Can someone explain the FWD Haldex system to me?

Think of the halfex as containing not the center diff and the rear diff in one unit. The slipping of the clutch pack determines the amount of power going to the rear. The output of the clutch pack is transmitted through the rear differential. This diff is an open type.

My rear tires were bald and fronts still had probably 3/32 left after using the race controller for DD and a track day... shit was good.

I'm still not understanding how the Haldex system can have front and rear driving (ie. Haldex clutch engaged) and go around a sharp corner without something slipping to make up for the difference in speed between the front and rear axles around the corner.

In a traditional 4x4 pickup with open diffs front and rear and a "part-time" transfer case, if you engage 4x4 on dry pavement it'll wind up the drivetrain and buck around corners. Compare this to say Audi Torsen based or Subaru AWD systems that do have a center differential. The AWDs with a centre diff can go around a corner without winding up the drivetrain (ie. no bucking).

It appears that our cars do not have a center differential, so to go around a corner smoothly, the Haldex clutch must disengage or at a minimum slip to make up for the lack of a center diff.

Is that what actually happens?

If the race controller forces the clutch to stay tightly engaged all the time, your tires would have to slip (like a 4x4 truck) which would be bad for cornering performance and drivetrain and tire wear...

Am I missing something?

I have no fucking clue, other than it was awesome and def worked.

the open rear diff works just like our front rear diff. Its not a locked axel like you would find in a 4x4 truck. When one side starts to slip all power along that rear axel gets sent to the other side.

Canadian_R said:

I'm still not understanding how the Haldex system can have front and rear driving (ie. Haldex clutch engaged) and go around a sharp corner without something slipping to make up for the difference in speed between the front and rear axles around the corner.

In a traditional 4x4 pickup with open diffs front and rear and a "part-time" transfer case, if you engage 4x4 on dry pavement it'll wind up the drivetrain and buck around corners. Compare this to say Audi Torsen based or Subaru AWD systems that do have a center differential. The AWDs with a centre diff can go around a corner without winding up the drivetrain (ie. no bucking).

It appears that our cars do not have a center differential, so to go around a corner smoothly, the Haldex clutch must disengage or at a minimum slip to make up for the lack of a center diff.

Is that what actually happens?

If the race controller forces the clutch to stay tightly engaged all the time, your tires would have to slip (like a 4x4 truck) which would be bad for cornering performance and drivetrain and tire wear...

Am I missing something?

Click to expand...

The clutch pack slips. The Haldex system reads the steering angle sensor output so I assume that even the competition controller allows a certain amount of slip.

Ask someone with an R or A3 with a race controller to drive in a tight circle on smooth concrete. If the clutch pack doesn't allow any slip, you'll see the rear wheels slipping, most likely the inside one.

As far as tire wear and vehicle dynamics, I daresay that the GTI chassis is set up for a bit of oversteer. Yes, the front end will plow, but in sweeping corners I've noticed the back end will come out even with a little throttle. Maybe it's weight distribution. :iono:

Next time I pop Forza 4 in, I'm going to play with a few of my AWD cars, especially the Golf R and RS3, lock the center diff, set torque split to 50:50, and see how it behaves.

Check this out...
http://www.audi-sport.net/vb/a3-s3-...l-question-any-haldex-controller-experts.html

Edit: Never mind, Forza 4 doesn't incorporate a locking center diff.

Seems the Haldex clutch must slip if it's not disengaged entirely when making a sharp turn.

When we get some snow, might have to do some experimenting.

Thanks for the link, good info there.

K...Set up my Golf R and GTI to B500 and did a couple races.
All mods are stock unless otherwise noted.

Golf R mods: B500, 287hp/275lb-ft, 3153lb, 54% front
Sport Air Filter
Sport Exhaust
Street Flywheel
Race Brakes
Race Springs and Dampers
Race Clutch
Street Transmission
Race Differential

Tune:
Tire pressure 32.0 PSI front and rear
Alignment default
Spring rate default, ride height reduced to 4.7" front/4.8" rear
Damping default
Braking default (though it seems to be a little heavy on the front brakes with 50:50)
Differential: Front 0% both ways, rear 0% both ways, center 50%.

Went and completed the Golf R Spec race, won easily. The car was rather neutral and predictable; oversteer on braking into corners and lift throttle, slight understeer on power, gentle transition from oversteer to understeer going from brake to throttle. No power oversteer at all, however I could do a short 4 wheel drift if started going into a corner...but the front end would always pull out. I'm going to play with the torque setting on this car, I have a feeling that 60% rear will reduce understeer under power while remaining fairly neutral.

GTI mods: B500, 299hp/297lb-ft, 3198lb, 63% front
Race Air Filter
Race Fuel System
Race Ignition
Race Exhaust
Race intercooler
Race oil and cooling
Race flywheel
Race brakes
Race springs
Race anti-roll bars front/rear
Street clutch
Street transmission
Race differential
BBS RE wheels

Tune:
Tire pressure 32/32
Alignment default
Anti roll bars default
Spring rate default, ride height 6.4/7.2
Damping default
Braking default
Differential 0%/0%

Results: Car is a little unpredictable, tail happy under braking and coasting, lights up the front tires on throttle (299hp though) and is much more difficult than the R to hold a line, despite similar suspension setup. Transitions from understeer to oversteer smoothly on lift throttle, transition the other way is much faster. Setting the diff to 50% on acceleration definitely helps get the power down coming out of corners.

Something worth noting is that in this game, the weight between the two cars is wrong. The mk6 GTI in Forza 4 is a 2010 2dr with sunroof, weight listed 3265lb. The Golf R is a 2010 2dr base, weight listed 3188lb. With the above mods, it's still lighter than the GTI by a fair amount. According to the VW site...2013 Golf R 2dr 3325lb vs 2013 GTI 2dr Convenience/Nav 3034lb...


BTW, anyone have Forza 4 and interested in getting into some VW/Audi/Porsche spec races?

Canadian_R said:

Seems the Haldex clutch must slip if it's not disengaged entirely when making a sharp turn.

Click to expand...

Can someone please explain this to me please? I understand the the outside wheels turn faster when cornering and how the differentials accommodate this, but is there a difference in angular velocity between the front and the rear axles as well? If the front and rear are moving at the same speed then there would be no reason for the clutch to slip when cornering right?

BTW, I am still pretty new to this but this has been one of the most informative and interesting threads I've ever read. Thanks for posting socal87!

boost pls said:

Can someone please explain this to me please? I understand the the outside wheels turn faster when cornering and how the differentials accommodate this, but is there a difference in angular velocity between the front and the rear axles as well? If the front and rear are moving at the same speed then there would be no reason for the clutch to slip when cornering right?

BTW, I am still pretty new to this but this has been one of the most informative and interesting threads I've ever read. Thanks for posting socal87!

Click to expand...

There is no center differential. The clutch pack allows for some slip because of difference in axle speeds. I imagine if you goose it while turning, the car will begin to understeer because there's no center diff to appropriate more torque (with less speed) to the rear.

socal87 said:

There is no center differential. The clutch pack allows for some slip because of difference in axle speeds. I imagine if you goose it while turning, the car will begin to understeer because there's no center diff to appropriate more torque (with less speed) to the rear.

Click to expand...

Right, I understand this. I guess I just asked the wrong question... The clutch in front of the rear differential allows slip, which accommodates differences in speed between the front and rear axles. I understand this difference would occur when either the front or rear was losing traction.

But the example of having someone drive around in a tight circle to see how the system behaves with the race controller.. If the clutch were fully locked and the person was driving slowly in a tight circle both the front and rear axles would be spinning at the same speed right? So there would be no issue with tire wear... or am I missing something?

So I'm confused... If the mechanics of the haldex system won't allow for greater than 50% torque to the rear wheels even if the front wheels are on ice, then how would a haldex controller upgrade allow for 60% or more to the rear wheels? Unless I'm just completely missing the point

Sent from my SCH-I545 using Tapatalk 2

boost pls said:

Right, I understand this. I guess I just asked the wrong question... The clutch in front of the rear differential allows slip, which accommodates differences in speed between the front and rear axles. I understand this difference would occur when either the front or rear was losing traction.

But the example of having someone drive around in a tight circle to see how the system behaves with the race controller.. If the clutch were fully locked and the person was driving slowly in a tight circle both the front and rear axles would be spinning at the same speed right? So there would be no issue with tire wear... or am I missing something?

Click to expand...

If the clutch pack is fully locked and doesn't allow for any slip between the two axles, then driving in a tight circle will result in bucking or scrubbing tires. So, if one were to drive with the clutch pack 100% locked 100% of the time, you'd definitely see increased tire wear and possibly drivetrain damage.

intoflatlines said:

So I'm confused... If the mechanics of the haldex system won't allow for greater than 50% torque to the rear wheels even if the front wheels are on ice, then how would a haldex controller upgrade allow for 60% or more to the rear wheels? Unless I'm just completely missing the point

Sent from my SCH-I545 using Tapatalk 2

Click to expand...

They advertise 100% to the rear axle; if the front wheels have no traction then indeed you will see close to 100% of available engine torque being used at the rear axle. However the front axle will spin at the same rate. Imagine a simple chain and sprocket system connecting the two axles. The system cannot direct ALL power to the rear axle as there is no center diff. Its either 100% to the front, or 50% to both. The angle drive for the propeller shaft to the rear is driven by an extension of the front differential case. So, you could completely remove both front halfshafts, and put a machined block in place of the clutch pack, and your Golf R would be RWD.

This would not be possible with a clutch pack because the system reads the ABS wheel speed sensors to determine when the front wheels are slipping. If the front wheels aren't driven...