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Why does Haldex work like this?

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jomagi

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Discussion starter · #1 ·
My understanding of Haldex is that the front wheels receive 100% of the power unless the wheels slip, in which case power is transferred to the rear wheels as well. The result is that Haldex avoids some of the problems of full time AWD (such as power loss) but still supposedly helps the handling. My question is, if this is the system they're going to use, wouldn't it make more sense to have the rear wheels powered full time and if they slip transfer power to the front wheels? Wouldn't everyone prefer this setup, since it is almost universally accepted that RWD cars are the most fun?


Modified by jomagi at 9:44 PM 5-10-2004
 
#4 ·
Re: Why does Haldex work like this? (jomagi)

If I am correct Haldex would never transfer more than 20% of its power to the rear, it's a simple AWD setup to use existing FWD setup on the VWs.
If it transfers too much power to the rear, my bet is the rear drive shafts would break, they are not designed to carry the whole car's weight and power load.
 
#5 ·
Re: Why does Haldex work like this? (jomagi)

No, that's not how it works.
Haldex is 50:50 until slip is detected. People get the mistaken notion that it's "FWD biased" because the nature of any 50:50 AWD system is to slip the wheels doing the most non-throttle-related work first. Since you're steering the front wheels, and the typical Haldex car has more weight up there and more momentum to overcome, the typical power-on cornering response will still be understeer. This is why a standard WRX with its 50:50 viscous coupling will understeer with power-on, also, though.
Nissan's ATTESA E-TS in the Skyline GT-R and Infiniti G35x is set up almost like you describe, with a high rear torque bias until slip occurs. It's not necessarily better - it's an excellent performance system that will step out the rear with throttle (albeit not as aggressively as a RWD car, since the AWD system will catch you if you get too stupid). But it's no better than the average 50:50 setup in the slippery stuff, and in fact to the non-enthusiast driver expecting AWD invincibility it could be very disconcerting.
 
#6 ·
Re: Why does Haldex work like this? (WReXinEFX)

Quote, originally posted by WReXinEFX »
If I am correct Haldex would never transfer more than 20% of its power to the rear, it's a simple AWD setup to use existing FWD setup on the VWs.

It can transfer 100% to either axle, or so VW claims.
The actual mechanism is a wet-clutch system just like ATTESA, just like the DCCD in an STi, just like the PSK setup in a Porsche 959! Anyone who has driven an Evo can tell you that mounting the engine transversely does not make for an inferior-handling AWD performance car, either.
 
#8 ·
Re: Why does Haldex work like this? (AKADriver)

Quote, originally posted by AKADriver »
No, that's not how it works.
Haldex is 50:50 until slip is detected.

This is incorrect. 50:50 split applies to the 'Torsen' set up of an Audi S3. On a MKIV with 4motion, it is indeed FWD until it detects slip, at which point power is transfered to the rear. This means two things:
1) I had to learn to drive it. Against all instinct, in my car you need to accelerate into a corner hard to get the rear wheels to kick in and push you round the corner - otherwise it understeers like a FWD and it sucks. When you force the 4motion to work, its bloody brilliant around a big roundabout
2) Its fast. 4Motion drivetrain loss is 35% or so - 100BHP on an R32. Yet my friends stock R32 is a dead fair match for a FWD 227BHP Revoed 1.8T. Why? Becuase a lot of the time its a FWD car. If it was 50:50 all the time, it would be 137BHP, my car would be 122BHP and we would suck. But we don't.
3) My wheels do not spin unless Im a fool or there is snow (whe its awesome). Caining it in the wet once a friend commented how there was no wheelspin - so I tried to get some. In standing water with ESP OFF, I couldn't get the light to flicker even with appaling 5000RPM starts...its just got loads of grip. In an R32 its slightly easier (40BHP more) but even so, its a pretty safe way to put down 204BHP.
I learnt about 4motion from browsing various forums trying to find out how it works - as AWD set ups go, its not the best but its pretty fast acting and highly reliable - failures are rare to zero.
To answer the initial question I would guess its FWD/AWD rather than RWD/AWD for a mass market perspective - an understeering car is more predictable and less easy to kill to Joe Public than an oversteering one?


Modified by Mikedav at 11:38 PM 5-10-2004
 
Discussion starter · #9 ·
Re: Why does Haldex work like this? (Mikedav)

Quote, originally posted by Mikedav »

To answer the initial question I would guess its FWD/AWD rather than RWD/AWD for a mass market perspective - an understeering ar is more predictable to Joe Public than an oversteering one.

I suppose that is the obvious answer, although I find it unfortunate (and counterintuitive). Apparently the infiniti/nissan AWD system is RWD/AWD. Makes more sense, especially for a car that is supposedly performance oriented.
 
#10 ·
Re: Why does Haldex work like this? (AKADriver)

Quote, originally posted by AKADriver »
No, that's not how it works.
Haldex is 50:50 until slip is detected..

This is true, but consider that it is a dynamic system which can react to different needs as quickly as a 15 degree wheel rotation (think about how short a span of time this really is)
Driving the R32 and the 337 back to back is a revelation, the R32 is as easy to drive as a FWD (and a natural progression if FWD has been your only driving experience) but it just sticks better, you feel the AWD kick in out back and sorting this out....and Haldex in the R32 can produce lift throttle oversteer...
Image
 
#11 ·
Re: Why does Haldex work like this? (Mikedav)

Quote, originally posted by Mikedav »
To answer the initial question I would guess its FWD/AWD rather than RWD/AWD for a mass market perspective - an understeering car is more predictable and less easy to kill to Joe Public than an oversteering one?

This is exactly what it boils down to, mass market appeal. Volvo's Haldex I unit is 90% front and 10% rear under no-slip conditions, but it can supposedly transfer 100% to either front or back. This is probably just fine for 99% of the people who buy these cars.
Volvo's R models which use Haldex II is a completely different story. It can send power to the back wheels before slip is detected, by throttle input sensors. Especially useful during quick standing starts. Here, the Haldex II system is programmed more for performance. It has more sensors, plus it interacts with the 4C suspension.
ATTESA ET-S is kind of like a reversed version of Haldex I, but during standing starts the G35x is 25/75 and then goes to 0/100 if conditions are good. But there is also a switch that can lock it at 50/50.
 
#12 ·
Re: Why does Haldex work like this? (AKADriver)

Quote, originally posted by AKADriver »

Haldex is 50:50 until slip is detected.

I'm pretty sure that is wrong. The haldex system is a clutchpack; it has no center differential. It can approximate a 50:50 split by locking the front and rear axles together, but in that case it can't accomodate different speeds between the two axles, so it probably never does that unless you are starting on ice or something.
Under normal circumstances the rear wheels on a haldex car are merely along for the ride. As the front tires start to lose traction, the haldex clutch can start to send torque to the rear wheels, but the only time the rear wheels will ever get 100% of torque is if the front wheels have zero traction.
 
#13 ·
Re: Why does Haldex work like this? (AKADriver)

Quote, originally posted by AKADriver »
No, that's not how it works.
Haldex is 50:50 until slip is detected.

No it isn't.
 
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#14 ·
Re: Why does Haldex work like this? (jomagi)

Quote, originally posted by jomagi »
My understanding of Haldex is that the front wheels receive 100% of the power unless the wheels slip, in which case power is transferred to the rear wheels as well. The result is that Haldex avoids some of the problems of full time AWD (such as power loss) but still supposedly helps the handling. My question is, if this is the system they're going to use, wouldn't it make more sense to have the rear wheels powered full time and if they slip transfer power to the front wheels? Wouldn't everyone prefer this setup, since it is almost universally accepted that RWD cars are the most fun?

well thats not really possible, All haldex cars are based off FWD cars, and therefore the transmissin will recieve the power first nomatter what. Also a nose heavy RWD car isnt a ton of fun either. Haldex works in a very efficiant way that is for the most part optimal for handling, and efficiancy.
 
#15 ·
Re: Why does Haldex work like this? (WReXinEFX)

Quote, originally posted by WReXinEFX »
If I am correct Haldex would never transfer more than 20% of its power to the rear, it's a simple AWD setup to use existing FWD setup on the VWs.
If it transfers too much power to the rear, my bet is the rear drive shafts would break, they are not designed to carry the whole car's weight and power load.

completly untrue, haldex can recieve up to 100% of the power to the rear wheels. Its much more adaptive than anyone gives it credit for. A simple haldex controller will give you a 50/50 torque split.
 
#16 ·
Re: Why does Haldex work like this? (AKADriver)

Quote, originally posted by AKADriver »
No, that's not how it works.
Haldex is 50:50 until slip is detected. People get the mistaken notion that it's "FWD biased" because the nature of any 50:50 AWD system is to slip the wheels doing the most non-throttle-related work first.

not true, haldex is 100/0 until it slips, but unlike viscous coupling systems it repsonds in 15 degree's of wheel rotation therefore mainating bith stability and efficiancy, something full time awd systems fail to do.
 
#18 ·
Re: Why does Haldex work like this? (Jettavr666)

The haldex system on the R32 is not 50-50. the haldex coupling seperates power from the rear axle until a difference in wheel speed happens, at wich point the clutch in the coupling closes and sends power to the rear wheels. The main advantage to this is that you can tow an R32 with the rear wheels still on the ground with no concequence, or run a spare, "donut" tyre, (wich the R32 doesnt come with, but whatever).
 
#19 ·
Re: Why does Haldex work like this? (Mikedav)

Quote, originally posted by Mikedav »
This is incorrect. 50:50 split applies to the 'Torsen' set up of an Audi S3.

Audi S3 has the same Haldex system as VW Golf and Audi TT.
Sorry, just had to make a correction.
 
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#20 ·
Re: Why does Haldex work like this? (J-Tim)

Quote, originally posted by J-Tim »

Audi S3 has the same Haldex system as VW Golf and Audi TT.
Sorry, just had to make a correction.

Oops! Sorry, is it the Passat 4Motion that runs Torsen then despite both tht car and mine being badged 4motion? I can't remember. If its not the S3 then I think its the Passat 4Motion
 
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