I'd like to get a full understanding of the 4wd system on the 8p just to satisfy my own curiosity really. I've looked on the haldex web-site and gone through the full technical description and read a few posts on here but am still struggling to get a couple of things straight in my mind, so if anyone could explian these I'd be most grateful
1) Why bother with such a system? having added the cost and weight of a 4WD system why would you only want it to operate on a part time basis? Is it purely to save fuel?
I think there are various evolutions of Haldex which behave differently depending on the technology available at the time. I don't know enough about this to pass an opinion I'm afraid. On the Audi site HERE there is a section about CENTRE LOCKING DIFFERENTIAL & EDL (Electronic diff lock), so it must have one.2) I presume there must be no centre diff as the disengagement of the haldex clutch would cause all the power to be spun away down the free rear prop shaft, unless it had a limited slip device. I'm guessing, but a more likely set up is no centre diff and a direct drive to both front and rear axles from the gearbox? This would make more sense as power would go through the front wheels with the haldex clutch open and to all 4 with it closed. If this is right with equal grip on all 4 tyres the torque split front to rear would be 50:50?
Pass! I think there are so many system algorithms, most of which are driven by readings from the ABS sensors detecting each individual wheel speed that I get the feeling it mostly electronic rather than mechanical intervention.3) I'm confused about the question regarding slip of the front wheels being required before torque is passed to the rear axle. The haldex 3d cut-away drawings and description make it fairly clear the rotational diferance between front and rear generates hydraulic pressure which in turn closes the haldex clutch, but I've also read that slip is not required?
Nicked from Audi UK site:4) How does the haldex unit send say 10% power to the rear, does the haldex clutch have to slip in this scenario? If I liken this to the main foot clutch, it's either engaged or disengaged most of the time and only when it slips, does partial power occur? I assume here that the power available is only partially going into tractive force and the rest being wasted in heat?
You've ****** my brain, I can't cope!!5) If I imagine driving down a straight road with the haldex clutch open so we're in front wheel drive only and there's no wheel spin. In this case the rear wheels are turning at the same speed as the front, so the input and output shafts of the haldex clutch are also turning together. If the haldex clutch were engaged at this point then torque can be sent to the rear, I can picture the full engagement in my mind as this is just like a direct drive as if the haldex wasn't there, but how does partial torque transfer work? Just by varying pressure within the clutch pack? My intuition says slip is required
The haldex system doesnt have a centre diff, the prop shaft connects to the front diff which then at the rear of the car connects to the haldex unit and rear diff.
Hmmm, but isn't Haldex a reactive system? And by it's nature waits for slippage of the front wheels (I have heard 1/7th of a turn and on later systems 15 degrees). It is at the point when the front wheels slip, that the clutch operates fully and the rear wheels are then "driven" and also aids fuel consumption, which is another selling point of Haldex. This would also seem to be bourne out by relative tyre wear on front and rear tyres of a Haldex equipped car and Audi's own blurb above "So if the front wheels begin to lose traction, the Haldex clutch channels power to the rear"?
Under normal circumstances, with normal traction, driving normally, isn't around 95% of the power going to the front wheels? I would say that makes it primarily front wheel drive in anyone's book!
PS I'm not knocking the system (I have had two Haldex equipped Audis, including an S3).
No the % of slip they talk about is the amount needed for the haldex unit for fully close the clutch pack and transfer maximum power. Theres no benefit to fuel consumption with haldex because even with the clutch pack fully open the rear driveshafts, diff, prop shaft are all still turning and causing drag not to mention the fact that it weighs and extra 80kg as well. As for tyre wear in my experience the two sets of tyres I changed on my A3 the front tyres were within 0.5mm of the rear wheels which again points to how much the rears are actually driven. You are right though under normal cruising only about 10% of power will go to the rear wheels but as you increase throttle input the amount of power transferred will go up. The system is front wheel drive biased and you can never get away fromn that due to the way it taps power of the front diff, the HPP upgrade will make the unit act more aggressively but it will never be able to move the bias to the rear.
Think we will have to agree to disagree! I think Haldex would have something to say about fuel economy; it has been one of their USPs (and still is) against a permanent solution...! Whilst the rears are turning, I do not believe they are being "driven" as such under normal conditions and to a lesser degree "freewheel", as the fronts get nearer to 95%.
Again, this has been the Haldex mantra. Yes, the prop shaft is rotating as it is permanently connected to the output drive. Under full load, there must surely be a greater drag on the engine's resources (=fuel) than when the clutch is open. Certainly on my last Haldex car (S3), the fronts tyres were noticeably more worn than the rears (millimetres), unless my Haldex was broken or I just drove like Miss Daisy!
This has always been my understanding of Haldex, unless you are a tech and I will bow to your expert words. Might have a visit to the Haldex site to see if there is some techy blurb on there...!
[Edit] Website: On their latest XWD "...can also adapt the torque distrubution [sic] down to 4% on the rear axle during normal crusing in order to save fuel and maintain stability".
The way it works is pretty straightforward, but one has to understand how it works and use it to your advantage. It is like the DSG. If you don't know how it predicts the next shift and just expect it to be on the next gear you think you want, it might be wrong some of the time. But if you know how it predicts the next gear, you can trick it to pre-select the gear you want and when you manually shift it, it will have a fast shift.I give up. The information is out there to read. The system really isn't that complex to understand.
What they are talking about slip is rotational difference between the front and rear axle in order for the differential pump to have pressure, which would allow the clutch pack to be closed. This rotational difference is almost always there, as they say, with the slight rolling radius between the front and rear wheel when driving straight, but also between the difference in path the front and rear wheels take during a turn, with the rear wheels taking a much shorter path. So it allows for significant pressure buildup on turns, even before there is actual tire slip taking place.Having looked into this a bit further I'm sold on the fact rotational speed difference is required between front and rear axles before any power can be transferred to the rear wheels. The ulitmte explanation comes from Haldex themselves in an earlier thread on this topic
"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 other point to consider is how would a friction plate clutch (which is what a haldex is) be able to vary the power transfer without slip? In the absence of slip it's either engaged or disengaged, so just like your left leg modulates the car's main clutch when pulling away from a standstill, the haldex unit varies it's own clutch depending on various factors BUT only when slip is occuring.
In the same response to questions Haldex also talk about fuel saving and drive train wear as the advantage of their system over a conventional 4WD set up. Interestingly they also describe the reason for a small power transfer to the rear during cruising, which is down to the reduction in effective tyre diameter at the front due to deflection under load (power). This causes the small rpm difference needed front to rear to transfer say 5% power.