press the gas, it will engage the clutches slowly and take off. The one difference from a conventional automatic is that it won't "creep" forward if you take your foot off the brake. You have to at least touch the throttle a little bit to make it move.
If you use the manual shift mode, you can go up one with the "+" and down one with the "-". You can use the paddle or the shifter on the VW/Audi, your choice. If you want to shift multiple gears, push it multiple times. If you ask for something impossible (try dropping to 1st at 70mph, for example), the VW/Audi system just ignores you. In "competition" modes, I hope Chevy queues up the requests, so if you request 3rd for example as you're braking for a corner you plan to exit in 3rd, it should get you to 3rd when it's appropriate for your speed, even if you asked for it when it wasn't possible. I wish the VW/Audi did that.
Again, thank you.
12-03-2018, 12:02 AM
#42
E-Ray, 3LZ, ZER, LIFT
Member Since: Sep 2007
Location: NE South Carolina
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In normal operation, the computer will select the "next" gear on the opposite shaft, and when the time comes to shift, the current clutch will disengage as the other clutch engages, transferring power nearly instantly from one shaft to the other. As long as you're doing what the computer expects, it all works good. But if you were mildly accelerating in 3rd gear (with it in "manual shift mode"), the computer is going to have 4th gear all engaged and ready to go, waiting on your to tap the "+" paddle or tap the stick in the "+" direction. Lets's say you're approaching a curve in the road, though, so instead, you left foot brake, taking your foot off the throttle pedal at the same time, and hit the "-" paddle (or tap the stick in the "-" direction), the shift might take longer than you expect, and perhaps longer than a modern automatic. First, the thing will have to move the gear forks/sliders around to select 2nd gear on the even shaft, probably having to do some "rev matching" and monkey motion with the electronically controlled throttle and the clutch on the even shaft to get things synced for the shift, before it finally swaps clutches and fully engages 2nd gear.
In short, it's awesome when you do exactly what it expects, and it's a bit weird and quirky when you don't. .
My reason was my favorite shifts with the M7, i.e. on an Interstate I go from 7th directly to direct drive 4th for an exit ramp. No need to go through two other overdrives to get where I want to be 4th gear. I understand the next gear the computer would have selected would be 6th (we'll call that Trans 2 with gears 2nd, 4th and 6th) and it would take a bit longer to get the shift forks in the 4th position but when I make that 7th to 4th selection now it's casually!
Also understood skipping one gear, also very common for me, 5th to 3rd would not be possible as both are in Trans 1(which for this discussion has 1st, 3rd, 5th and 7th.) Unless both clutches can be disengaged so the shift forks in Trans 1 can go to 3rd gear from 5th by disengaging that clutch, shifting and then engaging that same clutch while Trans 2 does nothing! Therefore if I had flipped an auto/manual two position switch (that indicated use the shift level position for gear selection,) when I put the lever from 5th to 3rd, the sequence would be:
1) disengage both clutches
2) place the shift fork in Trans 1 from 5th to 3rd
3) engage the clutch to Trans 1
4) the computer could do it's thing controlling clutch operation
TAKING OFF FROM A STOP:
The other issue was why have the slight drag in the clutch when waiting to take off from a light which I read is an issue when stopped and the Trans is in 1st gear, which I understand is what at least some DCT use. Rather have a 3rd pedal NOT mechanically liked only activated by sensors. It would NOT control how the clutch is engaged, which the computer can do, but when it should start. That "pedal" would only be used when starting not during gear shifts.
As my old boss would say, "I'm solving a problem not known to exist using a method known not to work," BUT when I hear the issues with starting and shifting it would look like replacing the computer algorithm logic with what I know I want to do could be an option. There would be a switch to select what I'll call this "manual mode."
Perhaps I'll just have to learn to press the paddles multiple times more quickly!
I can practice with the Rev Match paddles! Guess I'll have to wait for the C8 to see! Maybe GM and Tremic will have a good solution for launch.Pic is just a suggestion for a lever that is NOT connected mechanically just provides desired gear selection to the computer that still controls how it's activated.
Last edited by JerryU; 12-03-2018 at 12:41 AM.
12-03-2018, 06:41 AM
#43
Team Owner
I don't know everything about what they will use or do with the C8. I've done quite a bit of driving and coaching (autocross, track time) in VW/Audi cars with their DSG.
The driver interface, from the driver's point of view, it looks like any other automatic. You've got a shift lever with the normal "PRND" markings, and a gate to a "+/-" section. If you move it to the manual mode (slide it over so it moves in the "+/-" section, the paddles on the column are also activated. It seems the same as any other paddle shift automatic.
Operationally, and I'm talking about what's happening inside the transmission, it's much more like a manual transmission with a few key differences. On the Audi/VW system, there is no torque converter. There are just the 2 clutches. The biggest difference is that where there's one pinion/output shaft in a conventional manual gearbox, the DSG (their name for their DCT) has two shafts. One shaft has the odd numbered gears (1,3,5[,7]), and the other shaft has the even numbered gears (2,4,7[,8]). Each shaft has a clutch that is controlled by the computer. The gears on these shafts are very similar to gears in a manual transmission, with dog rings that slide to engage the gears. I'm not sure if there are synchronizers or not, but the operational movements inside are very similar to a manual transmission, with movement controlled by solenoids and hydraulics. The VW/Audi system in the transverse mounted gearbox has a pinion gear on each of the shafts (two pinion gears on the same ring gear). That simplifies things a bit and reduces some of the parts count and gear interfaces. If the ME C8 has the diff in the same case with the gears, they will probably have a similar arrangement.
In normal operation, the computer will select the "next" gear on the opposite shaft, and when the time comes to shift, the current clutch will disengage as the other clutch engages, transferring power nearly instantly from one shaft to the other. As long as you're doing what the computer expects, it all works good. But if you were mildly accelerating in 3rd gear (with it in "manual shift mode"), the computer is going to have 4th gear all engaged and ready to go, waiting on your to tap the "+" paddle or tap the stick in the "+" direction. Lets's say you're approaching a curve in the road, though, so instead, you left foot brake, taking your foot off the throttle pedal at the same time, and hit the "-" paddle (or tap the stick in the "-" direction), the shift might take longer than you expect, and perhaps longer than a modern automatic. First, the thing will have to move the gear forks/sliders around to select 2nd gear on the even shaft, probably having to do some "rev matching" and monkey motion with the electronically controlled throttle and the clutch on the even shaft to get things synced for the shift, before it finally swaps clutches and fully engages 2nd gear.
In short, it's awesome when you do exactly what it expects, and it's a bit weird and quirky when you don't. For autocrossing or track use, if you happen to hit max revs in a gear, the Audi/VW system automatically upshifts (whether you ask for it with the paddle or shifter or not). We had one autocross course last year with some very tight maneuvers out of the start, and the thing shifted to second right at the first point where you had to brake in the really tight stuff. It was doubly annoying. You're trying to slow down as it's dumping high revs onto 2nd gear for you, which means you're fighting the engine with the brakes. On top of that, you're now going through a corner that you want to be in first at the exit, and the thing is in second, so you have to do a downshift with the paddles (or shifter) after slowing enough that it will allow you to go back to first. There can be similar issues under braking near the end of any straight on a track or autocross course. If you go from full throttle to hard braking, at best you're going to face a longer downshift time. It may even ignore your request if you tap the paddle when you're moving too fast for your target gear. You may be able to tune around some of this, depending on the implementation.
To answer your other questions. If you put it in "D", it works like an automatic. When you stop, the thing is smart enough to disengage the clutches for you. When you press the gas, it will engage the clutches slowly and take off. The one difference from a conventional automatic is that it won't "creep" forward if you take your foot off the brake. You have to at least touch the throttle a little bit to make it move.
If you use the manual shift mode, you can go up one with the "+" and down one with the "-". You can use the paddle or the shifter on the VW/Audi, your choice. If you want to shift multiple gears, push it multiple times. If you ask for something impossible (try dropping to 1st at 70mph, for example), the VW/Audi system just ignores you. In "competition" modes, I hope Chevy queues up the requests, so if you request 3rd for example as you're braking for a corner you plan to exit in 3rd, it should get you to 3rd when it's appropriate for your speed, even if you asked for it when it wasn't possible. I wish the VW/Audi did that.
The driver interface, from the driver's point of view, it looks like any other automatic. You've got a shift lever with the normal "PRND" markings, and a gate to a "+/-" section. If you move it to the manual mode (slide it over so it moves in the "+/-" section, the paddles on the column are also activated. It seems the same as any other paddle shift automatic.
Operationally, and I'm talking about what's happening inside the transmission, it's much more like a manual transmission with a few key differences. On the Audi/VW system, there is no torque converter. There are just the 2 clutches. The biggest difference is that where there's one pinion/output shaft in a conventional manual gearbox, the DSG (their name for their DCT) has two shafts. One shaft has the odd numbered gears (1,3,5[,7]), and the other shaft has the even numbered gears (2,4,7[,8]). Each shaft has a clutch that is controlled by the computer. The gears on these shafts are very similar to gears in a manual transmission, with dog rings that slide to engage the gears. I'm not sure if there are synchronizers or not, but the operational movements inside are very similar to a manual transmission, with movement controlled by solenoids and hydraulics. The VW/Audi system in the transverse mounted gearbox has a pinion gear on each of the shafts (two pinion gears on the same ring gear). That simplifies things a bit and reduces some of the parts count and gear interfaces. If the ME C8 has the diff in the same case with the gears, they will probably have a similar arrangement.
In normal operation, the computer will select the "next" gear on the opposite shaft, and when the time comes to shift, the current clutch will disengage as the other clutch engages, transferring power nearly instantly from one shaft to the other. As long as you're doing what the computer expects, it all works good. But if you were mildly accelerating in 3rd gear (with it in "manual shift mode"), the computer is going to have 4th gear all engaged and ready to go, waiting on your to tap the "+" paddle or tap the stick in the "+" direction. Lets's say you're approaching a curve in the road, though, so instead, you left foot brake, taking your foot off the throttle pedal at the same time, and hit the "-" paddle (or tap the stick in the "-" direction), the shift might take longer than you expect, and perhaps longer than a modern automatic. First, the thing will have to move the gear forks/sliders around to select 2nd gear on the even shaft, probably having to do some "rev matching" and monkey motion with the electronically controlled throttle and the clutch on the even shaft to get things synced for the shift, before it finally swaps clutches and fully engages 2nd gear.
In short, it's awesome when you do exactly what it expects, and it's a bit weird and quirky when you don't. For autocrossing or track use, if you happen to hit max revs in a gear, the Audi/VW system automatically upshifts (whether you ask for it with the paddle or shifter or not). We had one autocross course last year with some very tight maneuvers out of the start, and the thing shifted to second right at the first point where you had to brake in the really tight stuff. It was doubly annoying. You're trying to slow down as it's dumping high revs onto 2nd gear for you, which means you're fighting the engine with the brakes. On top of that, you're now going through a corner that you want to be in first at the exit, and the thing is in second, so you have to do a downshift with the paddles (or shifter) after slowing enough that it will allow you to go back to first. There can be similar issues under braking near the end of any straight on a track or autocross course. If you go from full throttle to hard braking, at best you're going to face a longer downshift time. It may even ignore your request if you tap the paddle when you're moving too fast for your target gear. You may be able to tune around some of this, depending on the implementation.
To answer your other questions. If you put it in "D", it works like an automatic. When you stop, the thing is smart enough to disengage the clutches for you. When you press the gas, it will engage the clutches slowly and take off. The one difference from a conventional automatic is that it won't "creep" forward if you take your foot off the brake. You have to at least touch the throttle a little bit to make it move.
If you use the manual shift mode, you can go up one with the "+" and down one with the "-". You can use the paddle or the shifter on the VW/Audi, your choice. If you want to shift multiple gears, push it multiple times. If you ask for something impossible (try dropping to 1st at 70mph, for example), the VW/Audi system just ignores you. In "competition" modes, I hope Chevy queues up the requests, so if you request 3rd for example as you're braking for a corner you plan to exit in 3rd, it should get you to 3rd when it's appropriate for your speed, even if you asked for it when it wasn't possible. I wish the VW/Audi did that.
https://www.audi-technology-portal.d...es/s-tronic_en
12-03-2018, 07:23 AM
#44
Safety Car
I believe that you are wrong about Audi having two pinon gears feeding one ring gear in their DCT.
https://www.audi-technology-portal.d...es/s-tronic_en
https://www.audi-technology-portal.d...es/s-tronic_en
Your link is to the longitudinal engine, AWD version. In that one, you are correct.
I'm honestly not entirely sure about the current generation of the FWD, transverse engine versions (Golf GTI/Golf R). The earliest examples definitely had two pinion shafts in the gearbox, and as far as I know that hasn't changed, but I haven't checked up on it that closely.
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12-03-2018, 08:54 AM
#46
So you'd like to point out that some DCTs can automatically downshift more than one gear at a time? Thanks for proving my point about DCTs being automatic transmissions.
This is an automatic behavior; not something that you control. Unless your car has more than "+1" and "-1" paddles.
In my real manual transmission I can shift from 4th to 2nd by slotting that gear.
This is an automatic behavior; not something that you control. Unless your car has more than "+1" and "-1" paddles.
In my real manual transmission I can shift from 4th to 2nd by slotting that gear.
Good deflection. You had made the categorical statement that a DCT cannot skip gears.
It's no secret that a DCT is primarily an automatic. Tremec refer to their DCT as an "automated transmission" for instance. VW refer to the trans in my Golf R as a "7-speed automatic DSG".
12-03-2018, 09:27 AM
#47
Burning Brakes
Owner of a tuned 2011 BMW Z4 sDrive 35is here (7 speed Getrag DCT)
DCT is very fun and really takes a lot of work out of your hands, don't think of it as "soulless" but more like "efficient"
Like others have said, there is no torque converter, it's not like a typical Auto by any means - when you hit the gas it GOES just like a normal stick.
Frankly, I'm excited the C8 will get one however......it normally takes automakers a generation or 2 to get it right (BMW's 1st gen DCT vs the current Gen for example) so I'm hoping GM just sources from Getrag.
DCT is very fun and really takes a lot of work out of your hands, don't think of it as "soulless" but more like "efficient"
Like others have said, there is no torque converter, it's not like a typical Auto by any means - when you hit the gas it GOES just like a normal stick.
Frankly, I'm excited the C8 will get one however......it normally takes automakers a generation or 2 to get it right (BMW's 1st gen DCT vs the current Gen for example) so I'm hoping GM just sources from Getrag.
12-03-2018, 09:36 AM
#48
Team Owner
The key word in what I posted earlier is transverse engine (FWD).
Your link is to the longitudinal engine, AWD version. In that one, you are correct.
I'm honestly not entirely sure about the current generation of the FWD, transverse engine versions (Golf GTI/Golf R). The earliest examples definitely had two pinion shafts in the gearbox, and as far as I know that hasn't changed, but I haven't checked up on it that closely.
Your link is to the longitudinal engine, AWD version. In that one, you are correct.
I'm honestly not entirely sure about the current generation of the FWD, transverse engine versions (Golf GTI/Golf R). The earliest examples definitely had two pinion shafts in the gearbox, and as far as I know that hasn't changed, but I haven't checked up on it that closely.
Neither the Porsche or the Mercedes transaxle DCT's have two pinion gears either as that is not how DCT's are designed.
You had better check the link I posted again as the lower right hand drawing is for the Audi with a transverse engine/transaxle. Note that the only ring gear and pinion is for the PTO and it ONLY has a single pinion gear, not a double pinion gear as you said.
Last edited by JoesC5; 12-03-2018 at 11:54 AM.
12-03-2018, 10:14 AM
#49
Le Mans Master
Originally Posted by jim2092
The definition of when a thread is getting completely out of hand is when Bill Dearborn is being called a troll or a dolt.
Are others not entitled to their own opinions?
He is claiming the experience of playing with toy paddles is the same as driving a manual and he takes a jab at those who disagree with him. That is an opinion related item which I and others do not agree with.
12-03-2018, 12:07 PM
#50
Safety Car
In a FWD transverse engine application, there is no need for ring gear and pinion as both the engine and the front wheel drive axles are parallel to each other. You only need a ring gear and pinion if the power flow is changing direction at 90 degrees. In the case of the Audi, the transverse engine/transmission there is a PTO to provide power to a prop shaft for the rear axle differential for the Audi with the Quattro AWD system.
Neither the Porsche or the Mercedes transaxle DCT's have two pinion gears either as that is not how DCT's are designed.
Neither the Porsche or the Mercedes transaxle DCT's have two pinion gears either as that is not how DCT's are designed.
Now, the ring and pinion set in a FWD, transverse engine transmission is a pair of helical cut gears, because the shafts are parallel. That's slightly different than the bevel cut gears common in a differential assembly for a conventional (longitudinal) drive line. On VW/Audi transverse transmissions, the pinion gear is actually machined directly onto the end of the "output shaft" of the transmission section, and it's called a "pinion shaft."
But I assure you, there's a ring gear in every FWD transmission I've ever seen on the road, and bolted to the center of that ring gear is some type of differential.
Here's a video showing a VW/Audi transverse DSG partially disassembled, and it clearly does have two pinion shafts. He calls them 'output shafts" but they are the output of the transmission and they each end with a pinion gear that turns the ring gear and differential assembly.
Last edited by C6_Racer_X; 12-03-2018 at 12:13 PM.
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JerryU (12-03-2018)
12-03-2018, 12:56 PM
#52
Melting Slicks
Good deflection. You had made the categorical statement that a DCT cannot skip gears.
It's no secret that a DCT is primarily an automatic. Tremec refer to their DCT as an "automated transmission" for instance. VW refer to the trans in my Golf R as a "7-speed automatic DSG".
It's no secret that a DCT is primarily an automatic. Tremec refer to their DCT as an "automated transmission" for instance. VW refer to the trans in my Golf R as a "7-speed automatic DSG".
There may be other things that a DCT does in automatic mode that I did not mention in my previous comments. But writing detailed posts about such features only serves to bolster my remarks that a DCT is absolutely not a manual transmission. I see people are continuing to spread FUD about this. Probably because they want to believe in their hearts that it is true. It's not. Go back to bed.
12-03-2018, 01:23 PM
#53
You guys need to stop thinking about transmissions in "automatic" or "manual" sense. Today's transmissions all start to blur the line heavily that there are no longer "pure" automatics or "pure" manuals.
Today's transmissions breaks down simply into 3 categories. Planetary driven gear selectors, belt driven gear selectors, and mechanically driven gear selectors. Currently GM offers 2 out of the three on the Corvette, but I will try and explain all three if possible.
Planetary Gear
Planetary gear driven transmission is what most associate with an automatic transmission. There are multiple gear sets, with sun gear and planetary gears packed into a large rotating cylinder. Depending on which "clutch pack" is engaged in the gear, the outside cylinder spins at a different speed than the inside sun gear, thus the change in gear ratio. You can stack multiple planetary gear sets together to create more than 2 sets of ratios, for example, the outside cylinder may spin a second planetary gear set's sun gear, and that in turn gives you 2x2 =4 sets of different gear ratios. That's why typically you see automatic transmissions offer gears in multiples of 2. A series of complicated valves govern pressure to each clutch pack resulting in different gear changes and ratios.
Planetary gear driven transmissions have all the planetary gear sets engaged and spinning at all times, therefore it requires a torque converter to operate because you can't disengage a spinning mechanical gear. "Neutral" in an automatic simply means the driveshaft is disengaged at the torque converter, not the transmission's input shaft. The A8 equipped on the C7 Corvette is a conventional, planetary gear driven "automatic."
Mechanical Gear
This is where "automatic" and "manual" blur the line. This type of transmission has mechanical levers selecting 2 sets of spinning shafts. Depending on the position of the lever, a set of gears spins the intermediary shaft, which in turn spins the final output shaft. The relationship between the engaged gear by the lever on the input shaft, to the gear spinning on the intermediate shaft, basically determines the gear ratio. That's why mechanical gear transmissions can come in just about any number of gears, it really just depends on the number of gear sets packed into the transmission. This is typically the "performance" oriented transmission because parasitic power loss is minimized (all power transfers are done from mechanical gear to gear, gears not selected are not spinning thus less spinning mass to deal with compared to planetary gear sets). But since power transmission is done via mechanical gears, to go from one gear to another requires a momentary "disengagement" of the input shaft to the transmission because changing gear ratios means the engine and the gear speed is not synchronized.
Traditionally this is accomplished by the use of a clutch, a clutch pedal, and since the late 1970s, a synchronizer that spins up the next gear as it's selected. Lately, Formula racing technology has leaked down to the masses and beginning with sequential manual gearboxes, or SMGs, we have computer operated clutch and gear selectors that takes the operation of the clutch out of the hands of the driver. SMGs, unfortunately, still require a momentary disengagement of the gear and spinning up of the next gear in time for clutch engagement, therefore gear to gear shifting can never be as smooth as a planetary gear box set auto.
With the advent of dual clutch systems, modern transmissions based on mechanical gear sets have come much closer to a planetary gear set auto in terms of gear to gear transition while retaining the inherent performance advantages of mechanical gear boxes. Since the transition from gear to gear no longer involves a disengagement of one single clutch, then selecting the next gear, a dual clutch system (BMW's DCT. Porsche's PDK) keeps a clutch engaged at all times (unless stopped in neutral) and simultaneously engage the next clutch, and since the second intermediate shaft is connected already, the next gear to be selected is spinning at the same speed as the currently selected gear, thus seamless and instantaneous gear shifts that are as smooth as an "auto" and quick as lightning, while losing none of the energy transfer that's associated with spinning multiple sets of massive planetary and sun gears and its housing.
Currently dual clutch systems are the pinnacle of performance transmission technology, since mechanical gear set has zero limitations as to how many gears (well, size and packaging really), physical ratio, speed, efficiency, and power delivery. The lack of a torque converter also means lighter, more durable in the long haul. The drawback of a dual clutch system vs. a mechanically operated clutch is the computer control module are bank to replace, the dual spinning clutches are bank to replace once worn, but the benefit of speed and performance can not be overlooked. The benefit of a planetary driven system over dual clutch is again, the computer control module vs a mechanical valve body means significant cost, mechanical hydraulic actuators vs. switch gears in the valve body means cost to repair and replace, and a torque converter can handle significantly more engine torque output than your typical mechanical gears.
Belt Driven
I'm not going to go into details on this, but this is basically what you called CVT, or continuously variable transmission. Out of all three types of gearing systems, this is the newest innovation, but it is also fraught with the most issues. It's basically driven by a torque converter, and 2 spinning top looking gear set varies the width of the gear to change the size and ratio between the input and output shaft. There are no physical gear ratios as it can infinitely vary between the two. It is by far the most efficient gear set type, and it's far closer to a traditional "automatic" with planetary gear set than mechanical gear set. Its benefits are that you can literally keep the engine spinning at peak torque and just alter the gear ratio based on speed, load, and acceleration. For this reason alone, you see CVTs installed on more and more cars today because it can keep cars operating at peak fuel efficiency, or be tuned to offer higher multiplication and keep engine operating at peak POWER output. But unlike planetary gear sets and mechanical gear sets, the power transfer happens via a chain or belt drive and the amount of power and torque CVTs can handle today is but a fraction of what traditional planetary gear and mechanical gear transmissions can tolerate.
NOW THAT'S OUT OF THE WAY.
Dual clutch systems can operate JUST like a traditional automatic using planetary gear sets, since a computer control module can control the amount of clutch slip to make moving from a dead stop as smooth as a torque converter operated planetary auto. The control module can also be programmed to pick shift points based on engine load, throttle position, power output...etc. For performance purposes the shifts happen in a blink of an eye, no more engine surges between gear shifts, or time delay between torque converter surge and lock-up of the torque converter clutch. Porsche's PDK and BMW's DCT in automatic mode drives JUST LIKE a traditional planetary auto, but shifts like an F1 car when you demand it. The draw-back has always been cost, and the complexity of the control system. BMW's DCT is prone for the computer control module to overheat under track conditions, causing the shift to become unresponsive, or the hydraulic actuated gear selector needing service every 60,000 miles (conveniently just outside of the warranty period) to the tune of $2,000-4,000.
But for sheer performance, it can't be beat. It really is like having the best of both worlds, when it works. Lightning fast shifts like an F1 car, lazy, no touch operation like an auto.
Today's transmissions breaks down simply into 3 categories. Planetary driven gear selectors, belt driven gear selectors, and mechanically driven gear selectors. Currently GM offers 2 out of the three on the Corvette, but I will try and explain all three if possible.
Planetary Gear
Planetary gear driven transmission is what most associate with an automatic transmission. There are multiple gear sets, with sun gear and planetary gears packed into a large rotating cylinder. Depending on which "clutch pack" is engaged in the gear, the outside cylinder spins at a different speed than the inside sun gear, thus the change in gear ratio. You can stack multiple planetary gear sets together to create more than 2 sets of ratios, for example, the outside cylinder may spin a second planetary gear set's sun gear, and that in turn gives you 2x2 =4 sets of different gear ratios. That's why typically you see automatic transmissions offer gears in multiples of 2. A series of complicated valves govern pressure to each clutch pack resulting in different gear changes and ratios.
Planetary gear driven transmissions have all the planetary gear sets engaged and spinning at all times, therefore it requires a torque converter to operate because you can't disengage a spinning mechanical gear. "Neutral" in an automatic simply means the driveshaft is disengaged at the torque converter, not the transmission's input shaft. The A8 equipped on the C7 Corvette is a conventional, planetary gear driven "automatic."
Mechanical Gear
This is where "automatic" and "manual" blur the line. This type of transmission has mechanical levers selecting 2 sets of spinning shafts. Depending on the position of the lever, a set of gears spins the intermediary shaft, which in turn spins the final output shaft. The relationship between the engaged gear by the lever on the input shaft, to the gear spinning on the intermediate shaft, basically determines the gear ratio. That's why mechanical gear transmissions can come in just about any number of gears, it really just depends on the number of gear sets packed into the transmission. This is typically the "performance" oriented transmission because parasitic power loss is minimized (all power transfers are done from mechanical gear to gear, gears not selected are not spinning thus less spinning mass to deal with compared to planetary gear sets). But since power transmission is done via mechanical gears, to go from one gear to another requires a momentary "disengagement" of the input shaft to the transmission because changing gear ratios means the engine and the gear speed is not synchronized.
Traditionally this is accomplished by the use of a clutch, a clutch pedal, and since the late 1970s, a synchronizer that spins up the next gear as it's selected. Lately, Formula racing technology has leaked down to the masses and beginning with sequential manual gearboxes, or SMGs, we have computer operated clutch and gear selectors that takes the operation of the clutch out of the hands of the driver. SMGs, unfortunately, still require a momentary disengagement of the gear and spinning up of the next gear in time for clutch engagement, therefore gear to gear shifting can never be as smooth as a planetary gear box set auto.
With the advent of dual clutch systems, modern transmissions based on mechanical gear sets have come much closer to a planetary gear set auto in terms of gear to gear transition while retaining the inherent performance advantages of mechanical gear boxes. Since the transition from gear to gear no longer involves a disengagement of one single clutch, then selecting the next gear, a dual clutch system (BMW's DCT. Porsche's PDK) keeps a clutch engaged at all times (unless stopped in neutral) and simultaneously engage the next clutch, and since the second intermediate shaft is connected already, the next gear to be selected is spinning at the same speed as the currently selected gear, thus seamless and instantaneous gear shifts that are as smooth as an "auto" and quick as lightning, while losing none of the energy transfer that's associated with spinning multiple sets of massive planetary and sun gears and its housing.
Currently dual clutch systems are the pinnacle of performance transmission technology, since mechanical gear set has zero limitations as to how many gears (well, size and packaging really), physical ratio, speed, efficiency, and power delivery. The lack of a torque converter also means lighter, more durable in the long haul. The drawback of a dual clutch system vs. a mechanically operated clutch is the computer control module are bank to replace, the dual spinning clutches are bank to replace once worn, but the benefit of speed and performance can not be overlooked. The benefit of a planetary driven system over dual clutch is again, the computer control module vs a mechanical valve body means significant cost, mechanical hydraulic actuators vs. switch gears in the valve body means cost to repair and replace, and a torque converter can handle significantly more engine torque output than your typical mechanical gears.
Belt Driven
I'm not going to go into details on this, but this is basically what you called CVT, or continuously variable transmission. Out of all three types of gearing systems, this is the newest innovation, but it is also fraught with the most issues. It's basically driven by a torque converter, and 2 spinning top looking gear set varies the width of the gear to change the size and ratio between the input and output shaft. There are no physical gear ratios as it can infinitely vary between the two. It is by far the most efficient gear set type, and it's far closer to a traditional "automatic" with planetary gear set than mechanical gear set. Its benefits are that you can literally keep the engine spinning at peak torque and just alter the gear ratio based on speed, load, and acceleration. For this reason alone, you see CVTs installed on more and more cars today because it can keep cars operating at peak fuel efficiency, or be tuned to offer higher multiplication and keep engine operating at peak POWER output. But unlike planetary gear sets and mechanical gear sets, the power transfer happens via a chain or belt drive and the amount of power and torque CVTs can handle today is but a fraction of what traditional planetary gear and mechanical gear transmissions can tolerate.
NOW THAT'S OUT OF THE WAY.
Dual clutch systems can operate JUST like a traditional automatic using planetary gear sets, since a computer control module can control the amount of clutch slip to make moving from a dead stop as smooth as a torque converter operated planetary auto. The control module can also be programmed to pick shift points based on engine load, throttle position, power output...etc. For performance purposes the shifts happen in a blink of an eye, no more engine surges between gear shifts, or time delay between torque converter surge and lock-up of the torque converter clutch. Porsche's PDK and BMW's DCT in automatic mode drives JUST LIKE a traditional planetary auto, but shifts like an F1 car when you demand it. The draw-back has always been cost, and the complexity of the control system. BMW's DCT is prone for the computer control module to overheat under track conditions, causing the shift to become unresponsive, or the hydraulic actuated gear selector needing service every 60,000 miles (conveniently just outside of the warranty period) to the tune of $2,000-4,000.
But for sheer performance, it can't be beat. It really is like having the best of both worlds, when it works. Lightning fast shifts like an F1 car, lazy, no touch operation like an auto.
The following 5 users liked this post by The HACK:
dreamr616 (12-03-2018),
JerryU (12-03-2018),
range96 (12-03-2018),
thebishman (12-03-2018),
TheSenator (12-03-2018)
12-03-2018, 01:27 PM
#54
Instructor
I'm not going to add a bunch to what's already said, but as a former C6Z owner, who went to the darkside (GTR), and is looking to come back to the corvette in a C8, I couldn't be more excited about the DCT possibility. It's the ONE thing I love about the R35. It's an automatic, that feels like a manual. It's lightning fast, and reminds me of an air shifter on drag bikes.
Also, a few things to note, when the R35 has the GR6 transmission in R mode, it will not "shift for you", you can bang off the rev limiter as long as you want. Also, when I touch the "-" paddle to downshift twice, it doesn't "skip a gear", it switches through two gears, just very quickly.
TCM programming will become a HUGE thing for C8 if/when it is offered with this style transmission. I'm able to adjust my line pressure, clutch capacity, etc. to fine tune how I want the transmission to act.
Also, a few things to note, when the R35 has the GR6 transmission in R mode, it will not "shift for you", you can bang off the rev limiter as long as you want. Also, when I touch the "-" paddle to downshift twice, it doesn't "skip a gear", it switches through two gears, just very quickly.
TCM programming will become a HUGE thing for C8 if/when it is offered with this style transmission. I'm able to adjust my line pressure, clutch capacity, etc. to fine tune how I want the transmission to act.
Last edited by TheSenator; 12-03-2018 at 01:28 PM.
12-03-2018, 02:12 PM
#55
E-Ray, 3LZ, ZER, LIFT
Member Since: Sep 2007
Location: NE South Carolina
Posts: 29,476
Received 9,617 Likes
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6,624 Posts
Weird! I work on VW FWD manual transmissions all the time, and the two most common things I do to them is 1) upgrade the differential from an open diff to some type of limited slip diff and 2) change the ring and pinion set, usually to a numerically higher ("shorter") ratio.
Now, the ring and pinion set in a FWD, transverse engine transmission is a pair of helical cut gears, because the shafts are parallel. That's slightly different than the bevel cut gears common in a differential assembly for a conventional (longitudinal) drive line. On VW/Audi transverse transmissions, the pinion gear is actually machined directly onto the end of the "output shaft" of the transmission section, and it's called a "pinion shaft."
But I assure you, there's a ring gear in every FWD transmission I've ever seen on the road, and bolted to the center of that ring gear is some type of differential.
Here's a video showing a VW/Audi transverse DSG partially disassembled, and it clearly does have two pinion shafts. He calls them 'output shafts" but they are the output of the transmission and they each end with a pinion gear that turns the ring gear and differential assembly.
https://www.youtube.com/watch?v=mj1Vk7SE-TI
Now, the ring and pinion set in a FWD, transverse engine transmission is a pair of helical cut gears, because the shafts are parallel. That's slightly different than the bevel cut gears common in a differential assembly for a conventional (longitudinal) drive line. On VW/Audi transverse transmissions, the pinion gear is actually machined directly onto the end of the "output shaft" of the transmission section, and it's called a "pinion shaft."
But I assure you, there's a ring gear in every FWD transmission I've ever seen on the road, and bolted to the center of that ring gear is some type of differential.
Here's a video showing a VW/Audi transverse DSG partially disassembled, and it clearly does have two pinion shafts. He calls them 'output shafts" but they are the output of the transmission and they each end with a pinion gear that turns the ring gear and differential assembly.
https://www.youtube.com/watch?v=mj1Vk7SE-TI
Yep, spur gears, shift forks, synchro rings with hydraulically operated pistons defining which shift fork moves instead of my hand!
They repeat what other's have said, the glitch comes when the computer algorithm shift sequence prediction is NOT what you plan to do!
Last edited by JerryU; 12-03-2018 at 08:15 PM.
12-03-2018, 02:45 PM
#56
Race Director
I have a SMG now and have drive a McLaren 570s with a DCT.
Mine being a 2004 it isn’t as refined as the Dct I drove but there are a few things both have advantages.
E-gear is much closer to a Manual with an automated clutch. In Sport mode all it does is shifts when you tell it, it will bounce off the rev limiter, it doesn’t cut throttle automatically for smooth shifts the driver controls it(also why many complain that it is jerky). You can shift 2 gears without it shifting into the in between gear. Also I can shift into neutral at anytime.
DCT in the 570 shifted through each gear even in down shifts, could not put it in neutral easy. For example on my E-gear if in any gear and a light ahead is red I just pull both paddles and it goes to neutral. The 570 downshifted through all grears on a stop just like an automatic would.
Mine being a 2004 it isn’t as refined as the Dct I drove but there are a few things both have advantages.
E-gear is much closer to a Manual with an automated clutch. In Sport mode all it does is shifts when you tell it, it will bounce off the rev limiter, it doesn’t cut throttle automatically for smooth shifts the driver controls it(also why many complain that it is jerky). You can shift 2 gears without it shifting into the in between gear. Also I can shift into neutral at anytime.
DCT in the 570 shifted through each gear even in down shifts, could not put it in neutral easy. For example on my E-gear if in any gear and a light ahead is red I just pull both paddles and it goes to neutral. The 570 downshifted through all grears on a stop just like an automatic would.
12-03-2018, 03:28 PM
#57
I have a SMG now and have drive a McLaren 570s with a DCT.
Mine being a 2004 it isn’t as refined as the Dct I drove but there are a few things both have advantages.
E-gear is much closer to a Manual with an automated clutch. In Sport mode all it does is shifts when you tell it, it will bounce off the rev limiter, it doesn’t cut throttle automatically for smooth shifts the driver controls it(also why many complain that it is jerky). You can shift 2 gears without it shifting into the in between gear. Also I can shift into neutral at anytime.
DCT in the 570 shifted through each gear even in down shifts, could not put it in neutral easy. For example on my E-gear if in any gear and a light ahead is red I just pull both paddles and it goes to neutral. The 570 downshifted through all grears on a stop just like an automatic would.
Mine being a 2004 it isn’t as refined as the Dct I drove but there are a few things both have advantages.
E-gear is much closer to a Manual with an automated clutch. In Sport mode all it does is shifts when you tell it, it will bounce off the rev limiter, it doesn’t cut throttle automatically for smooth shifts the driver controls it(also why many complain that it is jerky). You can shift 2 gears without it shifting into the in between gear. Also I can shift into neutral at anytime.
DCT in the 570 shifted through each gear even in down shifts, could not put it in neutral easy. For example on my E-gear if in any gear and a light ahead is red I just pull both paddles and it goes to neutral. The 570 downshifted through all grears on a stop just like an automatic would.
Not really a good comparison to a well sored DCT.
My Gallardo was 06 and the car was horrible when driven in auto mode and the clutch wear was also horrible. Replaced at 7k miles.
The Egear also has a terrible delay from take off while waiting for clutch to engage. Car had to be in neutral when at a stop light. I don't recall but it would revert to neutral at a stop light somewhere between 5 and 10 second to attempt to control clutch wear.
My 15 458 has a Getrag 7 speed DCT. Car is absolutely the best transmission I ever driven.
In auto mode is smooth and seemless as the best traditional auto transmission. In manual mode the shift both up and down are lighting quick.
There is no delay from a dead stop and car can be left in 1st gear at a stop for as long as need be.
Also is similar to the Egear that putting in neutral is a simple as pulling both paddle back at same time.
The launch control also is well sorted and unlike the Lamborghini Huracan there is no lifetime limit as to how many launch control can be used. Lamborghini is programed to a max of 250 launch control activation and they will not allow futher use of launch control.
The Getrag 7spped DCT is absolutely the smoothest and quickest transmission I have ever driven.
Puts the A8 in my Callaway sc757 to shame in both auto mode and NO COMPARISON when in manual mode.
Last edited by vetteman41960; 12-03-2018 at 03:58 PM.
12-03-2018, 03:55 PM
#58
Melting Slicks
You guys need to stop thinking about transmissions in "automatic" or "manual" sense. Today's transmissions all start to blur the line heavily that there are no longer "pure" automatics or "pure" manuals.
Today's transmissions breaks down simply into 3 categories. Planetary driven gear selectors, belt driven gear selectors, and mechanically driven gear selectors. Currently GM offers 2 out of the three on the Corvette, but I will try and explain all three if possible.
Planetary Gear
Planetary gear driven transmission is what most associate with an automatic transmission. There are multiple gear sets, with sun gear and planetary gears packed into a large rotating cylinder. Depending on which "clutch pack" is engaged in the gear, the outside cylinder spins at a different speed than the inside sun gear, thus the change in gear ratio. You can stack multiple planetary gear sets together to create more than 2 sets of ratios, for example, the outside cylinder may spin a second planetary gear set's sun gear, and that in turn gives you 2x2 =4 sets of different gear ratios. That's why typically you see automatic transmissions offer gears in multiples of 2. A series of complicated valves govern pressure to each clutch pack resulting in different gear changes and ratios.
Planetary gear driven transmissions have all the planetary gear sets engaged and spinning at all times, therefore it requires a torque converter to operate because you can't disengage a spinning mechanical gear. "Neutral" in an automatic simply means the driveshaft is disengaged at the torque converter, not the transmission's input shaft. The A8 equipped on the C7 Corvette is a conventional, planetary gear driven "automatic."
Mechanical Gear
This is where "automatic" and "manual" blur the line. This type of transmission has mechanical levers selecting 2 sets of spinning shafts. Depending on the position of the lever, a set of gears spins the intermediary shaft, which in turn spins the final output shaft. The relationship between the engaged gear by the lever on the input shaft, to the gear spinning on the intermediate shaft, basically determines the gear ratio. That's why mechanical gear transmissions can come in just about any number of gears, it really just depends on the number of gear sets packed into the transmission. This is typically the "performance" oriented transmission because parasitic power loss is minimized (all power transfers are done from mechanical gear to gear, gears not selected are not spinning thus less spinning mass to deal with compared to planetary gear sets). But since power transmission is done via mechanical gears, to go from one gear to another requires a momentary "disengagement" of the input shaft to the transmission because changing gear ratios means the engine and the gear speed is not synchronized.
Traditionally this is accomplished by the use of a clutch, a clutch pedal, and since the late 1970s, a synchronizer that spins up the next gear as it's selected. Lately, Formula racing technology has leaked down to the masses and beginning with sequential manual gearboxes, or SMGs, we have computer operated clutch and gear selectors that takes the operation of the clutch out of the hands of the driver. SMGs, unfortunately, still require a momentary disengagement of the gear and spinning up of the next gear in time for clutch engagement, therefore gear to gear shifting can never be as smooth as a planetary gear box set auto.
With the advent of dual clutch systems, modern transmissions based on mechanical gear sets have come much closer to a planetary gear set auto in terms of gear to gear transition while retaining the inherent performance advantages of mechanical gear boxes. Since the transition from gear to gear no longer involves a disengagement of one single clutch, then selecting the next gear, a dual clutch system (BMW's DCT. Porsche's PDK) keeps a clutch engaged at all times (unless stopped in neutral) and simultaneously engage the next clutch, and since the second intermediate shaft is connected already, the next gear to be selected is spinning at the same speed as the currently selected gear, thus seamless and instantaneous gear shifts that are as smooth as an "auto" and quick as lightning, while losing none of the energy transfer that's associated with spinning multiple sets of massive planetary and sun gears and its housing.
Currently dual clutch systems are the pinnacle of performance transmission technology, since mechanical gear set has zero limitations as to how many gears (well, size and packaging really), physical ratio, speed, efficiency, and power delivery. The lack of a torque converter also means lighter, more durable in the long haul. The drawback of a dual clutch system vs. a mechanically operated clutch is the computer control module are bank to replace, the dual spinning clutches are bank to replace once worn, but the benefit of speed and performance can not be overlooked. The benefit of a planetary driven system over dual clutch is again, the computer control module vs a mechanical valve body means significant cost, mechanical hydraulic actuators vs. switch gears in the valve body means cost to repair and replace, and a torque converter can handle significantly more engine torque output than your typical mechanical gears.
Belt Driven
I'm not going to go into details on this, but this is basically what you called CVT, or continuously variable transmission. Out of all three types of gearing systems, this is the newest innovation, but it is also fraught with the most issues. It's basically driven by a torque converter, and 2 spinning top looking gear set varies the width of the gear to change the size and ratio between the input and output shaft. There are no physical gear ratios as it can infinitely vary between the two. It is by far the most efficient gear set type, and it's far closer to a traditional "automatic" with planetary gear set than mechanical gear set. Its benefits are that you can literally keep the engine spinning at peak torque and just alter the gear ratio based on speed, load, and acceleration. For this reason alone, you see CVTs installed on more and more cars today because it can keep cars operating at peak fuel efficiency, or be tuned to offer higher multiplication and keep engine operating at peak POWER output. But unlike planetary gear sets and mechanical gear sets, the power transfer happens via a chain or belt drive and the amount of power and torque CVTs can handle today is but a fraction of what traditional planetary gear and mechanical gear transmissions can tolerate.
NOW THAT'S OUT OF THE WAY.
Dual clutch systems can operate JUST like a traditional automatic using planetary gear sets, since a computer control module can control the amount of clutch slip to make moving from a dead stop as smooth as a torque converter operated planetary auto. The control module can also be programmed to pick shift points based on engine load, throttle position, power output...etc. For performance purposes the shifts happen in a blink of an eye, no more engine surges between gear shifts, or time delay between torque converter surge and lock-up of the torque converter clutch. Porsche's PDK and BMW's DCT in automatic mode drives JUST LIKE a traditional planetary auto, but shifts like an F1 car when you demand it. The draw-back has always been cost, and the complexity of the control system. BMW's DCT is prone for the computer control module to overheat under track conditions, causing the shift to become unresponsive, or the hydraulic actuated gear selector needing service every 60,000 miles (conveniently just outside of the warranty period) to the tune of $2,000-4,000.
But for sheer performance, it can't be beat. It really is like having the best of both worlds, when it works. Lightning fast shifts like an F1 car, lazy, no touch operation like an auto.
Today's transmissions breaks down simply into 3 categories. Planetary driven gear selectors, belt driven gear selectors, and mechanically driven gear selectors. Currently GM offers 2 out of the three on the Corvette, but I will try and explain all three if possible.
Planetary Gear
Planetary gear driven transmission is what most associate with an automatic transmission. There are multiple gear sets, with sun gear and planetary gears packed into a large rotating cylinder. Depending on which "clutch pack" is engaged in the gear, the outside cylinder spins at a different speed than the inside sun gear, thus the change in gear ratio. You can stack multiple planetary gear sets together to create more than 2 sets of ratios, for example, the outside cylinder may spin a second planetary gear set's sun gear, and that in turn gives you 2x2 =4 sets of different gear ratios. That's why typically you see automatic transmissions offer gears in multiples of 2. A series of complicated valves govern pressure to each clutch pack resulting in different gear changes and ratios.
Planetary gear driven transmissions have all the planetary gear sets engaged and spinning at all times, therefore it requires a torque converter to operate because you can't disengage a spinning mechanical gear. "Neutral" in an automatic simply means the driveshaft is disengaged at the torque converter, not the transmission's input shaft. The A8 equipped on the C7 Corvette is a conventional, planetary gear driven "automatic."
Mechanical Gear
This is where "automatic" and "manual" blur the line. This type of transmission has mechanical levers selecting 2 sets of spinning shafts. Depending on the position of the lever, a set of gears spins the intermediary shaft, which in turn spins the final output shaft. The relationship between the engaged gear by the lever on the input shaft, to the gear spinning on the intermediate shaft, basically determines the gear ratio. That's why mechanical gear transmissions can come in just about any number of gears, it really just depends on the number of gear sets packed into the transmission. This is typically the "performance" oriented transmission because parasitic power loss is minimized (all power transfers are done from mechanical gear to gear, gears not selected are not spinning thus less spinning mass to deal with compared to planetary gear sets). But since power transmission is done via mechanical gears, to go from one gear to another requires a momentary "disengagement" of the input shaft to the transmission because changing gear ratios means the engine and the gear speed is not synchronized.
Traditionally this is accomplished by the use of a clutch, a clutch pedal, and since the late 1970s, a synchronizer that spins up the next gear as it's selected. Lately, Formula racing technology has leaked down to the masses and beginning with sequential manual gearboxes, or SMGs, we have computer operated clutch and gear selectors that takes the operation of the clutch out of the hands of the driver. SMGs, unfortunately, still require a momentary disengagement of the gear and spinning up of the next gear in time for clutch engagement, therefore gear to gear shifting can never be as smooth as a planetary gear box set auto.
With the advent of dual clutch systems, modern transmissions based on mechanical gear sets have come much closer to a planetary gear set auto in terms of gear to gear transition while retaining the inherent performance advantages of mechanical gear boxes. Since the transition from gear to gear no longer involves a disengagement of one single clutch, then selecting the next gear, a dual clutch system (BMW's DCT. Porsche's PDK) keeps a clutch engaged at all times (unless stopped in neutral) and simultaneously engage the next clutch, and since the second intermediate shaft is connected already, the next gear to be selected is spinning at the same speed as the currently selected gear, thus seamless and instantaneous gear shifts that are as smooth as an "auto" and quick as lightning, while losing none of the energy transfer that's associated with spinning multiple sets of massive planetary and sun gears and its housing.
Currently dual clutch systems are the pinnacle of performance transmission technology, since mechanical gear set has zero limitations as to how many gears (well, size and packaging really), physical ratio, speed, efficiency, and power delivery. The lack of a torque converter also means lighter, more durable in the long haul. The drawback of a dual clutch system vs. a mechanically operated clutch is the computer control module are bank to replace, the dual spinning clutches are bank to replace once worn, but the benefit of speed and performance can not be overlooked. The benefit of a planetary driven system over dual clutch is again, the computer control module vs a mechanical valve body means significant cost, mechanical hydraulic actuators vs. switch gears in the valve body means cost to repair and replace, and a torque converter can handle significantly more engine torque output than your typical mechanical gears.
Belt Driven
I'm not going to go into details on this, but this is basically what you called CVT, or continuously variable transmission. Out of all three types of gearing systems, this is the newest innovation, but it is also fraught with the most issues. It's basically driven by a torque converter, and 2 spinning top looking gear set varies the width of the gear to change the size and ratio between the input and output shaft. There are no physical gear ratios as it can infinitely vary between the two. It is by far the most efficient gear set type, and it's far closer to a traditional "automatic" with planetary gear set than mechanical gear set. Its benefits are that you can literally keep the engine spinning at peak torque and just alter the gear ratio based on speed, load, and acceleration. For this reason alone, you see CVTs installed on more and more cars today because it can keep cars operating at peak fuel efficiency, or be tuned to offer higher multiplication and keep engine operating at peak POWER output. But unlike planetary gear sets and mechanical gear sets, the power transfer happens via a chain or belt drive and the amount of power and torque CVTs can handle today is but a fraction of what traditional planetary gear and mechanical gear transmissions can tolerate.
NOW THAT'S OUT OF THE WAY.
Dual clutch systems can operate JUST like a traditional automatic using planetary gear sets, since a computer control module can control the amount of clutch slip to make moving from a dead stop as smooth as a torque converter operated planetary auto. The control module can also be programmed to pick shift points based on engine load, throttle position, power output...etc. For performance purposes the shifts happen in a blink of an eye, no more engine surges between gear shifts, or time delay between torque converter surge and lock-up of the torque converter clutch. Porsche's PDK and BMW's DCT in automatic mode drives JUST LIKE a traditional planetary auto, but shifts like an F1 car when you demand it. The draw-back has always been cost, and the complexity of the control system. BMW's DCT is prone for the computer control module to overheat under track conditions, causing the shift to become unresponsive, or the hydraulic actuated gear selector needing service every 60,000 miles (conveniently just outside of the warranty period) to the tune of $2,000-4,000.
But for sheer performance, it can't be beat. It really is like having the best of both worlds, when it works. Lightning fast shifts like an F1 car, lazy, no touch operation like an auto.
This should be awarded post of the year, it’s that good.
Thanks for taking the time to post it for us.
Bish
12-03-2018, 04:09 PM
#59
Melting Slicks
There is nothing deflective about my comment. I was referring to the manual operation of a DCT, since there are some nut jobs here who keep saying it's an "automated manual", or some derivative. So then all the sudden some contrarian chimes in about DCTs skipping gears without mentioning that it can happen only in automatic mode. So I acknowledged the comment and stated that it proves my point.
There may be other things that a DCT does in automatic mode that I did not mention in my previous comments. But writing detailed posts about such features only serves to bolster my remarks that a DCT is absolutely not a manual transmission. I see people are continuing to spread FUD about this. Probably because they want to believe in their hearts that it is true. It's not. Go back to bed.
There may be other things that a DCT does in automatic mode that I did not mention in my previous comments. But writing detailed posts about such features only serves to bolster my remarks that a DCT is absolutely not a manual transmission. I see people are continuing to spread FUD about this. Probably because they want to believe in their hearts that it is true. It's not. Go back to bed.
A DCT is very much a manual transmission with an automated clutch pedal. The paddles let you shift just like you would in a manual. The reason they don't just give a manual gear lever to pick whatever gear is that would be stupid... you can keep both hands on the wheel and shift faster with the paddles.
12-03-2018, 04:49 PM
#60
Race Director
I have also owned a Gallardo with egear.
Not really a good comparison to a well sored DCT.
My Gallardo was 06 and the car was horrible when driven in auto mode and the clutch wear was also horrible. Replaced at 7k miles.
The Egear also has a terrible delay from take off while waiting for clutch to engage. Car had to be in neutral when at a stop light. I don't recall but it would revert to neutral at a stop light somewhere between 5 and 10 second to attempt to control clutch wear.
My 15 458 has a Getrag 7 speed DCT. Car is absolutely the best transmission I ever driven.
In auto mode is smooth and seemless as the best traditional auto transmission. In manual mode the shift both up and down are lighting quick.
There is no delay from a dead stop and car can be left in 1st gear at a stop for as long as need be.
Also is similar to the Egear that putting in neutral is a simple as pulling both paddle back at same time.
The launch control also is well sorted and unlike the Lamborghini Huracan there is no lifetime limit as to how many launch control can be used. Lamborghini is programed to a max of 250 launch control activation and they will not allow futher use of launch control.
The Getrag 7spped DCT is absolutely the smoothest and quickest transmission I have ever driven.
Puts the A8 in my Callaway sc757 to shame in both auto mode and NO COMPARISON when in manual mode.
Not really a good comparison to a well sored DCT.
My Gallardo was 06 and the car was horrible when driven in auto mode and the clutch wear was also horrible. Replaced at 7k miles.
The Egear also has a terrible delay from take off while waiting for clutch to engage. Car had to be in neutral when at a stop light. I don't recall but it would revert to neutral at a stop light somewhere between 5 and 10 second to attempt to control clutch wear.
My 15 458 has a Getrag 7 speed DCT. Car is absolutely the best transmission I ever driven.
In auto mode is smooth and seemless as the best traditional auto transmission. In manual mode the shift both up and down are lighting quick.
There is no delay from a dead stop and car can be left in 1st gear at a stop for as long as need be.
Also is similar to the Egear that putting in neutral is a simple as pulling both paddle back at same time.
The launch control also is well sorted and unlike the Lamborghini Huracan there is no lifetime limit as to how many launch control can be used. Lamborghini is programed to a max of 250 launch control activation and they will not allow futher use of launch control.
The Getrag 7spped DCT is absolutely the smoothest and quickest transmission I have ever driven.
Puts the A8 in my Callaway sc757 to shame in both auto mode and NO COMPARISON when in manual mode.
Gallardo will stay in first gear as long as your foot is on the brake, it is a safety thing not clutch wear. It will disengage 1st if you leave your foot off the brake so that you don’t accidentally accelerate thinking in neutral. I never drive mine in auto mode and my first clutch was replaced at 24k and the 2nd one is still going now at 61k.