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Haldex question

Discussion in 'A3/S3 Forum (8L Chassis)' started by Aky, Apr 27, 2007.

  1. Aky
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    Aky Aky

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    [Apr 27, 2007]
    When the haldex kicks in how much power does it send to the back wheels?

    Does the Performance haldex put more power to the rear or does it put the same but just faster?

    Is there anyway of getting a 70/30 split? (rear/front)
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  3. Booth_S4
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    Booth_S4 Riding on V8 power

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    [Apr 27, 2007]
    You can get one of the performance haldex controllers but i dont think it changes the split. From memory i think its more like 60/40 front.
    #2
  4. ChriS3
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    ChriS3 hud at ye bam

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    [Apr 27, 2007]
    I actually thought is was an even 50/50. Could be wrong....

    The Haldex performance controller won't change the torque split, it just changes the tollerences under which the diff clutch engages.
    #3
  5. Booth_S4
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    Booth_S4 Riding on V8 power

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    [Apr 27, 2007]
    Maybe its me thats wrong.... ?!
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  6. madvw
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    madvw Active Member

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    [Apr 27, 2007]
    the maximum you can transfer to the rear wheels is 50%. All that different controllers do, is change the circumstances in which the power is transfered and removed, or even fix it 50/50.
    #5
  7. colinra
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    colinra Member VCDS Map User

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    [Apr 27, 2007]
    The haldex system can put 'near 100%' of available power to the rear wheels in the right circumstances. I have an indepth explanation of what it can do on my PC but it came from this forum at some point. Try a search and if not I will post it up.
    #6
  8. madvw
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    madvw Active Member

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    [Apr 27, 2007]
    its physically impossible for it to transfer any more that 50% of the power to the rears.
    #7
  9. colinra
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    colinra Member VCDS Map User

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    [Apr 27, 2007]
    Ok here is the article it goes on a bit:

    Hello.

    As the owner of an Audi S3 I make use of the Haldex LSC everyday. I am also
    a member one particular Audi forum (www.audi-sport.net). We have discussed
    the issues of Haldex, torque transfer and such like. In particular regard
    comparing to Torsen.

    The Haldex LSC solution is often seen as inferior and is often said to be
    "not real 4WD". I disagree and believe it's abilities go beyond that of
    Torsen when used in real driving scenarios.

    I would greatly appreciate it if you could take the time and answer my
    questions. I'll try to be as brief and as clear as possible. Thanks.

    1) Haldex claim 100% torque transfer to the rear is possible. Is it? Some
    people dispute this, but I believe it requires 100% slip of the front wheels
    for this to be the case. i.e. In real conditions it would never happen, the
    more realistic split is 50-70% under full load and some slip. I came to this
    conclusion because the front wheels are always driven.

    2) There is much dispute over whether the Haldex LSC delivers torque to the
    rear if and only if the front wheels slip. It's my belief that the torque
    transfer can and does occur in response to wide throttle openings, even
    before any slippage occurs. Can you confirm?

    3) With particular regard to the Audi S3 (or TT). Do you know if the
    steering angle is taken into account when deciding whether to increase or
    decrease the torque transfer? i.e. is it purely throttle/slip related or is
    actually much more complex? If possible, some explanation would be useful.
    For example, if there is 10% to the rear and I turn a corner, would it
    increase to 30 or 40% to provide increased stability. If so, would be that
    due to a natural left/right speed difference or a result of steering angle
    change (ESP sensor)?

    4) There is much debate on the "constant speed" torque transfer to the rear.
    i.e. travelling on a road at 30 or 70MPH, how much torque is transferred to
    the rear? Is it speed dependant?

    5) Does the torque transfer under acceleration depend on the current vehicle
    speed? If so, can you provide characteristic details?

    Any further information you can provide would be appreciated, in particular
    how the Haldex LSC operates in conjunction with the ESP and whether the ESP
    can affect the transfer of torque.

    The folk on the forums are an enthusiastic bunch who appreciate technical
    explanations to technical problems. If there's anything else you can provide
    beyond the questions I've asked, that would be very welcome. Once again, I
    thank you again for taking the time to read my questions.


    Cheers.

    Mark.

    -----------


    Dear Mark,

    I am pleased to see that you like your Audi S3 with the Haldex AWD system
    and that you disagree with your Torsen friends when it comes to the
    excellent abilities of the Haldex AWD System. It is a fact that the
    electronically controlled Haldex AWD system provides a much wider range of
    possibilities over a purely mechanical system. We are also convinced, after
    having delivered 500.000 Haldex AWD systems to the market since our start in
    1998, that we also have the best electronically controlled AWD system on the
    market.

    We have put together some answers on your questions that I hope will be
    helpful in your discussions in the Audi forum.



    1. There are situations where near 100% torque transfer to the rear axle
    occur. An example is if the front wheels are on ice and the rear wheels are
    on tarmac. In that case the front wheels have (almost) no grip. In that
    case, the Haldex coupling will transfer all torque to the rear axle and
    prevent front wheel spin. On uniform surfaces however, the coupling can not
    transfer all torque to the rear axle. See below.

    2. We need slip over the coupling in order to be able to transfer torque.
    That slip (rotational speed difference between the front and rear axle) is
    created by different tyre rolling radius (front to rear) and drive slip
    between the tyre and road. The rolling raduis difference can be created by
    differently worn tyres (or different dimensions, something that should be
    avoided) and different load. In most cars, the front axle has a greater load
    than the rear axle, which causes the roll radius of the front tire to be
    smaller than the one for the rear tyres (given the same nominal size). This
    gives us the possibility to transfer torque to the rear axle also when no
    slip occurrs on the front tyres.

    If you have differently worn tyres on the front and rear axles, the new
    tyres should always be on the rear axle. This is true no matter if the car
    is FWD, RWD or AWD, since you otherwise risk heavy and uncontrollable
    oversteer in situations such as aqua planing. In this case, putting the worn
    tyres at the front also helps not to reduce the maximum transferable torque
    (maximum rear axle torque).

    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
    less the same torque distribution as the dynamic weight distribution.
    Generally speaking, depending on the vehicle somewhere around 60-70% is
    possible to achieve during a full acceleration. Note that we are still
    talking about a uniform surface, with no spin on the front wheels.

    When cornering , there is in most cars a tendency for the inner front wheel
    to lift and spin. In that situation, we can increase the torque transfer
    even further.

    So far I have only spoken about what possibilities there is to transfer
    torque. How much is actually transferred depends also on how the Haldex
    coupling is controlled. The engine torque and gas pedal position are
    together with the wheel speeds and the engine speed the most important
    signals that are used in the control. Brake, ABS and ESP signals are also
    very important for enabling co-existance between the AWD system and the
    ABS/ESP system. We control the coupling in order to prevent wheel spin as
    well as removing it quicky if it should occur.

    3. Steering angle is not a signal used in the control of the coupling in VW
    group cars. The reason for this is that the steering angle is not available
    in most cars as it is only present when an ESP system is mounted. We do
    however calculate the curve radius from the wheel speeds. We have software
    ready using more signals as the steering angle that we offer to the vehicle
    manufacturers. This enables further optimisation of handling performance.

    4. It may vary a bit with speed (and road surface), but without going into
    details a figure of around 10-15% would be typical. It is enough to help
    stabilising the car while at the same time saving fuel and reducing the
    temperature of certain driveline components. As soon as the driver starts to
    accelerate or decelerate, more torque is transferred.

    5. Yes it does. In general, a higher percentage of the torque is transferred
    to the rear axle in low velicities than in high ones. This is partly due to
    the fact that the total available driveline torque is larger at lower speeds
    (and lower gears), thus causing more weight transfer to the rear axle. In
    order to achieve consequent handling characteristics (as well as optimised
    traction), more torque must then be transferred to the rear axle.

    6. The Haldex coupling is completely compatible with ABS and ESP systems.
    In order to optimise the performance of the ABS/ESP system, it is possible
    to open the Haldex couplng during ABS braking or a stability control brake
    intervension. The ABS/ESP antispinn and stability control also depend very
    much on being able to calculate the vehicle velocity. That is very easy with
    2WD, but as soon as you have the possibility of four wheels spinning it gets
    very complicated. The Haldex coupling and ABS/ESP system interface make it
    easier to obtain a good reference velocity.
    In the cars where the Haldex coupling is available today, additional signals
    available with ESP are not used. We do however have software using these
    signals. This enables further optimisation of handling performance and life
    span of driveline components.


    During calibration of the Haldex coupling , we try to optimise the traction
    and handling performance of the car. These are however not the only aspects
    that are important. The final calibration is alway a compromise between
    traction, handling, the life span of driveline components, temperatures in
    driveline components, fuel consumption and more. If the car manufacturer
    wants the same calibration to be used in several different cars, a new
    compromise has to be made. Different manufacturers do have different
    strategies about this. Some tend to let many cars share the same calibration
    while others want to optimise each car individually.

    I hope that this answers your questions.

    Best Regards

    Ulf Herlin
    Vice President, Marketing
    #8
  10. madvw
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    madvw Active Member

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    [Apr 27, 2007]
    explain this one more tomorrow when back from pub ;)

    essentially, the fronts are always driven, they cannot be disengaged - therefore upto 50% of the power that is being delivery to the front when wheelspinning is lost.
    #9
  11. jojo
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    jojo Looking for Boost! Staff Member Moderator

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    [Apr 28, 2007]
    I'm with you on this one Madvw, it's actually very simple, yet so complex. The detailed explanation and answers you have posted up makes for interesting reading Colinra, as it's all true and from the horses mouth, but we have to keep things simple for most of us to understand.
    My basic explanation is that with the front wheels being directly connected to the transmission, and the rears taking power from a transfer box on the gearbox, the maximum 'drive' to both the front and rear axles is 50:50 with the haldex fully engaged. But in the extreme condition, where the front axle/wheels is slipping on ice or off the ground(free wheeling), then the car would have to rely on the rear wheels to push it along, which is where the 100% torque to the rear wheels come in.
    You have to differentiate between drive between the front and rear axles and torque distribution, where grip is available.
    #10
  12. edward_harris
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    edward_harris s3 gone now a shiney red evo 6

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    [Apr 28, 2007]
    so if accelerating hard and there is no slip at the front would all the power still be going through the front wheels, meaning there is not as much transmission loss?
    #11
  13. james0808
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    james0808 Active Member

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    [Apr 28, 2007]
    Off topic i know,happy birthday ED.
    #12
  14. james0808
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    james0808 Active Member

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    [Apr 28, 2007]
    I wouldn't worry about transmission loss with the power yours will have.
    #13
  15. ChriS3
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    ChriS3 hud at ye bam

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    [Apr 28, 2007]

    Generally yes, but if you had the performance controller, then you'd be able to get power to all four wheels on throttle position.
    #14
  16. madvw
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    madvw Active Member

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    [Apr 28, 2007]
    on full acceleration, the OEM haldex will transfer 50% to the rear even though the fronts aren't spinning.
    #15
  17. colinra
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    colinra Member VCDS Map User

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    [Apr 28, 2007]
    cheers jojo that makes it more clear to me. I think we will all agree that it is a very clever system.
    #16
  18. ChriS3
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    ChriS3 hud at ye bam

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    [Apr 28, 2007]
    Nope, I'm afraid it won't. Its the performance controller that uses throttle position, the OEM version only uses front wheel slip.
    #17
  19. madvw
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    madvw Active Member

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    [Apr 28, 2007]
    If the OEM controller doesnt transfer on full power regardless, i'd expect load more squat from the rear when accelerating harshly? Theres never any wheelspin when flooring it from a standing start, barely even a chirp from the tyres.

    I thought that OEM was 4wd on full power(or wheelspin), whereas the performance controllers shifted more to the rears earlier on (less throttle position), giving a more predictable front-> rear shift and partial (or full?) 4WD when on part throttle whilst cornering?

    happy to be proved wrong as always, because i really want to know better how these things work!! :icon_thumright:
    #18
  20. jojo
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    jojo Looking for Boost! Staff Member Moderator

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    [Apr 28, 2007]
    AFAIK, the OE Haldex controller will fully engage the Haldex coupling to 50:50 drive on full throttle, or engage partial drive to the rears on part throttle upon a difference in axle speeds of the front and rear, as little as 4degrees in slip, so very quick to respond, but still a delayed response. Where as the uprated Haldex controller is more sensitive to throttle input and would lock up the Haldex fully on half throttle or more, so makes it more responsive and predictable?
    #19
  21. edward_harris
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    edward_harris s3 gone now a shiney red evo 6

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    [Apr 28, 2007]
    so on a dry day with the oem version and no slip on the front you would have more power at the wheels as it's not in 4wd mode! so really power wise id be better keeping the oem version?
    #20
  22. jojo
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    jojo Looking for Boost! Staff Member Moderator

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    [Apr 28, 2007]
    Realistically Edward, the most power a hatchback should have going through the front wheels was suppose to be 200BHP, but since modern hatchback are bigger and heavier, we are seeing 250BHP, and you will have 370 horses to hold back!
    I personally don't think you will benefit from a performance haldex controller if straight line performance is your main target, as you probably wouldn't feel the difference. It's more of a handling thing by having predictable traction mid corner, knowing what the haldex will do when you boot it mid turn. :racer:
    #21
  23. edward_harris
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    edward_harris s3 gone now a shiney red evo 6

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    [Apr 29, 2007]
    lol, well im gonna try get one chucked in from aps, see what they say!
    #22
  24. god_thats_quick
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    god_thats_quick Numptie of the highest order

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    [Apr 29, 2007]
    For those of you that say it can't be more then a 50:50 split to the rear, I think you are missing what was said by haldex themselves, it is possible to be nearly 100% going to the rear in for example the situation where both fronts were on ice and the rears had full grip. Then the fronts would only be using the percentage of the torque required to keep the fronts moving, very unlikely but possible.
    #23
  25. madvw
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    madvw Active Member

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    [Apr 29, 2007]
    yeah, I was trying to put it into words what i was thinking, but it never came out...as usual! I understand what they're saying - that if the fronts have 0% grip and are spinning without any resistance, and the rears DO have grip, then 100% of power is therefore being applied through the rears..

    The more scientifically minded will see instantly that some of the torque will already be mopped up in the effort needed to make the fronts spin - so i guess its no suprise that the response from haldex was from the VP of MARKETING.....
    #24
  26. edward_harris
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    edward_harris s3 gone now a shiney red evo 6

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    [Apr 29, 2007]
    so when the OEM version does kick in to 4wd IE when there is slip at the front. when does it convert back to front wheel again? IE say you were going round a corner and it went into 4wd when would it switch back to front wheel, half way through the corner, or when?!
    #25
  27. madvw
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    madvw Active Member

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    [Apr 29, 2007]
    whenever the fronts had sufficient grip - be it after the corner or mid corner. Thats why its called unpredictable, and why people go for the performance controller.
    #26
  28. edward_harris
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    edward_harris s3 gone now a shiney red evo 6

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    [Apr 29, 2007]
    oh ok so the uprated version goes on throttle position does it come in at full throttle or half way?
    #27
  29. jojo
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    jojo Looking for Boost! Staff Member Moderator

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    [Apr 29, 2007]
    I thought I did explain it quite clearly in my earlier post, obviously not lol

    You have to differentiaite between drive and Torque distribution! the drive will always be 50:50, with the Haldex fully engaged it impossible for it to be anything else. And as I mentioned earlier, you can have 100% of the torque going through the rear axles if the fronts was freewheeling....
    #28
  30. god_thats_quick
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    god_thats_quick Numptie of the highest order

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    [Apr 29, 2007]
    You did jojo, but it would appear I didn't bother reading every post in the thread... oops :sorry:
    #29
  31. dultanur
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    dultanur all promises, no action :)

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    [Apr 30, 2007]
    edward, the haldex doesnt work like a switch, it will start so transfer torque to the rear axle until the speed difference between the axles are 0...so realistically the only time 50:50 drive spilt occurs is during full throttle drop clutch starts, or under slippery conditions.

    heres a good link:

    http://www.haldex-traction.com/default.htm
    #30

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