# Haldex Difference Between S3 and RS3



## DaLeadBull (Feb 15, 2011)

I've heard that there is a difference between the Haldex systems in the S3 and the RS3. The Haldex is the RS3 is supposed to be more "aggressive", whatever that means. Its all just clear as mud!

I just wanted to make this thread so that those in the know can come and shed some light on this topic. I'm sure there are other wondering about the same thing.

I think mechanically both cars have the same hardware but there are programming differences? I've heard the RS3 shares its programming with the TT models. 

So, anyone know what exactly these software changes are? More rear power bias? Quicker power transfer?


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## reyoasian (Feb 22, 2015)

Haldex Quattro on the S3 is the same as the Golf R, TT/TTS, and other VW vehicles. It's an electronic AWD system that's 100% FWD by default, and transfer up to 50% of power to the rear wheels upon loss of traction.

I don't know much about the RS3 Quattro but I would assume it's something similar to the one from Touareg, which is a much better system. Possibly mechanical, with torque vectoring, or something else fancy


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## araemo (Dec 9, 2012)

reyoasian said:


> Haldex Quattro on the S3 is the same as the Golf R, TT/TTS, and other VW vehicles.


While mechanically they are all at least very similar, it's been stated to the press that there are software and calibration differences between the systems in the different applications. (see link at bottom)

TT-S gets a more 'aggressive' software/calibration than the S3 does. RS3 gets an even more aggressive one.



> It's an electronic AWD system that's 100% FWD by default, and transfer up to 50% of power to the rear wheels upon loss of traction.


You're partially correct. It will transfer power to the rear wheels upon loss of traction, or when the software tells it to, which can be when the throttle is pressed aggressively. It does not need to wait for wheel slip like the gen1 haldex systems that used a mechanical linkage to activate the clutch pack when there was a speed difference between the axles. But if the wheels slip, it definitely will cut in. Before wheel slip, it's up to how the system is programmed.

Also, it's incorrect to refer to the haldex system as 'transfering power' - it is not a biasing differential, it is a simple clutchpack that can vary between free-spinning (rears are not being driven by the driveshaft) and fully engaged (rears are turning at the same speed as the fronts). When the fronts and rears are turning at the same speed, the torque distribution is based on the available traction. The wheels with the most traction receive the most torque through simple physics. If the rear wheels are capable of holding 100% of the torque, and the fronts are not capable of holding any of it (for example: they're on ice), the rears get 100% of the torque. But more likely is simply that you'll be able to apply a higher % of the available engine torque to the ground at any given time.



> I don't know much about the RS3 Quattro but I would assume it's something similar to the one from Touareg, which is a much better system. Possibly mechanical, with torque vectoring, or something else fancy


Nope, RS3 still uses haldex, but as stated by VW/Audi... there are both software (the actual code running on the haldex control unit), and calibration (the data that code uses as reference points to compare to live readings) differences between the different vehicles using the gen5 haldex systems. This is how it can feel more 'all wheel drive' - locking the rear wheels to the front wheels much more proactively. This will hurt fuel economy, but it will increase the satisfaction of most of the drivers of the car, most likely. 

As far as I've seen reported, any side-to-side torque vectoring in the RS3 is ESC-style brake-mediated, not a separate clutch unit in the diff like the 'sport diff' in the S4s/etc.

Edit: Source: https://www.youtube.com/watch?v=9fCpMMURclE&t=313


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## DaLeadBull (Feb 15, 2011)

^ Thanks for the explanation. I've heard all that before in bit and pieces but reading it again makes a lot more sense.

So, in essence the programming in the RS3 is pretty much like an aftermarket Haldex controller when compared to the S3/A3.


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## reyoasian (Feb 22, 2015)

araemo said:


> While mechanically they are all at least very similar, it's been stated to the press that there are software and calibration differences between the systems in the different applications. (see link at bottom)
> 
> TT-S gets a more 'aggressive' software/calibration than the S3 does. RS3 gets an even more aggressive one.
> 
> ...


Awesome, so Haldex really stepped up their game.

I question this part:


> it is a simple clutchpack that can vary between free-spinning (rears are not being driven by the driveshaft) and fully engaged (rears are turning at the same speed as the fronts)


When fronts are on ice and have 0 traction, is 100% of torque actually sent to the rear wheels, or it's still a 50/50 (as all wheels are spinning at the same speed), but since the front has no traction, that front 50% torque become power loss, and the rear 50% torque become 100% of power that's pushing the vehicle?


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## araemo (Dec 9, 2012)

reyoasian said:


> Awesome, so Haldex really stepped up their game.
> 
> I question this part:
> 
> ...


You're confusing rotational motion with torque (rotational force). The fronts will move at the same speed, but it will take basically 0* torque to rotate them at that speed. So, no the power isn't 'lost' to the front. If the engine pushes against the wheels with 100 lb-ft of torque, and it only takes 1 lb-ft to move them, they're going to move REALLY fast REALLY suddenly.... but since they're linked to the rears that are on tarmac, that can't happen, and all the torque is used to try to move the rears. It would be 'wasted' if the wheels started spinning freely, which could only happen if the haldex clutchpack was disengaged.. or the rears were also on ice.

For a better explanation of how the fronts being linked to the rears can produce something different from a 50/50 torque split, read this, statements from haldex directly: http://www.audiworld.com/forums/aud...x-system-got-reply-haldex-themselves-1999304/


> During cruising which a constant velocity, we have the possibility to
> transfer up to 40-45% of the torque to the rear axle, given nominal tyres.
> During acceleration, the weight transfer increase the front tyre slip and
> decreases the rear axle slip, giving us the possibility to achieve more or
> ...


That's about Gen II haldex, so a lot is out of date and not at all accurate to the MK3 A3/S3, but the 'torque split' discussion applies. But it really boils down to there is no actual 'torque split' between front and rear axles. Once the clutch is 100% engaged they're treated as one axle, and it's just a matter of how much traction you have available between all 4 wheels. (The front and rear open differentials can still cause issues if one wheel on each axle is on ice, and then you rely on brake-mediated traction control... but most of the torsen-based and crown gear-based quattro cars have open front/rear diffs these days too, except some S/RS models.)

And yes, haldex stepped up their game with their gen III units (they added electronic pre-tensioning, so they could lock up before slip.. but the pump was still operated by 'slip'), and even more so with gen IV, which was fully electronically controlled hydraulics. Gen V is just improvements on the gen IV electronically controlled unit... The first gen A3/S3 used the gen II haldex, and the second gen A3/S3 used gen IV haldex.

*: technically not 0, but if they're really on ice, the only torque that can be 'used up' to turn them is the torque needed to overcome their friction coefficient with the ice and to overcome their momentum.. which will be a very small % of the overall torque the engine is putting out.. Torque doesn't 'disappear' - it has to be used to do work to go anywhere, and the work in this case is turning the wheels to move the car, whether they're pushing against ice or tarmac.


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## reyoasian (Feb 22, 2015)

araemo said:


> You're confusing rotational motion with torque (rotational force). The fronts will move at the same speed, but it will take basically 0* torque to rotate them at that speed. So, no the power isn't 'lost' to the front. If the engine pushes against the wheels with 100 lb-ft of torque, and it only takes 1 lb-ft to move them, they're going to move REALLY fast REALLY suddenly.... but since they're linked to the rears that are on tarmac, that can't happen, and all the torque is used to try to move the rears. It would be 'wasted' if the wheels started spinning freely, which could only happen if the haldex clutchpack was disengaged.. or the rears were also on ice.
> 
> For a better explanation of how the fronts being linked to the rears can produce something different from a 50/50 torque split, read this, statements from haldex directly: http://www.audiworld.com/forums/aud...x-system-got-reply-haldex-themselves-1999304/
> 
> ...


I guess what I meant to ask was: if the vehicle had rear wheels on the ground, and front wheels in the air...and you accelerate...are the front wheels gonna spin? If so, that would be really the engine pushing 50% torque to the front, and the rear (or whatever % they claim). If the front don't spin at all as the system senses no traction, then all of the torque are going to the rear. Not sure which is the case here?


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## roblove (Apr 18, 2014)

reyoasian said:


> I guess what I meant to ask was: if the vehicle had rear wheels on the ground, and front wheels in the air...and you accelerate...are the front wheels gonna spin? If so, that would be really the engine pushing 50% torque to the front, and the rear (or whatever % they claim). If the front don't spin at all as the system senses no traction, then all of the torque are going to the rear. Not sure which is the case here?


I think the point araemo is trying to make is, if the front wheels are in the air, it doesn't take any torque to spin them.

Maybe this is just arguing about wording given the physics, but if the wheel doesn't have any traction, it can't consume nontrivial torque. And thus all of the _available_ torque _is_ going to the rear wheels.


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## reyoasian (Feb 22, 2015)

roblove said:


> I think the point araemo is trying to make is, if the front wheels are in the air, it doesn't take any torque to spin them.
> 
> Maybe this is just arguing about wording given the physics, but if the wheel doesn't have any traction, it can't consume nontrivial torque. And thus all of the _available_ torque _is_ going to the rear wheels.


Yes, but will the front wheels be spinning from the gas pedal?

Maybe I'm being very confusing that's why...but let me try again

If the car has rear wheels on the ground with traction, and front wheels on ice or air with no traction...and you accelerate....are all 4 wheels getting power sent to them like a subaru, or only the rear wheels are getting power to them (like a RWD car)?


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## lilmira (Feb 4, 2014)

Have you been stuck in snow when your wheels are spinning and your car is not moving? That's because there is no traction available. It doesn't mean that your car can't produce torque, it's just there is nothing to apply to. It's like telling a heavyweight lifter to lift without giving him the weight, how much is he lifting now? nothing.


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## reyoasian (Feb 22, 2015)

lilmira said:


> Have you been stuck in snow when your wheels are spinning and your car is not moving? That's because there is no traction available. It doesn't mean that your car can't produce torque, it's just there is nothing to apply to. It's like telling a heavyweight lifter to lift without giving him the weight, how much is he lifting now? nothing.


My question revolves around engine sending torque to the wheels, not WHP (power sent to the ground).
Just trying to understand how the system works.

In the case with the lifter...he's still sending energy/power to the arms to lift. He doesn't have to send as much power to lift them up, but energy is still required...out of this energy, is 100% of it on the right arm, or left arm, or 50% left and 50% right?

Note when I question 50% torque, 100% torque, etc. I'm not talking about full power from the engine...I'm asking about the torque split ratio between front and rear that the engine is sending out at that specific moment. (e.g. if the engine is sending out 20% if its power considering 100% is full WOT...out of that 20%...are they fully to the rear wheels, or 10% to the front and 10% to the rear aka 50/50 split?)


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## lilmira (Feb 4, 2014)

An engine can't just send torque where there is nothing to apply to regardless of how the machine works, that's the end condition. 

What if I lift three wheels off the ground, now the most you'll have is all the torque going to the remaining wheel. Depending on the drive train, if you have open diff which limit your torque to even distribution, now you have 0 output because 3 wheels are free wheeling, just like getting stuck in snow. That's why offroaders have locking differential to prevent free wheeling.

Watch some videos on how differentials work, it's quite interesting.


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## reyoasian (Feb 22, 2015)

Just read through this: http://www.ozaudi.com/forums/a3-s3-rs3/18442-haldex-vs-torsion-awd.html#post153342
Good read.

some of the things that araemo mentioned in a little more detail.

To answer my own question -- if front lost all traction, then all power sent to the wheels and the car will "behave" like a RWD car, not a subaru.


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## araemo (Dec 9, 2012)

reyoasian said:


> If the car has rear wheels on the ground with traction, and front wheels on ice or air with no traction...and you accelerate....are all 4 wheels getting power sent to them like a subaru, or only the rear wheels are getting power to them (like a RWD car)?


Yes, the front wheels will spin, but that doesn't mean that they received 50% of the torque. There's a big difference between 'they received enough torque to spin them when they have no friction stopping them' and 'they received 50% of the torque produced'. On a car with an open center diff, the total torque delivery is limited by the axle with the least traction, but as I said before, the haldex unit is not a diff, so it doesn't have that limitation. So, the torque delivery is limited by total traction available to all 4 wheels.

On a vehicle with a torsen center diff, the torque delivery is limited to a multiple of the torque required to overcome friction on the axle with the least traction (Which is how it was easy to get early audi's with quattro stuck in the situation we're discussing, if the fronts or rears were on ice.. it was pretty useless. This is also why in those situations, applying the brake slightly while also accelerating gently would let you get out, because it would increase the torque delivered to the slipping wheels (by delivering it to the brakes instead of the ice), allowing more torque to be transferred to the wheels with grip). On vehicles with viscous coupling centers (most subarus), there's usually a minimum % of the torque that goes to each axle.

So, like I said before.. the fronts spinning in that situation is not a bad thing, because they can only spin as fast as the rears can in that situation, they can't spin faster (and thus waste torque).


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## reyoasian (Feb 22, 2015)

araemo said:


> Yes, the front wheels will spin, but that doesn't mean that they received 50% of the torque. There's a big difference between 'they received enough torque to spin them when they have no friction stopping them' and 'they received 50% of the torque produced'. On a car with an open center diff, the total torque delivery is limited by the axle with the least traction, but as I said before, the haldex unit is not a diff, so it doesn't have that limitation. So, the torque delivery is limited by total traction available to all 4 wheels.
> 
> On a vehicle with a torsen center diff, the torque delivery is limited to a multiple of the torque required to overcome friction on the axle with the least traction (Which is how it was easy to get early audi's with quattro stuck in the situation we're discussing, if the fronts or rears were on ice.. it was pretty useless. This is also why in those situations, applying the brake slightly while also accelerating gently would let you get out, because it would increase the torque delivered to the slipping wheels (by delivering it to the brakes instead of the ice), allowing more torque to be transferred to the wheels with grip). On vehicles with viscous coupling centers (most subarus), there's usually a minimum % of the torque that goes to each axle.
> 
> So, like I said before.. the fronts spinning in that situation is not a bad thing, because they can only spin as fast as the rears can in that situation, they can't spin faster (and thus waste torque).


Cool. Much better understanding now. Gona watch some videos too.
This will be my first German/Audi/VWCorp/Quattro/Haldex so trying to understand how the system works.


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## A3SBQ (Aug 26, 2013)

Bumping this. 
It seams that the RS3 8V Haldex unit is different in size. New left and right driveshafts are needed to if a retrofit is due. 
This also explains there is not only sw changes but also hw. 
I wonder what was changed?


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## thebc2 (Apr 15, 2016)

They have previously said that the RS3 has an improved Haldex pump aside from the programming changes.


Sent from my iPhone using Tapatalk


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## The Wiry Irishman (Apr 14, 2016)

https://www.audiusa.com/models/audi-rs3/performance

At the link above and a couple of other places, Audi has referred to the new RS3 as having permanent all-wheel drive. I'm curious as to whether this is disingenuous marketing BS like claiming it can send 100% of the power to the rear, or if the Haldex is going to be engaged all the time.


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## steve111b (Jun 2, 2011)

Permanent AWD (50/50) has been available on A3/S3/RS3 Haldex cars since 2009. There may be exceptions that I am not aware of, but I suspect all cars have this feature.

Your first choice is to turn off ESP, this will activate 50/50 AWD.

Second choice is to put the car in Sport mode, this will activate 50/50 AWD.

The FL S3 has an option for the Haldex in the MMI. When set to dynamic it changes something. My guess is it engages the AWD 50/50 like the two choices above. No one has confirmed this, but it would make sense.

Most information about the Haldex states that the application of any braking action will disengage the Haldex. My brief seat time in a FL S3 a couple of months ago leads me to belief that the ESP will activate at the same time as the car goes into oversteer. In other words a front wheel can brake while the rears continue to power the car into oversteer.


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