Feature: Dual Clutch Technology

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Dual Clutch Technology Dual Clutch Technology Dual Clutch Technology
Dual Clutch Technology Dual Clutch Technology Dual Clutch Technology
Dual Clutch Technology Dual Clutch Technology Dual Clutch Technology

They're the latest rage in new performance cars but are whizz-bang dual-clutch boxes as good as they seem? Martin Donnon investigates

Feature: Dual Clutch Technology
Dual Clutch Technology

 

Dual Clutch Technology

There's always been resistance when it comes to accepting new fangled gearshift technology.

From SMG through to Tiptronic, even the big names like BMW and Porsche struggle with a market base that seems tied to the manual gearstick.

But slowly, with a whimper, over the past couple of years the semi-automatic gearboxes with paddle shift options, released with so much fanfare and glory, have slowly died away.

The conventional manual can’t rest easy though, as out of the ashes of the failed semi-autos has risen the new generation Dual Clutch Transmission (DCT) that takes a conventional manual gearbox and robotizes it without the clunking, banging, slow shifting and general jerkiness that has affected earlier paddle shift options.

Where other options have failed, DCT will be the one to finally put the H-pattern manual to the sword.

 

PERFECT SENSE

Adopting a DCT style transmission gives manufacturers and end users a much wider and more balanced choice.

For the very first time there's the chance of a truly dual purpose transmission, allowing normal ‘D’ for Drive automatic shifting through the daily grind, and sharp edged paddle shifting on track days.

It allows narrow focused track cars like the R35 GTR to be liveable in traffic, all the while moving more modest performers like the VW Golf further up the performance tree.

So good and precise has the shift mapping of the modern day DCT become that stalwart automatic drivers are struggling to pick the difference or notice anything untoward other than a slight firmness and positive shift engagement, which really is a hallmark of these gearboxes.

A great example is my partner Mrs Donnon; unable to drive the R32 GTR because of its manual transmission she readily jumps behind the wheel of the R35 and rolls on down to the shops...

HOW IT WORKS

Referring to a DCT transmission as an ‘auto’ then is totally wrong. Yes there's an automatic mode where the gears are automatically shifted by the computer.

But the reality is that when all of the electrics and actuation systems are dug away from a DCT gearbox, it's nothing more than a set of conventional manual gears, an electro-hydraulic actuation system, and a pair of clutches rather than a single manual clutch.

It’s this pair of clutches that sets the DCT apart from earlier style paddle shift gearboxes, greatly reducing the shift time (which decreases the delay between hitting the paddle and shifting a gear).

The principle of operation is quite simple, with the gearbox being divided into two halves, with gears one, three, and five on one clutch, while two, four and six share the other clutch.

When first gear is selected the transmission not only releases the clutch on first gear, but also engages the second clutch, selects second gear, and sits there waiting for a shift command.

The moment the driver selects a shift there's no time wasting gear engagement to take place, just a simple ‘dropping’ of the second clutch.

When second is selected, third is readied on the next clutch, and on it goes.

In other words driving a DCT is like being at a shooting gallery with ‘one in the chamber’ the whole time. Rapid fire? No problems!

 

SMARTS

While the stripped down DCT gearbox looks pretty much like any other manual when its pulled down, the genuine smarts live in the TCM (Transmission Control Module).

It’s this separate transmission computer that can make the distinction between full manual mode where the driver controls the shift process, or a full auto selection to trick the conventional auto driver.

There are normally a couple of different levels of ‘aggression’ in DCT programming, so don’t be surprised to have a switch on the dash giving different ‘sports’ modes.

Once a paddle is pulled or an auto shift commanded solenoids channel hydraulic fluid pressure to shift actuators that move the gear linkages in the DCT transmission just as they are in a conventional manual when you pull the gearstick.

These shifts are slow and precise, using normal run of the mill synchro hubs on standard helical cut gears. The shift process you feel as a driver though isn’t this slow and deliberate action occurring, rather the lightning fast ‘jab’ of the clutch disengaging on the pre-selected gear you have just shifted into.

Last but not least are the twin dual clutch packs. Rather than a single disc arrangement standard to most conventional manuals the DCT transmission with have five or six clutch discs for each of its small clutches.

This high number discs gives the same surface area as a big single clutch, but with far more compact packaging potential, which is important when you consider the clutches are attached directly to the end of the gear shafts and form part of the transmission casing.

 

PROBLEMS

With any quantum leap in technology there is a fair share of associated issues that come along for the ride.

Put them down as ‘newness’ if you will, but regardless of what you might read on the internet or hear around the traps, DCT style transmissions are indeed every bit as reliable as their more primitive forebears.

The logic behind this is simple too, as with the human directly removed from the shifting process there is little chance of graunched shifts, damaged synchro hubs or abuse destroying parts, as the TCM simply won’t allow it.

What of the GTR that was capable of destroying the teeth on first gear when the inbuilt Launch Control was used too much? This comes down to a genuine programming error where the limits of ‘what the driver was allowed to do’ versus the ability of the transmission to absorb the load was grossly miscalculated.

A later 2010 year model software update reduced the savagery of the Launch Control system and eliminated the problem.

Other issues with DCT transmissions that have shown themselves have been clutch seal failure.

This problem only tends to affect one kind of Borg Warner seal (however Borg Warner are a major DCT technology supplier), and is not endemic, but when the problem occurs the result will be the same as a constantly slipping clutch in a conventional manual transmission arrangement.

The greatest issues DCT transmissions suffer at the moment are a lack of individual parts supply and service, meaning that at dealer level a small seal failure can result in the replacement of the whole transmission.

Factor in that most DCT transmission assemblies come off the manufacturer price list at upwards of $10K and you can see why some of wise industry heads are concerned on ownership costs long term.

This whole situation is changing very quickly though, with the early GTR transmission failures providing a tremendous boost to clever aftermarket developers who have manufactured replacement parts from complete gears through to seal kits and fluid to equip the R35 with the first truly aftermarket serviceable DCT transmission.

It won’t be long till this Godzilla inspired repair technology filters across to other generation DCT gearboxes.

 

FLUIDS

The last link in the DCT shock and awe campaign is the cost of the fluid in the things. The GTR comes out on top of the fluid price list with the factory transmission fluid coming in at around $100 a litre.

Even base model Mitsubishi Ralli-arts are equipped with $60 a litre fluid in their SST; with all of these transmissions holding upwards of six litres that’s a fair bit of fluid to change.

Service intervals vary with make and model with once more the GTR being the worst (most frequently serviced) at 3000kms when driven hard, yet some VW and soon to be released BMW offerings will feature a lifetime transmission fill that’s never swapped.

The reality is that the transmissions that have regular servicing of every 20,000km or so for normal street driving will live the longest and happiest lives. It’s all about finding the correct balance.

Differing a proper DCT transmission fluid from a conventional Gear Oil or Automatic Transmission Fluid is its unique requirement to not only protect the gears and mechanical parts, but also lubricate the wet clutches and provide a strong coefficient of friction to avert any potential clutch slipping.

These factory fluids are carefully developed to ensure no problem with the bonding of the clutch material occurs…which means you need to be very careful indeed with what you pour in your DCT transmission.

At this stage the only oil supplier that makes aftermarket performance DCT fluid in Australia is South Australian based Willall Racing. At over $60 a litre it’s a similar price to most factory offerings, but has greater heat resistance and longer rated life.

This is just another example how the aftermarket is changing, and with their adoption of DCT technology the days of the manual gearstick equipped performance car are as good as over.

 

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