The Haldex implementation in the new Audi TT (Mk2)
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    1. #1
      Member Razor1973's Avatar
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      Jul 2005
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      The Haldex implementation in the new Audi TT (Mk2)

      Any of you guys know what the differences between this Haldex implementation and that in our Volvos? Here's what someone in the Audi forums has to say:

      - the system is Haldex
      - it sends about 5-10% of the torque to the rear on an ideal surface under straight driving
      - when slip is detected (something like 15 degrees difference in rotation angle! - i.e., almost instantaneously), the clutch pack engages and can almost lock up fully (this was not the case with earlier, heavier Volvos and their units)
      - although Gen. II is implemented, VW/Audi does not use the extra pump (Prex) that allows to pre-charge the system for yet faster available torque to the back
      - the system relies on the differential rotation mechanically (the pump is driven by it), so it cannot engage via electronic impulses. A separate pump and reservoir could help with that --- then the system could be fully electronic, pro-actively engaging based on steering input, yaw angle, throttle, speed, rpm, etc. --- like some of Subaru's systems
      - still, under most driving situations the system seems to be doing just fine, as per reviews. In particular, when not driving in a straight line, there is almost always sufficient differential rotation to engage the system
      - the 50:50 vs. 0:100 number confusion stems from a misconception what constitutes transmitted torque. The only useful definition is the torque transmitted to the road via the wheels (or axles). Therefore, the Haldex system, since it can (almost) lock the differential, can transmit ~100% to the back although the front wheels are permanently driven (attached to the drive train). This can happen when there is virtually no traction up front, and good traction in the back. Both axles will turn at the same rate, but only the one with traction is actually transmitting torque to the road.
      - marketing brochures of the R32, A3, or TT are not engineering specifications.


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    3. #2
      Global Moderator tmtalpey's Avatar
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      Re: The Haldex implementation in the new Audi TT (Razor1973)

      This info seems almost identical to the Volvo, to me. But to clarify the 0-100% torque transfer, the Haldex system can in fact transfer 100% to the rear, in the case where the front wheels have zero traction and the rears have good traction. This pretty much never occurs on the road, of course. But it can happen when stuck.

      Tom.

      2005 V50 T5 AWD 6MT w/Heico tune, Heico exhaust, Bell intercooler, IPD TCV, Viva Forge CBV, Elevate rear swaybar and torque mount, Snabb shift kit, etc.
      2016 XC60 T6 AWD
      1956 PV444 complete, running
      1956 PV444 very original, very rough

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