would this haldex gen 2 ecu put more most power to the back wheels for more fun at weekend .
even thow its a front wheel drive quattro
would this haldex gen 2 ecu put more most power to the back wheels for more fun at weekend .
even thow its a front wheel drive quattro
Its not a front wheel drive quattro. Its a myth that the car is FWD until the front wheels spin then the back wheels get power.The car does have a large front wheel bias on the drive though. If you car is an 8P it will already have the gen 2 haldex fitted or gen 4 if its newer than 2007. What you are probably referring to is the HPP upgraded haldex ecu(make sure you get the right one for the right version on the car) this has three different switchable modes. It enables the car to send more power to the rear wheels and earlier than the stock system would. It gives the car a more rear biased drive. It cannot send more power to the rear wheels than the front though due to the way the system taps into the front diff for the rear wheel power.
The Gen II HPP upgrade will in Sport mode, engage the clutch faster and transfer torque to the rear wheel much faster than the stock unit. When the clutchpack are fully engaged, the rear wheels are overdriven relative to the path they are taken (since the rear will be turning at the same rate as the front wheel, but taking a shorter path), this will allow the rear wheel to follow more closely the path of the front wheel, giving the feeling of a livelier rear end.
Audi's sports differential takes this model and apply it to even more extreme level.
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?
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?
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?
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?
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
My brains fading now so any clarification woul be most appreciated
HERE there is a section about CENTRE LOCKING DIFFERENTIAL & EDL (Electronic diff lock), so it must have one.
The Haldex clutch is an electronically-controlled multi-plate clutch. It performs the function of the Torsen centre differential in cars with transverse engines, such as the Audi A3, A3 Sportback and Audi TT.
It ensures that engine power is permanently distributed between the front and rear wheels as and when required.
The Haldex clutch works by reacting to differences in the rotating speed between the front and rear wheels. This causes variations in the system’s hydraulic pressure, which in turn compress the clutch plates together to balance the distribution of power between the front and rear wheels. So if the front wheels begin to lose traction, the Haldex clutch channels power to the rear. And the greater the difference in rotational speed, the higher the pressure applied to the plates – which means that more engine power can be transmitted to the rear wheel.
Some good questions there, and it is a very complex system. The Haldex world has moved on quite a way from the pre-facelift S3 to the new MY10 ones, so there are a mix of truths and technologies. Would like to drive a new one to see if I can actually feel a difference, as those who go for the Gen II haldex switch upgrade really rate it (that I've read).
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. The haldex unit detects slip like you said but slip is not wheel spin like alot of people seem to think and were the myth its front wheel drive most of the time comes from. Slip is the difference between torque on the front and rear axles. If your driving along at a steady speed the front wheels are being driven and the rears are dragging in effect, the difference in torque between the two axles is the slip and to counter this the haldex unit will start to close the clutch until the torque difference is the same on both axles. If you accelerate harder the slip becomes greater as the fronts wheels start to put the power down and the haldex unit closes the clutch even more to compensate and keep the slip the same.
The clutch pack doesn't work like an on/off switch its variable and is constantly changing the amount of power tapped to the rear wheels. It also uses other inputs such as throttle position, steering angle, brake inputs as well to work out how much power to transfer rearward. There are times though that it will totally disengage such as when the ABS kicks in, if the handbrake isn't off etc.
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).
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".
Theres a copy of an email form Haldex themselves in this post. Although the system at the time was gen 2 rather than the gen 4 now most of it still stands. http://www.audi-sport.net/vb/showthr...089#post306089
Yes, they are being driven by around 5% (or 4%) in normal circumstances, however, that would mean that 95% (to 96%) is going to the front wheels? That's a pretty high proportion in anyone's book and would characterise the car as front wheel drive?
To "drive" 4 wheels requires more energy to drive 2 wheels. Yes, there is the constant added weight, however, there is additional drag on the system surely to also drive the rears - friction required to do so for one. Again, economy over permanent systems is a Haldex selling point and they make constant reference to it in these green, bunny hugging times.
The XWD system simply allows more apportioning of power front/rear and left/right (for rear only); the actual principle on which Haldex operates remains the same: it is a reactive four wheel drive system which will operate (albeit at 95-96%) in front wheel drive mode under normal driving conditions. I am not saying it not a four wheel drive system, I think that is a misunderstanding, my tenet is that it operates, under normal circumstances, in front wheel drive bias. And 95% (now 96) is a pretty large bias effectively making it a front wheel drive car, under those circumstances!
I don't follow your comment about "...awd system that is only awd when the front wheels have lost control is next to useless..." If the front wheels have lost traction (not sure what you mean by "control") then having AWD must surely be an advantage as the rear wheels could provide the necessary momentum?
The post from Haldex, which I had read when it was first posted, talks technically about it's operation under different load situations. Point 4 touches on normal usage (although is inconlusive), and indicates 10-15%, still a heavy bias towards front wheel drive though. I will have words with a mechanic friend of mine and gauge their opinion/view...
I don't really get what your trying to disagree about. You posted above that you believed the system was fwd now your saying its front wheel biased which is what I've said all along. The system is reactive so the % of power going rearward varies all the time. If your cruising at a steady speed and 10% of power is being transferred rearwards and you accelerate the % transferred will go up so more power reaches the rear wheels, the harder you accelerate the more power is moved aft it works the same when cornering with power constantly being moved about.
Thanks for the answers guy's I'm still not 100% with it, but I guess the 1st issue we need to agree on is the pupose of this arrangement. I can't see that the haldex unit is any less complex / costly than a centre differential so I don't think it's a "cheap and cheerful" 4WD system in fact it seems more complex if anything. It doesn't seem to have a handling / high performance advantage over a full time 4WD system, in this area it seems to be less effective if anything (none of the cars evolved for rallying have it?).
The only thing which seems to make sense is the "only 4WD when you need it" reason. Even for the most enthusiastic drivers, in reality we don't spend much time %age wise where 4WD is needed. I think you must save energy only driving 2 wheels even with all the 4WD gubbins on board and not driving. The transmission of energy through the rear diff and tyres will incur losses so there must be some effiency gain by only driving the rears 5% of the time (or something).
I think we are talking semantics here; for bias read preference! You seem to have disagreed with the initial post that Haldex is not a "front wheel drive quattro". I have said that the car acts primarily in a front wheel drive mode (or bias if you prefer) until such a time as power is moved to the rear, depending on driving conditions, traction, acceleration, etc. Under normal circumstances, the primary drive (95-96%) is at the front. My observations of my two cars and their tyre wear confirmed this operation to me, as does most of what Haldex themself say when they talk about fuel economy and 4% rear use under normal circumstances.
I have never said that the car is front wheel drive. Part of my first post read "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!"
That's it really!
Haldex is only reactive when you go from no throttle then to giving it throttle in which that application of the throttle would lose traction. Scenarios for such an even to take place would be braking into the apex of a corner on a relatively low traction surface, then applying throttle right afterward and the front wheel losing traction. What happens in that case is the haldex clutch was completely disengaged, then as soon as traction is lost in the front, the clutch tightens up and torque is transferred to the rear.
In situations where it is pre-emptive, you enter a increasing radius turn and put more and more gas on the throttle to go faster and faster around the curve until you reach traction limit. Since you are already in the turn, the clutch has engaged already and are already transferring torque to the rear wheel. The result is that it should not feel like understeer. If you are doing the same thing in low traction situation, you should fee the rear of the car steering you around.
So it could be reactive or pre-emptive, depending on how you drive it. Once you understand how the car should react in a certain type of situations and your input, then you should be able to make the car behave the way you want. If you don't want to know how to make the car do what you want, but just want it to go where you point it, then you should just leave ESP on.
Haldex is constantly reactive, except at certain times like zero throttle when the clutch is open. They even state themselves as pointed out above that at a constant cruise between 4-10%(depending on the version) of power is transferring to the rear wheels. The gen2 system is torque reactive across both axles so any throttle increase will increase the power transfer as the torque increases on the front axle the clutch closes to match the rear axle. The gen 4 system works slightly differently as it doesn't require a difference in torque to power the pump but will still transfer power to even torque across the axles. Neither system requires a loss of traction to work.
Haldex might be reactive to your input, but that translates to preemptive action against what the car might do. It is only reactive the car's trajectory if it didn't predict that was going to happen like loss of traction from sudden transition from braking to hard throttle. In situations where the haldex is reactive to your car's trajectory (not your input), you will notice instability, extreme understeer, etc, and frequently the haldex cannot correct in time what has gone wrong already.
You also seem to freely interchange power, torque, rotational difference, etc, causing your explanation to not make sense.
The system is constantly reacting to both the driver inputs and the cars inputs and constantly changing the power delivery to the rear wheels(except under conditions where the system will disengage i.e zero throttle, ABS activating etc.), its not delivering a constant % of power rearward nor is it constantly disengaged. That makes it reactive IMO.
I give up. The information is out there to read. The system really isn't that complex to understand.
Ever tried pushing one of these things? Try it, you will find out then how much drag they have compared with a front wheel drive similar car.
So slightly playing with the rate you apply the throttle and at what point will lead to different AWD behavior. You could go from disappointing feeling of it being a FWD until it loses traction to one that feels like it is always being driven by 4 wheels.
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.
One simple experiment is to drive in a low slip condition (I have snow here right now, so it is easy, but you guys in the ever raining UK could try it too) is to drive in a constant low to moderate speed in a circle, then floor it. AWD should feel relatively seamless with the tail pushing out. Then come to a stop, with the steering wheel in the same angle, floor it. This time, you should feel the front wheel pull and lose traction first, go into understeer, then after a split second, the rear catches on. In first scenario, the pump has been primed and AWD is likely partially engaged, so loss of traction occurs on all wheels. In second scenario, the pump is not primed and clutch is not engaged, and traction loss occur on the driving wheel, which is at the front.
I had a go at this tonight on a wet road, giving it a boot full away from a junction with the steering wheel at a slight angle, produced noticable wheel spin at the front for a couple of seconds, a similar exercise whilst travelling at 30mph on a roundabout and the car felt planted with a real 4WD grippy feel. I take your point about learning how the system behaves and using it to adapt your driving, any other tips?? I'm off for another play now!!!!
Another situation where the difference between puttting down power with the front wheel as opposed to all the wheels is when cornering through the apex. When coming into the apex, you usually are braking into the corner and once you hit the apex, you go full throttle to accelerate out of the corner. In dry road, you might feel a little understeer, which is easily correctable, but in low grip situation, your front wheel will slip a bit before the rear catches on, but by that time, the momentum of the car will continue to cause it to understeer and if you have ESP on, it will start to kick in. This is due to the clutch going from fully disengaged mode then engaging once the wheel slips. In order to minimize the slip, one has likely be not so hard on the throttle right after hitting the apex or ease up on the brake slightly before the apex, rid the throttle over the apex and then go hard on the throttle.
I haven't played around with this too much because there is usually cars, either waitng on the light or parked, so if I lose grip, my car will plow into them. But the point is to try to be in a situation where the haldex clutch pack has engaged somewhat.