Twin (DSG type?) clutches

[MeldrewsMate]

Trying to educate myself here.
These semi-automatic "clutchless" gearboxes seem to have certain advantages over the standard gearboxes, but having looked at diagrams online I have a few question regarding wear and tear on them.

Q1. If one clutch is always engaged doesn't this cause extra drag (extra engine load)?
Q2. Does one release bearing always have load on it when going through the gears?
Q3. If cruising on the motorway in top gear, again is the unused clutch release bearing loaded up for several hundred miles?
Q4. Does the reaction to the release bearing being loaded load both the crankshaft axially and the crankshaft thrust washers?
Q5. Do these types of gearbox suffer extra cost in terms of clutch and thrust bearing wear?

Regards, MM

 

[Keithmac]

Theres two significantly different twing clutch designs, one that use "wet" clutches bathed in oil (good design, long clutch life) and more recent "dry" twin clutches that imho are a very poor idea.

1, the most efficent transfer is then the clutch is fully engaged, that's why modern "normal" autos lock up the torque converter. The Idle disengaged clutch will have slight drag due to being in oil but not significant.

2, they work backwards to a normal geabox, oil pressure is used to clamp the clutch packs shut (engaged).

3, no.

4, it's not like a "normal" clutch, there is no force acting on the crankshaft thrust washers.

5, the wet clutch boxes can last 100's of 1000's of miles if serviced on time (very important!). Dry twin clutch gearboxes have provent to be less reliable (basically forcing a dry clutch to "creep" or slip is a very bad idea from a wear point of view.

I own a MK2 Kuga with the twin clutch Powershift gearbox (Getrag built), use it for towing our 1500kg caravan and have no complaints at all.

Don't be frightened off, just make sure the services have all been done (and it's not a dry clutch variant!).

 

[MeldrewsMate]

Q3. If cruising on the motorway in top gear, again is the unused clutch release bearing loaded up for several hundred miles?

3, no.

Thanks for your answers, but referring to Q3, why not? Does it select the equivalent of neutral on the unused side of the 'box? Or are you saying that it keeps the unused side clutch engaged but, because it's bathed in oil (on a wet plate clutch) there is little wear....and hence the critical need for regular oil changes?

 

[Keithmac]

Thanks for your answers, but referring to Q3, why not? Does it select the equivalent of neutral on the unused side of the 'box? Or are you saying that it keeps the unused side clutch engaged but, because it's bathed in oil (on a wet plate clutch) there is little wear....and hence the critical need for regular oil changes?

Because the gearboxes use oil pressure to clamp the clutch packs, the release springs are not required to overcome any real force.

It's not like coasting along in a normal manual with the clutch pedal down forcing the pressure plate open (which by design requires a significant amount of force and loading on the release bearing and crankshaft).

When released (oil pressure removed) the clutch release springs push the plates apart up to the stops on the cluch pack, they have nothing to fight against so relatively little force is required.

The Odd and Even shafts will always have a gear selected, the none used clutch will be disengaged for long periods as you say but there is no detriment to the clutch pack.

Because the clutchpacks cause friction and oil shear, and the clutch material will gradually mix in with the oil (normal wear and tear) it needs changing, the Mechatronic unit also requires clean oil to avoid clogging up the oilways or solenoids. The oil has to be in a predictably state to allow the correct hydraulic clamping of the clutches.

Servicing wise I did my Powershift myself, required 7l of oil and paper element filter (both genuine Ford), took an hour or so. Required precise final level setting but wasn't rocket science.

Luckily it has an external filter which is worth bearing in mind if you plan on doing your own service.

I think the DSG requires a special pump to fill the gearbox and set level but could be wrong?.

This link maybe worth a read?, Mitsubishi but same pricipals..

https://www.jackstransmissions.com/pages/sst-how-it-works-problems-upgrades