VAG 4x4 systems

[Sammycarp]

I saw the Audi Quattro advert on TV the other night, which got me thinking about the 4x4 systems available in other VAG vehicles. :think:

 

I know that Skoda's Octavia 4x4 (and I guess also Scout, Yeti, Superb) uses a viscous coupling/Haldex unit, and I presume that Audi's Quattro system has a proper (lockable) centre differential?

 

What appears in the VW 4Motions?   Is it different depending on the model (Golf, Passat, Tiguan, Touareg, Amarok, Transporter)?  I'm not aware that Seat produce a 4x4 model?   What do Porsche use?    What about Bugatti?

 

In my mind (a dangerously inaccurate place!), VW also use a Haldex type unit (maybe lockable diff in Amarok?), and Porsche use a centre differential (possibly LSD depending on model?), but without a locking function? No ideas on Bugatti, probably something bespoke and expensive!

 

How far off the mark am I?   Please be gentle, these are only the idle thoughts of someone who should be concentrating on other things!

[XLBaconDoubleCheese]

I thought they were all the same

[mannyo]

My 2004 A8 has full time 4x4, not a Haldex in sight, same setup as the 2001 A8 I recently sold.

 

However even the modern Audis have changed to the Haldex setup, partly due to improved fuel consumption when compared to the Torsen system in my A8.

[Huskoda]

The cars with transverse engines have Haldex clutches. Longitudinal engines use Torsen centre differentials.

 

In 99% of cases.

 

So for example:

 

Haldex:

Audi A3

Golf

Tiguan

Passat

 

Torsen:

A4-8

Toureg

[snow_muncher]

The old Quattro system doesn't have a 'locking' central diff, but instead uses a torsen LSD

The Haldex system uses a viscous clutch pack type diff whereby the lockup ratio can be adjusted via a computer adjusting the pressure of the oil in the diff

Such systems have been designed to out-perform mechanical central diffs by being able to adjust the torque sent forward/rear through the drivetrain

The extact nature of how a system like haldex behaves is largely down to the software logic loaded into it

Companies like Land Rover, BMW & Mercedes tend to use software which makes a haldex simulate a torsen

VAG use logic which makes the diff behave more like a very passive viscous coupling

The software logic of the haldex unit can be remapped by changing the control unit: HPA Controllers

Mitsubushi use the same tech in the Evo 9 and later, their ACD system comes switchable maps allowing aggressive, medium and passive torsen simulation

The AWD system found in Audi R8's/Lambo's and Bugatti's is very similar, but designed round a mid-engine RWD biased basis

That's only the central diff though - 4x4 systems have two further diffs, one of the front axle and one on rear axle

The old Quattro system tends to be setup with an open front diff and a rear torsen LSD - this gives a more traction across the rear axle giving a rwd edge to the driving experience and aiding oversteer

The Haldex system normally has an open-diff on both axles, this can be compensated to greater/lesser extent by electronic-LSD programmes in the ABS system - whereby the brakes are used to simulate the effect an LSD can have

Mitsubishi like put LSD's into the front axle of the higher end Evo's, and either another on the rear axle, or a second haldex like transaxle system

ps

There's a Seat Leon 4 - just never sold in UK

Edited 28 March, 2014 by snow_muncher

[WeirdKerr]

Seat Altea Alltack  was sold in the uk same 4x4 system as the 2nd gen Octavia 4x4 and Octavia Scout I believe

[Volvomeg]

Haldex clutchs are basically like a wet clutch similar to what are found in motorbike clutch pack at the rear end of the propshaft,

 

As said above this system is better on fuel and a little more forgiving when it comes to differant tread depths on tyres as there not full time 4wd,

 

Volvo use a similar system and also used to use a similar older system with visious coupling and LSD rear diff and used to have big issues winding up the propshafts and couplings due to tyres with differant depths

[Huskoda]

I believe Volvo actually use Haldex. At least according to wikipedia It even says the Bugatti Veyron uses it and I'm told Porsches do, but in reverse, ie rear wheels are direct drive with the fronts through the coupling. Though I imagine its a more powerful variant not available in normal cars.