Ok here is the article it goes on a bit:
Hello.
As the owner of an Audi S3 I make use of the Haldex LSC everyday. I am also
a member one particular Audi forum (
www.audi-sport.net). We have discussed
the issues of Haldex, torque transfer and such like. In particular regard
comparing to Torsen.
The Haldex LSC solution is often seen as inferior and is often said to be
"not real 4WD". I disagree and believe it's abilities go beyond that of
Torsen when used in real driving scenarios.
I would greatly appreciate it if you could take the time and answer my
questions. I'll try to be as brief and as clear as possible. Thanks.
1) Haldex claim 100% torque transfer to the rear is possible. Is it? Some
people dispute this, but I believe it requires 100% slip of the front wheels
for this to be the case. i.e. In real conditions it would never happen, the
more realistic split is 50-70% under full load and some slip. I came to this
conclusion because the front wheels are always driven.
2) There is much dispute over whether the Haldex LSC delivers torque to the
rear if and only if the front wheels slip. It's my belief that the torque
transfer can and does occur in response to wide throttle openings, even
before any slippage occurs. Can you confirm?
3) With particular regard to the Audi S3 (or TT). Do you know if the
steering angle is taken into account when deciding whether to increase or
decrease the torque transfer? i.e. is it purely throttle/slip related or is
actually much more complex? If possible, some explanation would be useful.
For example, if there is 10% to the rear and I turn a corner, would it
increase to 30 or 40% to provide increased stability. If so, would be that
due to a natural left/right speed difference or a result of steering angle
change (ESP sensor)?
4) There is much debate on the "constant speed" torque transfer to the rear.
i.e. travelling on a road at 30 or 70MPH, how much torque is transferred to
the rear? Is it speed dependant?
5) Does the torque transfer under acceleration depend on the current vehicle
speed? If so, can you provide characteristic details?
Any further information you can provide would be appreciated, in particular
how the Haldex LSC operates in conjunction with the ESP and whether the ESP
can affect the transfer of torque.
The folk on the forums are an enthusiastic bunch who appreciate technical
explanations to technical problems. If there's anything else you can provide
beyond the questions I've asked, that would be very welcome. Once again, I
thank you again for taking the time to read my questions.
Cheers.
Mark.
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Dear Mark,
I am pleased to see that you like your Audi S3 with the Haldex AWD system
and that you disagree with your Torsen friends when it comes to the
excellent abilities of the Haldex AWD System. It is a fact that the
electronically controlled Haldex AWD system provides a much wider range of
possibilities over a purely mechanical system. We are also convinced, after
having delivered 500.000 Haldex AWD systems to the market since our start in
1998, that we also have the best electronically controlled AWD system on the
market.
We have put together some answers on your questions that I hope will be
helpful in your discussions in the Audi forum.
1. There are situations where near 100% torque transfer to the rear axle
occur. An example is if the front wheels are on ice and the rear wheels are
on tarmac. In that case the front wheels have (almost) no grip. In that
case, the Haldex coupling will transfer all torque to the rear axle and
prevent front wheel spin. On uniform surfaces however, the coupling can not
transfer all torque to the rear axle. See below.
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 rolling raduis difference can be created by
differently worn tyres (or different dimensions, something that should be
avoided) and different load. In most cars, the front axle has a greater load
than the rear axle, which causes the roll radius of the front tire to be
smaller than the one for the rear tyres (given the same nominal size). This
gives us the possibility to transfer torque to the rear axle also when no
slip occurrs on the front tyres.
If you have differently worn tyres on the front and rear axles, the new
tyres should always be on the rear axle. This is true no matter if the car
is FWD, RWD or AWD, since you otherwise risk heavy and uncontrollable
oversteer in situations such as aqua planing. In this case, putting the worn
tyres at the front also helps not to reduce the maximum transferable torque
(maximum rear axle torque).
During cruising which a constant velocity, we have the possibility to
transfer up to 40-45% of the torque to the rear axle, given nominal tyres.
During acceleration, the weight transfer increase the front tyre slip and
decreases the rear axle slip, giving us the possibility to achieve more or
less the same torque distribution as the dynamic weight distribution.
Generally speaking, depending on the vehicle somewhere around 60-70% is
possible to achieve during a full acceleration. Note that we are still
talking about a uniform surface, with no spin on the front wheels.
When cornering , there is in most cars a tendency for the inner front wheel
to lift and spin. In that situation, we can increase the torque transfer
even further.
So far I have only spoken about what possibilities there is to transfer
torque. How much is actually transferred depends also on how the Haldex
coupling is controlled. The engine torque and gas pedal position are
together with the wheel speeds and the engine speed the most important
signals that are used in the control. Brake, ABS and ESP signals are also
very important for enabling co-existance between the AWD system and the
ABS/ESP system. We control the coupling in order to prevent wheel spin as
well as removing it quicky if it should occur.
3. Steering angle is not a signal used in the control of the coupling in VW
group cars. The reason for this is that the steering angle is not available
in most cars as it is only present when an ESP system is mounted. We do
however calculate the curve radius from the wheel speeds. We have software
ready using more signals as the steering angle that we offer to the vehicle
manufacturers. This enables further optimisation of handling performance.
4. It may vary a bit with speed (and road surface), but without going into
details a figure of around 10-15% would be typical. It is enough to help
stabilising the car while at the same time saving fuel and reducing the
temperature of certain driveline components. As soon as the driver starts to
accelerate or decelerate, more torque is transferred.
5. Yes it does. In general, a higher percentage of the torque is transferred
to the rear axle in low velicities than in high ones. This is partly due to
the fact that the total available driveline torque is larger at lower speeds
(and lower gears), thus causing more weight transfer to the rear axle. In
order to achieve consequent handling characteristics (as well as optimised
traction), more torque must then be transferred to the rear axle.
6. The Haldex coupling is completely compatible with ABS and ESP systems.
In order to optimise the performance of the ABS/ESP system, it is possible
to open the Haldex couplng during ABS braking or a stability control brake
intervension. The ABS/ESP antispinn and stability control also depend very
much on being able to calculate the vehicle velocity. That is very easy with
2WD, but as soon as you have the possibility of four wheels spinning it gets
very complicated. The Haldex coupling and ABS/ESP system interface make it
easier to obtain a good reference velocity.
In the cars where the Haldex coupling is available today, additional signals
available with ESP are not used. We do however have software using these
signals. This enables further optimisation of handling performance and life
span of driveline components.
During calibration of the Haldex coupling , we try to optimise the traction
and handling performance of the car. These are however not the only aspects
that are important. The final calibration is alway a compromise between
traction, handling, the life span of driveline components, temperatures in
driveline components, fuel consumption and more. If the car manufacturer
wants the same calibration to be used in several different cars, a new
compromise has to be made. Different manufacturers do have different
strategies about this. Some tend to let many cars share the same calibration
while others want to optimise each car individually.
I hope that this answers your questions.
Best Regards
Ulf Herlin
Vice President, Marketing
