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xRIEx

Original Poster:

3,411 posts

34 months

[news] 
Friday 24th January quote quote all
In a couple of discussions about A3 against A4/A5/A6 etc quattros (and VWs/other VAG 4WD), it's been stated that the A3 has to have a Haldex system rather than a torsen centre diff because the engine is transverse rather than longitudinal. Why is that the case?

Mitsubishi used a transverse engine in the Evo but I assume they didn't use a Haldex system - what's the difference? How did they take the power off the gearbox for the rear axle?

motco

5,737 posts

132 months

[news] 
Friday 24th January quote quote all
Audi TT has a transverse engine and a Haldex, but I don't know if it's necessary.

jamieduff1981

3,998 posts

26 months

[news] 
Friday 24th January quote quote all
It's not a necessity. There are other cars with transverse engines and AWD which use different technology. Haldex is just one way of doing it (and a way I've never been impressed with, tbh).

Krikkit

5,120 posts

67 months

[news] 
Friday 24th January quote quote all
Early Haldex wasn't very impressive - iirc it was something like a 97.5% front bias until the wheels slipped for it to lock up, so pretty useless in normal cornering etc.

In the newer systems (Gen IV and V IIRC) it has a fully controllable pressure system, so it can push that around and lock up without wheel slip apparently, thanks to clever software controlling the lock-up, including a 50/50 split during spirited driving.

Engineer1

10,429 posts

95 months

[news] 
Friday 24th January quote quote all
xRIEx said:
In a couple of discussions about A3 against A4/A5/A6 etc quattros (and VWs/other VAG 4WD), it's been stated that the A3 has to have a Haldex system rather than a torsen centre diff because the engine is transverse rather than longitudinal. Why is that the case?

Mitsubishi used a transverse engine in the Evo but I assume they didn't use a Haldex system - what's the difference? How did they take the power off the gearbox for the rear axle?
Possibly as Mitsubishi always intended AWD while VAG built their cars then decided an AWD version needed engineering, so went for the easiest option with regard to implementation.
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Max_Torque

8,152 posts

103 months

[news] 
Friday 24th January quote quote all
Because the Haldex unit can be made integral to the rear differential, this frees up space at the front of the car for the bevel gearbox required to turn drive by 90 degrees. A typical production FWD car does not have a large 'transmission tunnel' and space is very limited in this area, so using a rear mounted haldex coupling means 4x4 versions of transverse (and normally just fwd) powertrains is possible.


Max_Torque

8,152 posts

103 months

[news] 
Friday 24th January quote quote all



Notice how little extra space is required at the rear of the transverse engine & main gearbox.

Scuffers

14,262 posts

160 months

[news] 
Friday 24th January quote quote all
Max_Torque said:
Because the Haldex unit can be made integral to the rear differential, this frees up space at the front of the car for the bevel gearbox required to turn drive by 90 degrees. A typical production FWD car does not have a large 'transmission tunnel' and space is very limited in this area, so using a rear mounted haldex coupling means 4x4 versions of transverse (and normally just fwd) powertrains is possible.
Come again?

Haldex is not in the rear diff, it's mounted ontonthe gearbox.

Reason it's used us it's a cheap way to a 4wd To a fwd platform.

AER

370 posts

156 months

[news] 
Saturday 25th January quote quote all
Scuffers said:
Come again?

Haldex is not in the rear diff, it's mounted ontonthe gearbox.

Reason it's used us it's a cheap way to a 4wd To a fwd platform.
What? Like this...?


DVandrews

717 posts

169 months

[news] 
Saturday 25th January quote quote all
Having just changed the haldex filter and oil on the wife's A3 I can confirm the coupling is attached to the rear diff.

Dave

SuperchargedVR6

1,103 posts

106 months

[news] 
Friday 31st January quote quote all
Krikkit said:
Early Haldex wasn't very impressive - iirc it was something like a 97.5% front bias until the wheels slipped for it to lock up, so pretty useless in normal cornering etc.

In the newer systems (Gen IV and V IIRC) it has a fully controllable pressure system, so it can push that around and lock up without wheel slip apparently, thanks to clever software controlling the lock-up, including a 50/50 split during spirited driving.
A common misconception Gen 1 Haldex. Front wheel slip isn't the decisive factor in engaging the coupling. The car's dynamic state is also a factor, as is driver demand - via pedal speed measurement. And it's not just on or off either. The Annular pistons clamping force is varied to provide anything from 10 - 50% drive.

If that's not good enough for some people, the aftermarket has methods of turning Gen 1 Haldex into full-time 50/50, at the expense of component lifespan.

Edited by SuperchargedVR6 on Friday 31st January 11:26

xRIEx

Original Poster:

3,411 posts

34 months

[news] 
Friday 31st January quote quote all
I think previously I misunderstood what Haldex was; I thought it was a fluid coupling but from images it looks like a clutch pack - is that right?

Max_Torque

8,152 posts

103 months

[news] 
Friday 31st January quote quote all
Unfortunately most armchair experts misunderstand how a center diff must operate to provide positive yaw control, and there is much talk of "torque biasing" when you actually need a "speed bias"..........

Scuffers

14,262 posts

160 months

[news] 
Friday 31st January quote quote all
Max_Torque said:
Unfortunately most armchair experts misunderstand how a center diff must operate to provide positive yaw control, and there is much talk of "torque biasing" when you actually need a "speed bias"..........
this ^^^^^

a (single) clutch pack cannot do the job of a differential

SuperchargedVR6

1,103 posts

106 months

[news] 
Friday 31st January quote quote all
xRIEx said:
I think previously I misunderstood what Haldex was; I thought it was a fluid coupling but from images it looks like a clutch pack - is that right?
Wasn't the fluid coupling in the Golf Rallye? A 'viscous coupling'. I seem to remember drawings of input and output plates suspended in some kind of oil. The faster the input plates spin up initially, the more drive is sent to the back, or something along those lines.

Haldex uses an electric pump to 'pre-charge' a fluid up to a specific pressure and runs all the while the engine is running. Internal valves governed by the aforementioned monitors then direct that pressurised fluid into the clutch pack when necessary. Haldex used to sell their own 'sport' version of the controller (faster ramp rate), but rather than making the ECU flashable, they sell you the whole unit for £500+VAT instead. Cunning.

I don't profess to be a 4WD expert (or an armchair expert) but I have noticed a significant difference using the sport controller on my car, compared to the standard controller that shipped with the car.


Test driver

109 posts

10 months

[news] 
Saturday 1st February quote quote all
From personal experience the haldex 4wd is very good; much better than any fwd and most rwd setups. The sport/race upgrades do transform the balance though, well worth the money.

Cars like the focus rs would be much better with haldex 4wd than fwd.
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