Haldex Upgrades – Questions
I am trying to obtain feedback from anyone who has used one of these or has access to a more detailed technical spec than is available on most websites.
Let me explain my understanding of the Haldex, as this will then set my questions into perspective.
The Haldex is a part-time four wheel drive system , consisting of a normal front wheel drive system, with a part time connection to the rear wheels made by the Haldex clutch pack. This connection is basically triggered by a speed differential between the front and rear ABS sensors. This means that the system is at its best in muddy or snowy conditions. Similarly it should cut in when front wheel spin occurs from a standing start. This arrangement means that, despite PR artistic licence, without front wheel spin it is impossible for more than 50% of the torque to go to the rear wheels. Similarly this also implies that it will only help to combat understeer once a front wheel starts slipping (much too late).
By contrast a true 4WD first feeds power to a centre diff and then to the front and rear axles – this normally gives a 50/50 split. In the case of the rightly much praised RS4, not only is there a centre diff with a limited slip function (Torsen), but it also incorporates an epicyclic gear which gives a rear bias of 60/40. For circuit use an even more extreme bias is often used (I have seen 72/28).
The only way in which I can see the Haldex function being more performance orientated is by supplementing the ABS sensors with Yaw sensors and accelerometers, some of which I believe are now fitted to the current generation of Haldex systems as fitted to the TT. I would guess that the after market upgrade simply takes this one step further.
Now to my specific questions:
- Has anyone used one of these upgrades (Haldex or MTM) and if so does it reduce understeer?
- What are the conditions under which it will activate the clutch pack?
- If it completely locks the clutch pack, does it create any problems by forcing the front and rear axles to run at the same speed? Normally in cornering the rears run slightly slower as the corner radius is normally less at the rear – possibly only an issue on tight corners.
- If it does not completely lock the clutch pack, then the plates will slip which can cause overheating or premature wear – any known problems?
Apologies for such a long post, but any help will be much appreciated.