An Old DCT Lag Rant

[CanAutM3]

Originally Posted by swamp2

I did some testing of M-DCT shift times and posted the data here long ago. The data was not very consistent nor high quality and noise was an issue. I never claim in these type of discussions that the M-DCT can shift in zero time. However, some data, in some modes indicated that there may be some zero time shifts where the vehicle acceleration is not interrupted but actually surges during the shift with a very carefully orchestrated clutch overlap period and we feel this as a surge.

This makes sense

Originally Posted by swamp2

I think the best estimate, based on a variety of sources is 50-100 ms for a typical aggressive upshift. Also, shift times will vary depending on the Drivelogic mode and the level of agressiveness in the current driving.

Why would acceleration be interrupted for 50-100ms on an aggressive DCT upshift ?

[swamp2]

Originally Posted by CanAutM3

Why would acceleration be interrupted for 50-100ms on an aggressive DCT upshift ?

I'm not sure I can answer that, probably only Getrag and/or BMW engineers can... It's probably the exact same group of engineers who can answer these questions about lag as well!

Again my hunch is that some shifts do have an effective zero shift time, however, clearly not all shifts in all modes have this. There is a fine balance in being able to phase 2 clutches without dangerously having both partially engaged at the same time. Also, there is the question of dumping excess crank/flywheel angular momentum into the drive train. This will help performance albeit probably minimally. This momentum dump can probably be controlled as to it being fast or sharp or slower and steady again depending on the Drivelogic setting. It also may be the case that the mode that feels the fastest might not actually be the best performer. I suspect it is the former but certainly cannot prove that.

I certainly feel a wide range of shift times myself, again depending on level of aggressiveness in driving along with Drivelogic mode. My estimate of 50-100 ms for WOT upper Drivelogic modes is based on a variety of quotes from OEMs, along with CarTest simulation, observations about vehicle performance as well as my somewhat crude testing.

[DreamChaser26]

quick question. im new to the website but it wont let me post my own thread but have anyone of you had problems with the DCT m3's i just had a warning that came up on my navigation and said i had a transmission malfunction. NEED ADVCIE OR HELP

[CanAutM3]

Originally Posted by swamp2

I'm not sure I can answer that, probably only Getrag and/or BMW engineers can... It's probably the exact same group of engineers who can answer these questions about lag as well!

Again my hunch is that some shifts do have an effective zero shift time, however, clearly not all shifts in all modes have this. There is a fine balance in being able to phase 2 clutches without dangerously having both partially engaged at the same time. Also, there is the question of dumping excess crank/flywheel angular momentum into the drive train. This will help performance albeit probably minimally. This momentum dump can probably be controlled as to it being fast or sharp or slower and steady again depending on the Drivelogic setting. It also may be the case that the mode that feels the fastest might not actually be the best performer. I suspect it is the former but certainly cannot prove that.

I certainly feel a wide range of shift times myself, again depending on level of aggressiveness in driving along with Drivelogic mode. My estimate of 50-100 ms for WOT upper Drivelogic modes is based on a variety of quotes from OEMs, along with CarTest simulation, observations about vehicle performance as well as my somewhat crude testing.

IMO, if the most performance oriented shift on a DCT would interrupt power delivery for 50-100ms, it would entirely negate the whole point of DCT transmissions. If the best single clutch can do it in 40-50ms, where is the performance benefit of DCT ?

This is my interpretation of a DCT upshift:

It is physically impossible to have the two clutches simultaneously engaged. Either one, or both are slipping. Static friction is stronger than dynamic friction. This implies that the first clutch remains fully engaged even if the second clutch is progressively applied (and slips). As the second clutch is applied with more pressure, it transmits more and more torque to the transmission. As soon as the first clutch is sufficiently released, it starts to slip and the gearbox gets driven only by the second clutch. The second clutch then continues slipping until the gearbox spins at the same speed. With good timing of clutch engagement and disengagement, the inertia of the engine can be recuperated in the process. With the power of electronic control, this can be (relatively) easily achieved.

I do not see any physical reason for both clutches to be disengaged simultaneously and interrupt power delivery during an upshift. Even if both clutches are simultaneously partially engaged, we are talking milliseconds here. IMO, the slippage on a standing start is far more severe. To minimize wear in less aggressive shift modes, I can see a combination of engine management adjustments (a very slight cut in engine power output to accelerate the RPM drop) and clutch application to make the shift smoother and reduce the severity of the clutch slippage.

I believe that the quoted DCT shift times is the time it takes from one clutch being fully engaged to the second one being fully engaged. However, contrary to a single clutch transmission, power continues to be transmitted in the process. Hence the "zero shift time" for the purpose of the discussions in this thread.

IMO, it is possible to make the transmission of power truly seamless with a DCT .

From the Getrag website:

The GETRAG Powershift dual clutch turns the dream of fully seamless gearshifting into reality. While one gear is engaged, the system has already preselected the next. Once the relevant rpm has been reached, one clutch is opened while the second is closed simultaneously, precluding any interruption in tractive force. The intelligent software control means that sportiness and convenience are guaranteed alongside high efficiency.

I could not find any significant technical data though...

[MikeBlaszczak]

Originally Posted by gio26

quick question. im new to the website but it wont let me post my own thread but have anyone of you had problems with the DCT m3's i just had a warning that came up on my navigation and said i had a transmission malfunction. NEED ADVCIE OR HELP

If the problem persists, you should contact a BMW service center as soon as possible.

[sensi09]

Originally Posted by gio26

quick question. im new to the website but it wont let me post my own thread but have anyone of you had problems with the DCT m3's i just had a warning that came up on my navigation and said i had a transmission malfunction. NEED ADVCIE OR HELP

Had this happen once before and went away with a restart. Hasn't reoccurred in over 2 years since.

Try a search. It was awhile ago, but for a few it went away quickly, but some may even had their DCT replaced due to this. The issue with going to the dealer is having the warning actually appear on idrive....a hassle if the problem is intermittent.

[DreamChaser26]

Originally Posted by sensi09

Had this happen once before and went away with a restart. Hasn't reoccurred in over 2 years since.

Try a search. It was awhile ago, but for a few it went away quickly, but some may even had their DCT replaced due to this. The issue with going to the dealer is having the warning actually appear on idrive....a hassle if the problem is intermittent.

so they got a whole new transmission to fix the problem? or was it a computer glitch

[sensi09]

Don't know exactly, like I said it was a long time ago. Could have been unrelated. Search man.

[M3takesNYC]

The bottom line is the dct clearly shifts in an imperceptable amount of time that the cars acceleration is uninterupted. Look at any video's side by side of almost any non dual clutch car. Even SMG or whatever the manufactuers name of the single clutch manuamatic. They all have a slight hesitation beteween gear changes that lose ground ever so slightly.

The m3 dct simply pulls forward while shifting and does not give a single inch up doing so.

Its a great tranny or else they would not have picked it for the best sports supercar in the world-ferrari 458!

[US///M3]

Originally Posted by M3takesNYC

The bottom line is the dct clearly shifts in an imperceptable amount of time that the cars acceleration is uninterupted. Look at any video's side by side of almost any non dual clutch car. Even SMG or whatever the manufactuers name of the single clutch manuamatic. They all have a slight hesitation beteween gear changes that lose ground ever so slightly.

The m3 dct simply pulls forward while shifting and does not give a single inch up doing so.

Its a great tranny or else they would not have picked it for the best sports supercar in the world-ferrari 458!

Mmmmmh now I'm confused how the MT Corvette ZR1,Z06,Dodge Viper ACR etc. Spank the dual clutch auto tranny Ferrari Italia 458,Nissan GT-R, M3 GTS etc. on most race tracks.

[CanAutM3]

Originally Posted by US///M3

Mmmmmh now I'm confused how the MT Corvette ZR1,Z06,Dodge Viper ACR etc. Spank the dual clutch auto tranny Ferrari Italia 458,Nissan GT-R, M3 GTS etc. on most race tracks.

Just imagine what a Corvette ZR1,Z06,Dodge Viper ACR would do with a DCT

Kidding aside, I am not sure any of the cars you quoted "spank" a 458 on a road course. Maybe a little faster on some courses and slower on others, but "spank"

[MikeBlaszczak]

Radiation Joe bought a car with a semi-automatic transmission that doesn't shift the way he likes it to ... even though he modified the car. Since then, he's had it fixed and that wasn't good enough. After that, he bought a different car with a manual transission.

He posted about the problems the had almost three years ago, but posted again this week just to rant. He admits to being drunk when posting, just wanting to publicly humiliate the BMW engineers. To what end?

What's the real point of this thread, other than for Radiation Joe to cry about something that's long fixed? Is this whole thing just some play for attention, or is there something that Radiation Joe actually expects someone to do for him?

[Radiation Joe]

Originally Posted by MikeBlaszczak

Radiation Joe bought a car with a semi-automatic transmission that doesn't shift the way he likes it to ... even though he modified the car. Since then, he's had it fixed and that wasn't good enough. After that, he bought a different car with a manual transission.

He posted about the problems the had almost three years ago, but posted again this week just to rant. He admits to being drunk when posting, just wanting to publicly humiliate the BMW engineers. To what end?

What's the real point of this thread, other than for Radiation Joe to cry about something that's long fixed? Is this whole thing just some play for attention, or is there something that Radiation Joe actually expects someone to do for him?

Not the engineers. The jerks at BMWNA that refused to make it right. If you think driving that car for almost two years with a @#%&ed up transmission and being told there is nothing wrong is ok, then more power to you. I thought the rant was kind of funny, and thought I'd bring it out again so that others could see how BMWNA treats their customers. I doubt you were around when this problem was plaguing us, so I'm not sure your perspective is quite right for chiming in.

[M3takesNYC]

Originally Posted by Radiation Joe

Not the engineers. The jerks at BMWNA that refused to make it right. If you think driving that car for almost two years with a @#%@ed up transmission and being told there is nothing wrong is ok, then more power to you. I thought the rant was kind of funny, and thought I'd bring it out again so that others could see how BMWNA treats their customers.

Your still complaining or crying over a stupid transmission on a car that had some lag?

Must really suck to be consumed by such minutia in life.

Sure it bugs me if my car is not working exactly right but the thought lasts a day if that. 2 or 3 years later and this still brings out this much emotion in you-scary

[Vic311]

Originally Posted by M3takesNYC

The bottom line is the dct clearly shifts in an imperceptable amount of time that the cars acceleration is uninterupted. Look at any video's side by side of almost any non dual clutch car. Even SMG or whatever the manufactuers name of the single clutch manuamatic. They all have a slight hesitation beteween gear changes that lose ground ever so slightly.

The m3 dct simply pulls forward while shifting and does not give a single inch up doing so.

Yet Lambo and the F1 cars using single clutch technology are the fastest shifting transmissions in the world.

And the real reason DCT were not incorporated into these cars? Because it is a complex engineering mess, fraught with flaws and problems.

It's never about simply being fastest, its about who is consistently fastest without breaking down.

Simplicity is its own reward.

Originally Posted by M3takesNYC

Its a great tranny or else they would not have picked it for the best sports supercar in the world-ferrari 458!

Probably the most subjective statement of this thread.

[sensi09]

Originally Posted by Vic311

Yet Lambo and the F1 cars using single clutch technology are the fastest shifting transmissions in the world.

And the real reason DCT were not incorporated into these cars? Because it is a complex engineering mess, fraught with flaws and problems.

It's never about simply being fastest, its about who is consistently fastest without breaking down.

Simplicity is its own reward.

No DCTs originally in lambos because of extra weight and at the time, it could not handle all the power. Even though the aventador is single clutch, the next gallardo will offer a DCT.

And others like the LFA and I believe pagani, choose single clutch as it's more "mechanical" and "visceral" then its dct counterpart.

As for F1, dual clutch is banned. Weight penalty is probably an issue as well.

[sensi09]

Originally Posted by swamp2

Somewhere other than here? Thanks.

Well from this thread, there was my post here http://www.m3post.com/forums/showpos...7&postcount=69

and I think a few others.

[US///M3]

Originally Posted by Vic311

Yet Lambo and the F1 cars using single clutch technology are the fastest shifting transmissions in the world.

And the real reason DCT were not incorporated into these cars? Because it is a complex engineering mess, fraught with flaws and problems.

It's never about simply being fastest, its about who is consistently fastest without breaking down.

Simplicity is its own reward.



Probably the most subjective statement of this thread.

+1

[CanAutM3]

Originally Posted by Vic311

Yet Lambo and the F1 cars using single clutch technology are the fastest shifting transmissions in the world.

And the real reason DCT were not incorporated into these cars? Because it is a complex engineering mess, fraught with flaws and problems.

It's never about simply being fastest, its about who is consistently fastest without breaking down.

Simplicity is its own reward.

^^ This is the most subjective post of this thread

[M3takesNYC]

F1 actually uses a dual clutch don't they?

[CanAutM3]

Originally Posted by M3takesNYC

F1 actually uses a dual clutch don't they?

F1 use automated single clutch sequential gearboxes.

[SenorFunkyPants]

Originally Posted by CanAutM3

F1 use automated single clutch sequential gearboxes.

Actually F1 use a manually operated single clutch sequential gearbox.

The DCT essentially shifts in zero ms as there is no moment when torque is not being applied to the output shaft. During the "shift" there will be some loss to heat as one clutch pack is disengaging while the other is engaging but there is no break in torque output...which is why your passengers head doesn't move during the shift (best observed in S2).