Haldex / AWD limit handling

[DocSteve]

Hi,

Apologies if this appears like a stupid question but I have been struggling with it. I have a Golf R, which is clearly not a car that someone vaguely competent should easily be losing control of but driving it has thrown up some issues hat I have not been able to pin down. I am used to driving RWD cars, including my track MX5, although I do enjoy driving a decent FWD motor from time to time.

The main issue, I guess, with the Golf R and its Haldex system is this:

When the stability control is fully disengaged and driving on/past the limit then it is very hard to know what the system is going to do i.e. if it starts to oversteer then what throttle inputs should be used to either deliberately sustain it or deal with it. I had assumed that it would behave somewhat like a FWD and more gas would during oversteer conditions transfer power to the front wheels and alleviate it. What seems to happen is that it actually continues to generate rear-bias and further provoke oversteer in most settings. Obviously aspects of handling that are not dependent on the driven wheels are independent of this e.g. the effects of weight shift resulting from throttle inputs and the effect of steering. The issue is more when traction is being broken and how the Haldex deals with it.

Any thoughts or expertise welcome. I should probably take it to a track and really get to grips (pardon the pun) with it but to be honest it's my road car and I don't really want to do that.

Cheers
Steve

[DocSteve]

Hi Reg,

Thanks for your thoughts - they are all quite reasonable, including your last points. Rest assured I do not perform dangerous manoeuvres on the public road and that I believe any experimentation has been conducted safely in areas where I have had enough available vision, grip and space to do so. Part of the reason I am asking the question is that the opportunity to explore this on the public road safely is rare - mostly in low grip conditions. Certainly, I have not provoked the car to move around simply due to excess corner entry speed - that would be madness.

In addressing your other main point, you are of course correct that in most modern vehicles switching off the stability control system rarely means that. The Golf R, however, is one of those rare beasts where you can either put it into an intermediary mode or completely switch it off (this is not possible in the GTI, for example). Therefore I am quite sure that the Haldex is the matter for debate.

I perhaps have not come across very well in my post, but I am trying to ask what I believe to be a genuine question that could have some relevance on the public road - especially for example in snow or ice (where I do think stability control systems still do not always work so well). I think you are correct in that you can feel what it is doing but my question was more about what I should expect it to do, which seems difficult to gauge.

I may not be as experienced as you but I do have a reasonable amount of on-road AD training under my belt and appreciate where you are coming from with your concerns!

Steve

Edited by DocSteve on Friday 29th January 21:35

[DocSteve]

Reg Local said:

You'll need to describe the issue in a bit more detail.

Is the car oversteering when you experience the problem? Is it oversteering because you've induced oversteer with a lift of the throttle mid corner? If so, are you lifting off to counter understeer? Or are you deliberately lifting off to provoke oversteer?

Does the torque transfer to the rear feel like it comes in too suddenly? Does it feel like it's removed suddenly?

How does the car react to throttle inputs when it's oversteering? And how does it react to steering inputs?

The first time I encountered it was exiting a wet roundabout in cold weather - the car understeered briefly under throttle input which almost instantly resulted in the power being transferred to the rear and an oversteer which then became more pronounced with sustained throttle input; it then corrected after coming off the throttle (smoothly I hope..). Of course it is possible that I applied extra steering lock during the initial period of understeer although I don't think I did - I deliberately tried not to do much with the steering in order to understand what the system was doing. I then encountered it again in snow where I applied a fairly decent amount of throttle to the car when it was moving slowly with some steering lock on - I expected understeer but what happened was oversteer that showed no signs of abating before I called it a day. I would have expected that it would transfer power to the front under these circumstances.

The car does as expected have a natural tendency to understeer at higher speeds with throttle and steering input which tightens up when coming off the power but I suspect that is more to do with weight transfer and the characteristics of the chassis/suspensions setup that most road cars have rather than anything to do with the Haldex (i.e. I would expect that with almost any car)

I hope that makes some sense!

[DocSteve]

Reg Local said:

DocSteve said:

The first time I encountered it was exiting a wet roundabout in cold weather - the car understeered briefly under throttle input which almost instantly resulted in the power being transferred to the rear and an oversteer which then became more pronounced with sustained throttle input; it then corrected after coming off the throttle (smoothly I hope..). Of course it is possible that I applied extra steering lock during the initial period of understeer although I don't think I did - I deliberately tried not to do much with the steering in order to understand what the system was doing.

Understeer in a front-wheel drive car is generally caused by a combination of excess corner speed and/or excess throttle application. The best way to correct understeer is to gently lift off the throttle - this will reduce the torque going to the front wheels and transfer some weight forward, slightly increasing front tyre grip and reducing or removing the understeer.

If you accelerate in an understeering FWD car, the understeer will generally just get worse. In your Haldex-equipped car, the car has recognised that the front wheels are losing traction, but because you've switched off the stability, the car will only deal with the traction issue, which it does by transferring some drive to the rear wheels. If your inputs are harsh, this torque transfer will be sudden and in a car which is already at the limit of adhesion at the front axle, it will easily transition quickly (snap) into oversteer.

Your best approach is, of course, always to avoid the understeer in the first place.

Your second best approach would be to correct the understeer in the normal way with a slight throttle lift.

If its all gone tits-up and you're oversteering, keep some throttle applied (not a bootfull - just enough to keep the car sliding at a managable angle) and just steer in the direction you want to go. Lifting off the throttle at this stage will remove drive from the rear, transfer weight forward (and away from the already sliding rear wheels) and make the oversteer much worse.

DocSteve said:

I then encountered it again in snow where I applied a fairly decent amount of throttle to the car when it was moving slowly with some steering lock on - I expected understeer but what happened was oversteer that showed no signs of abating before I called it a day. I would have expected that it would transfer power to the front under these circumstances.

You're expecting too much from the system in this case - some power is always being directed to the front wheels, it's just that, when traction is limited, the system will transmit some of the power to the rear wheels as well. Snow is one surface which should prompt the system to direct drive to all four wheels at once. If your throttle inputs are harsh, the wheels will keep spinning and the drive will continue to go to all four wheels. If you left the stability switched on, the car will do much more to help you sort it out, but with it off, it'll just deal with the traction and leave you to sort out the slide. Once all four wheels are being driven, the car is doing everything it can to give you maximum traction - at this point it will not start directing more torque back to the front wheels.

DocSteve said:

The car does as expected have a natural tendency to understeer at higher speeds with throttle and steering input which tightens up when coming off the power but I suspect that is more to do with weight transfer and the characteristics of the chassis/suspensions setup that most road cars have rather than anything to do with the Haldex (i.e. I would expect that with almost any car)

This is normal in any car - I'm yet to drive a car which isn't sensitive to weight transfer when pressing on.

Thanks again Reg for your detailed reply.

In the first example, I did deliberately apple excess throttle whilst exiting the corner in order to assess what might happen. What you have said makes a lot of sense and fits with what I experienced. I guess that in either a RWD or FWD car it would have been obvious to me what was going to happen but in this vehicle it really wasn't / isn't. Overall, I am quite sure if I gave the Golf R to a 18 year old keen but inexperienced driver they would stay out of trouble in it compared with a powerful RWD or even FWD car but, still, I think that for someone with a reasonable amount of experience it is an odd steer that is not that intuitive.

With regards to your advice about oversteer, I dare to take issue with some of that - I think it is valid advice if the oversteer is not primarily occurring due to a loss of traction at the driven rear axle but perhaps not so true if the principle cause of the oversteer is loss of rear axle traction on a low grip surface, especially at lower speeds - where the effect of traction on the trajectory of the vehicle is much more significant than any weight transfer process, if you agree?

In the snow, I take your point about leaving the stability on, but for the purposes of the discussion I think your description of the situation without the stability on has probably nailed it. If all 4 wheels are losing traction and then there is more grip available at the rear and traction is gained there then the power may shift to the rear, which in a very low grip situation like that may be enough to induce significant oversteer. On reflection the situation in the snow I described was on an incline (going up) so that could well be part of the explanation there.

On the last point, with regards to road cars and expected handling I meant the tendency to understeer on corner entry (I'm sure we've all driven some track cars that don't quite do that if set up in certain ways...!)

I genuinely have an interest in limit handling (and think I'm reasonable at it, rightly or wrongly) and this car has challenged my expectations. Very much appreciate you taking the time to reply and I have enjoyed your articles so far - PH is great sometimes!

Thanks
Steve

[DocSteve]

Alpinestars said:

I agree re power oversteer as in my earlier post.

Throttle
Slide
Catch
Either show boat by applying throttle and holding it sideways
Or straighten the wheel to avoid a tank slapper.

With lift off I still can't see that applying throttle helps. Except in very limited circumstances. Eg a fwd car will pull itself out of a slide, or as you say an old school 911 where there is weight over the rear wheels might give some traction back to the slipping rear wheels.

I can't see that if someone has barrelled into a corner too quickly, lifted off, rear slides, that the right advice on a road is to apply throttle. Surely the first thing to do is recover the slide, and only if you get it back apply some throttle? (Fwd, rwd rear engine possibly aside). Applying more throttle on the road is going to end in an even bigger crash 9 times out of 10.

The discussion seems to have moved on from the effects of Haldex, although I will say that my original post related to throttle induced over/understeer rather than excess cornering speed as Reg suggests above.

I agree with most of what you say - advising people to apply more throttle when they don't really know what they are doing is likely to just make a bad situation even worse. However, I was experimenting with this on the track with Mark Hales (great coach btw) recently in my track car which is front engined and RWD. On one particular corner I was setting myself up for it by braking into the turn-in (trail braking, if you will) and the result was oversteer which I then corrected before getting back on the power. What I learnt was that I could get around the corner faster by getting straight back on the power (not enough to break traction at the rear though) which settled down the oversteer with minimal steering input and resulted in a higher speed on the straight before the next corner.

I think the key point as you and Reg allude to is what has actually caused the oversteer - if it is excess throttle and loss of traction then applying more throttle is not likely to help, although the issue of over-correction is what causes most problems, I think. On track if this happens I find briefly letting go of the steering whilst correcting usually prevents the problem as if you keep hold of the wheel most of the time you will have too much corrective steering lock on just as the grip recovers at the rear and as above - tank slapper/slide in opposite direction/everything going green :-)

Steve