Mercedes-Benz GLA Forum banner

321 - 330 of 330 Posts

·
Registered
Joined
·
83 Posts
so the RPM falls when the friction is removed..... Gotcha

Can you articulate the mechanism that engages the clutch as the ECU sees a commanded throttle position opening above idle? Also, how much clutch engagement is being commanded when in gear, brake hold engaged?
It would seem that there would be more effective clutch control, than introducing an eco stop feature, but I am often naive
Well, let's start with looking in depth to the wear action of a classic clutch, before we extrapolate it to the automated dual clutch action:
There's a category of manual gearbox drivers, which some affectionally call clutch-raiders.
What does a clutch raider do?

A newbie will semi-slip the clutch all the time and cause premature wear, polishing of the friction areas and heat that will in time kill the throw-out bearing causing bearing chatter in neutral.

An experienced, intent clutch rider, will just not yank the shifter to neutral. He will step on that clutch early on, as the traffic slows down in a traffic jam, and keep the pedal stepped all the way down. He will not semi slip the clutch, he will let the car free-wheel in the traffic jam, only gingerly tapping the brake to adjust the speed of the car, to what the others are doing.
Some will even wait every single traffic light, with the transmission engaged in first gear, with the clutch stepped to the floor.
What kind of wear do these guys cause?
Well, the throw-out bearing will be spending a lot more time pressed against the diaphragm of the clutch plate. This will definitely speed up the wear of the throw-out bearing.
Also, the teeth of the clutch diaphragm of the clutch plate, of such a clutch rider, end up with deep wear rings, around where the bearing sits on them and it can be (seen it in Europe), as bad as the bearing literally starting to cut the teeth off of the diaphragm of the clutch plate, before the clutch disc is worn.
And of course this generates extra heat.
In time, the clutch of a clutch rider's car, starts to deactivate. Those teeth become less elastic and cannot completely disengage the clutch disc. So the gear changes become grabby and crunchy.

Honestly, I think the dual clutch tends to want to act like a clutch rider person. There is no torque converter, to allow the car to sit without wear with the gear engaged. This means that the clutch deactivation mechanism remains engaged as long as the car is not in neutral.

You saw that when you bring the car in neutral, the revs drop to the same point be it in comfort or sport+. To me this means that the clutch is deactivated.

Now, if you ask me exactly which parts are going to wear with this scenario in the dual clutch gearbox, I have to research a bit. It won't necessarily be the discs themselves, but it will be one or another clutch activation component, and I would think the associated clutch bearing.
 

·
Registered
Joined
·
83 Posts
But just like the classic clutch action, as time goes by and the clutch deactivation mechanism gets increased wear, its ability to deactivate deteriorates, then the clutch disc wear can begin. They don't separate properly or fast enough anymore, which causes increased wear..
 

·
Premium Member
Joined
·
183 Posts
Uh huh,.......
If the throw out bearing was driven by a mechanisim totally under the designers control, and all the actions wear known, the number of expected cycles, the rate of engagement, dwell time, etc all known.

ya think they threw a LUK in there and hoped, really?

I'm quite certain it is correctly sized, loaded, and lubricated to run as long as any other internally lubricated part.




Your discounting of published advice is balanced on a flawed presumption
 

·
Registered
Joined
·
83 Posts
I understand your comments. Also you’re entitled to your opinion, that the clutch aggregate is designed to operate reliably with either choice of handling idle.

Also, my comments are not in response to a bad wear situation where a solution needs to be found to an imminent problem, there are not many reports of dual clutch failures in our cars.
No, my comments are in terms of which is a better choice out of two ways to handle the idle based on the already posted info.

I would prefer a diagram or a schematic to support the operation advice found, an engineering way to say that I’d like to see for myself and draw my own conclusion.

I’m going to look for dual clutch actuation diagrams, try to learn the action and post my opinion about it.

Also, I don’t mind being wrong about this.
 

·
Registered
Joined
·
83 Posts
So far, I found that the Ford DCT 6 speed, made by Getrag, a dry clutch unit, is direct action.
Being direct action changes everything:
A manual car clutch is reverse action. Meaning that pressing on the actuator mechanism disengages the clutch.
this is why you can have a neutral with gear selected and ride the clutch
Or a neutral with no gear selected and clutch engaged.
Now with direct action, the clutch is disengaged when the actuator doesn’t press it and engaged when actuator presses it.
So both main gearbox shafts can have gears selected, as long as the actuators are not energized, the clutch’s are disengaged.
This is a totally different type of neutral.

I still don’t know the staging that the DCT does, but potentially the only difference between the “advertised” neutral, meaning the N lamp is lit on the shifter facia
And the Drive neutral with D lamp lit

is that
for the N neutral, no gear is selected, the clutch is disengaged.

For the D neutral, a gear is selected, the clutch is disengaged.

I’m starting to understand how the DCT works and starting to believe the “best advice” line items.

here is a link to the Ford DCT

 

·
Super Moderator
Joined
·
1,850 Posts
Porsche's PDK (their term for DCT) doesn't have a neutral position on either gear shaft. Odd gears (1, 3, 5, and 7) are on one shaft while even gears (2, 4, and 6) are on the other shaft. One gear or the other is always selected on each shaft. It's just a matter of which if any clutch is engaged. At a stop with brake pedal applied both clutches are disengaged. When starting the computer feathers the clutch of the appropriate shaft based upon need, much in the same way as a human with a third pedal would. The failure mode of the clutch is disengaged. This is opposite of a single manual clutch system. That's important to avoid a catastrophic failure mode ...

50 some years ago in my youth I rebuilt a motorcycle transmission. My shims weren't quite right. Shifting into fourth I also caught fifth. This resulted in a contention of desired RPMs with a resultant engine lock up, complete with an adrenaline rush. This is the kind of failure you do not want!!!
 

·
Premium Member
Joined
·
183 Posts
I understand your comments. Also you’re entitled to your opinion, that the clutch aggregate is designed to operate reliably with either choice of handling idle.

Also, my comments are not in response to a bad wear situation where a solution needs to be found to an imminent problem, there are not many reports of dual clutch failures in our cars.
No, my comments are in terms of which is a better choice out of two ways to handle the idle based on the already posted info.

I would prefer a diagram or a schematic to support the operation advice found, an engineering way to say that I’d like to see for myself and draw my own conclusion.

I’m going to look for dual clutch actuation diagrams, try to learn the action and post my opinion about it.

Also, I don’t mind being wrong about this.
Right on, any rabbit hole of learning is primarily, learning.
I am by blood, an engineer, but my paper maths were never good enough. but my nose calcs are easy good to an order of magnitude on hard stuff, an 5~7% on mundane stuff. That an 20 years in medical give me a pretty decent "snapshot" of any given devicey, electromechano situation.

I predict that in the first gen iterations of production DSG on any manufacturer you look at, you will find specific, condition peculiar failures, of various parts. Depending of the variables between Sally cooking an input shaft seal, and Dieter convincing Toto that the redesign must be pushed back 6 months....that stuff was pretty solved by the time gen 2's came out. I dunno what the lineage of the box in my 2019 is exactly, and there may well be ways to **** up the reverse selector. proly involving a sequence of events that would have NTSB cats saying "shits gonna happen", but there. but not from what you do at a light, as a "non-grossly negligent" operator.

Wrong...
If one does not expect to be wrong at times thru the day, and seek self improvement over deflection of the truth as it whizzes by on whatever conveyance be handy, well one of the stickers on my car is meant specifically for that aggregate of human, those blissfully confident of any opinion gleaned from a source they've bookmarked in the browser.

But Dive in, read some white papers, etc all good if it serves a valid use of your time. I have said many times that I dearly wish I do not know, so intrinsically, by part number and function. the mazda 3 air conditioning system. But I do, and had gages, and a vacuum pump. but Eff that, now I have a warranty, and time to play
26723
 

·
Registered
Joined
·
83 Posts
Warranty for me too. Not used to having one either.
As a known youtuber says "I am the warranty".
And I am a mech. engineer. And many of us are grumpy, it's true.

As far as the DSG:
I have a new link.
Our DSG is a wet clutch. The wet clutch is a prepackaged unit that must be replaced as a module.
Wet clutch units don't have throw-out bearings, or clutch forks.
They have "rotating pistons"
Am not yet there with understanding the rotating pistons.

But here's another thing:
It does not at all ... appear to me.. that our dual clutch units have only two clutches.
It more and more seems to me that, especially the wet clutch units are actually multi-clutch units.
Maybe there are two sets of multi-clutches. One for one set of gears, another for the other set.


It seems that you have the front canards on the 2019, I cannot find a picture showing the area between them and the plastic wheel well trim, when they're installed, they cover the paint under them?
 

·
Registered
Joined
·
83 Posts

I think that the rotating pistons are units that are mounted under the dual clutch sealed pack, that have oil passages. These turn with the input shaft, but when the transmission control unit opens a clutch solenoid, oil is being fed to a rotating piston, which then extends a piece towards the clutch pack and actuates the clutch this way.
In one of the pictures in the article, you can see the sealed wet clutchpack drum and under it at the center, the two roatating pistons and you can even see the oil feed holes.
 
321 - 330 of 330 Posts
Top