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Old 08-27-2013, 10:07 PM   #1
socal87
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Can someone explain the FWD Haldex system to me?

I feel this question would be easily answered by a schematic or diagram of the R32/CC VR6/Passat VR6/Tiguan 4motion transmission, but my question is this:

The Haldex system supposedly can deliver up to 100% of the power to the rear axle. Now I'm probably wrong, but I thought the PTO for the prop shaft to the rear axle got its power from the front differential housing - so there's a flange on the right side for the PTO, with a spline socket in the center for the right side halfshaft. The PTO and prop shaft are always driven, and a clutch pack on the rear differential input determines how much power the rear axle receives.
Like this:


And this:


If I'm right, then the rear axle can only receive 50% of the power, as there is no "center" differential - the clutch pack allows slip between the front and rear axles to prevent binding, but there is no way to direct 100% of the power to the rear axle...short of removing the front halfshafts and making the car RWD.
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Old 08-27-2013, 10:28 PM   #2
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You are correct. You can send 100% of available power, and since there is no center differential to control that, the most that can be sent is 50% of total torque.
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Old 08-27-2013, 10:34 PM   #3
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You are correct. You can send 100% of available power, and since there is no center differential to control that, the most that can be sent is 50% of total torque.
Right. And since torque is the force applied, and power is work performed with the force applied, then TECHNICALLY 100% of power can go to the rear...if the front wheels have absolutely no traction. Still, all 4 wheels would be driven equally if the clutch pack were completely locked up. Shame, considering much fun can be had with a 70/30 rear split...

Yes, I've seen the APR MaxR drifting videos, but the GTI will step out the rear too...just not under throttle.
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Old 08-28-2013, 11:42 PM   #4
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Yes, I've seen the APR MaxR drifting videos, but the GTI will step out the rear too...just not under throttle.
What about with a Haldex controller?
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Old 08-29-2013, 02:05 AM   #5
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What about with a Haldex controller?
As I said before, even at full lockup, the rear axle will only receive 50% of available torque...so a comp/race controller will get you true AWD, but the car won't want to oversteer under throttle because the rear axle isn't being overdriven. Rather, as engine torque overwhelms available traction at all four corners, the car will slide. Watching the BFGoodrich MaxR videos, you can tell by the way the car behaves that the rear wasn't coming out because of throttle - you can do the exact same thing with a FWD GTI, which will behave much the same way when you get on the throttle - the car will transition from oversteer to understeer as the front end pulls out.

The Audi mechanical quattro system (as seen in the B7 RS4,RS6/B8 S4 S6) has a mechanical bias of 60/40 rear. This is done using a planetary center differential, driven by the tailshaft of the longitudinally mounted transmission. The propeller shaft to the front diff is actually contained within the transmission, as is the front diff. This is how the base Audis have their engines mounted the American way, but are still front wheel drive. Subaru is very similar.

The system as used in the S3, Golf R, 4motion, et al uses an extended flange off the front differential to drive the bevel gear that powers the propshaft to the rear. There is no center differential, so it basically acts like a transfer case - and the ONLY difference there MIGHT be in torque between the two axles would be determined by gearing in the rear pinion, which I am assuming is just 1:1.

If you eliminate the clutch pack, you'll get driveline binding when turning. If you kept the clutch locked up all the time, you'd have full time AWD, except it differs from the common interpretation of AWD where power can be directed to any one wheel. In our case, any front AND any rear wheel will be constantly driven.

The schematics and diagrams I've seen suggest that a potentially cheaper way of doing things could be bolting a right angle PTO to the right side output of the transmission...basically driving the rear axle off the right front wheel. But that's just silly.
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Old 08-29-2013, 02:16 AM   #6
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Watch this:



Notice how he doesn't powerslide out of a corner...the back end comes out as he turns into a corner, often following understeer.
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Old 08-29-2013, 10:24 AM   #7
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ok so here is my question. How can the 4motion transfer case send a 100% torque power to the rear with a front open differential. Does a 4motion front differential have a sleeve that goes over the right side of the passenger side output shaft from the diff. This sleeve attaches to the ring gear and sends power thru the transfer case so its constantly driven especially when power is diverted from one front wheel to the other during a loss of traction event?

Just trying to see if there are differences between a 4motion front diff and a FWD front diff.
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Old 08-29-2013, 12:14 PM   #8
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ok so here is my question. How can the 4motion transfer case send a 100% torque power to the rear with a front open differential. Does a 4motion front differential have a sleeve that goes over the right side of the passenger side output shaft from the diff. This sleeve attaches to the ring gear and sends power thru the transfer case so its constantly driven especially when power is diverted from one front wheel to the other during a loss of traction event?

Just trying to see if there are differences between a 4motion front diff and a FWD front diff.
Yes, this is exactly how they do it. The Haldex-prepared transmissions (DSG and 6MT) are mechanically different from the FWD versions, wherein the right side output actually has two splines - the inner shaft for the front right wheel, and the outer sleeve for the bevel gear drive. So, there is no differential in the flow of power from the gearbox to the rear axle...just the clutch pack.

Imagine if the front differential ring gear had a sprocket on it, with a chain running to a similar sprocket on the rear differential, with a clutch pack mounted between the rear sprocket and ring gear There is no way to shift power back and forth between the axles, other than using the clutch pack to vary power at the front between 95 and 50%.

The beauty of this is, when the clutch pack is locked up, the front wheels cannot spin independently of the rear. So (assuming we have locking differentials) if the front axle has absolutely no traction, but the rear axle does, the axles will rotate at the same speed. With the standard controller, it doesn't work this way...the front axles MUST rotate a certain amount more than the rear axle before the clutch pack locks up.

Conversely, if the rear wheels have no traction but the front wheels do, you'd see the same thing in either case because a locked up controller would drive the rear wheels at the same speed as the front...while an open controller would allow the rear wheels to function as if the car is FWD.

Now, if we look at the system as designed from the factory (standard controller and open diffs front and rear) then things become interesting. If ONE of the front wheels doesn't have traction, it will spin freely when power is applied, only interrupted by the XDS system. The rear axle will eventually get power and the car will move. If ONE of the rear wheels doesn't have traction, the car will immediately move because power isn't sent to the rear by default.
If ONE of the front wheels AND one of the rear wheels doesn't have traction...the car won't move at all, because the front will spin a little until power is sent to the rear, then BOTH will begin to spin.

It's probably a good thing that the system doesn't allow for any rear bias anyway, because the way the chassis and suspension is set up, the car won't handle oversteer well. Just look at the W650 concept. So, the 4Motion system is designed with traction in mind, less so than performance, since it essentially piggybacks the rear axle off the front. So, to simplify and summarize...imagine the Golf R AWD system as a regular part time 4WD system with open differentials.
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Old 08-29-2013, 02:45 PM   #9
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^ Thanks that finally explains how the front part of the haldex system works. I could find any information about it in the self study guids. Been search every corner of the web for this kind of explaination with all the talk about AWD conversions going on in the forum these days. Always knew the general concept that haldex is part time awd full time fwd.

Yea I remember when the W650 was on top gear. It was just going donuts when it entered a corner.
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Old 08-29-2013, 07:54 PM   #10
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^ Thanks that finally explains how the front part of the haldex system works. I could find any information about it in the self study guids. Been search every corner of the web for this kind of explaination with all the talk about AWD conversions going on in the forum these days. Always knew the general concept that haldex is part time awd full time fwd.

Yea I remember when the W650 was on top gear. It was just going donuts when it entered a corner.
Maybe if it had Haldex it would do better...
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Old 08-30-2013, 05:39 PM   #11
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I have *heard* that the reason Audi didn't put a Quattro setup under the S3, was because it wouldn't fit under the car. Is this correct?
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Old 08-30-2013, 11:04 PM   #12
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I have *heard* that the reason Audi didn't put a Quattro setup under the S3, was because it wouldn't fit under the car. Is this correct?
Yes, because the A3 is transverse FWD, so to reduce packaging complexity, they used the same system they used in the Golf.

The A4 is longitudinal, so it lends itself easily to mechanical AWD. Or, if someone were to replace the center diff with simple gears and remove the front halfshafts, it would be RWD. Same for the A6.
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Old 09-05-2013, 03:01 PM   #13
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So if you're hard on the throttle around a sharp corner (so that the front wheels would loose traction if power wasn't going to the rear) how does the system control the power to the rear and yet still allow the slip needed between the front and rear axles?
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Old 09-05-2013, 03:09 PM   #14
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as already stated, get a comp or race controller.



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