[Audi A3/S3] Is Quattro much better than 1.8T Sport?

[Scotty]

You may be right but why would they engineer that in. The Haldex will activate when needed so why hardcode something. If you're correct then it means every time your on the road doing a reasonable speed (i.e. you won't get wheel spin) then you're wasting energy by invoking the Haldex.

Also when a Haldex car is on a rolling road and being max'ed i.e. WOT why does the Haldex pulse in an out if what you say is correct? The normal explanation is that the rears have been powered up to the same speed as the fronts hence power is removed. They then start slowing so power is again supplied to the rear.

I'm interested to find out the real answer though as I'm only 90% on this and could well be proved wrong.

[Viraje]

Whilst the Haldex system is variable and allows up to 50% torque to be sent to the rear, and can kick in in less than an 1/8th of a wheel rotation, this kind of sudden transfer of torque split isn't ideal when you're cornering. And even in WOT conditions, if you were to suddenly throw the car into a bend, and the rears step in to lend a hand, the result is a very unbalanced and unpredictably handling car. It's important to remember that the Haldex ECU is controlling the torque split, so although the system runs 4wd for the majority of the time, the front wheels nearly always have more torque. Unless you're ice racing of course!

[Scotty]

Oh dear I seem to be disagreeing with everyone.

I don't believe that it is running 4wd for the majority of the time. I believe the cars are 100% FWD until it detects the rotation difference and then it starts putting some power to the rear.

Just to be really picky I believe this happens after 15degrees of rotation and not 1/8th (45degrees).

[Drillslinger]

Dammit Scotty, you're "Supposed" to say "Yes Drill, whatever you say Drill"

I've a wild idea, why don't we go look at the Haldex website?? Wouldn't that say? Or would it just tell us what the system is capable of??

[Scotty]

[ QUOTE ]

The Haldex® clutch has advantages for all-wheel drive in conjunction with a transverse-mounted engines. A lot of different information is needed to distribute power to the wheels – for example, the engine speed, torque, wheel speed and anti-lock braking system (ABS) function.

*) Trademark of Haldex AB, Landskrona, Sweden

[/ QUOTE ]

No mention of throttle setting.

[Simonl]

answers on a postcard please!!

[Drillslinger]

[ QUOTE ]

answers on a postcard please!!

[/ QUOTE ]

OK

[Viraje]

Knew it was in here somewhere, 14th post down.... linky.

[StephV6]

From what I understand about Haldex the torque transfer is only possible when there is a speed difference between front and rear axle which is about 95% of the time. Or you can also say it is 100% of the time when accelerating, constant speed or cornering.

that only allows the system to transfer but the amount of torque transfered depends on all the different data reported to the haldex ECU like speed, angle cornering, throttle position, slippage (of course) etc...

Basically I think the car is 85%FWD 15%RWD most of the time and going to as much as 50/50 unless you wheelspin the front wheels when it can go down to as much as 10/90.

[Viraje]

I think that pretty much sums it up. Unless you are cruising at a fairly constant speed, there is always some difference between axle speeds, so the system is usually doing something.

[Scotty]

Viraje - I can't work out from that post your refer to howe much is fact and how much is your intepreation of it. It says in one bit "The ESP detects the following circumstances:

Wide Open Throttle

Throttle closed while cornering

Throttle ‘Delta V’ while cornering (an increase or decrease in throttle position while cornering)"

which is fine but Haldex and ESP are separate (but inter operating devices). Just because ESP (or the ECU for that matter) knows you're at full throttle, doesn't imply that the Haldex locks to give rear drive.

StephV6 - why is there a difference between front and rear 95% of the time. This would mean that either the tyres are slipping all the time or that your car is stretching and contracting!!

I just found this quote in "Haldex News" !!

"The unit can be viewed as a hydraulic pump, where the housing and the annular piston are connected ti one shaft and a piston actuator connected to the other. When both shafts are rotating at the same speed, there is no pumping action. As soon as a speed difference occurs, the pumping and oil flow start. As a piston pump, there is virtually instant reaction with no low-speed pumping loss. The oil flows to a clutch piston, compressing the clutch pack, braking the speed difference."

It also says different cars can be setup in different ways simply by adjusting the pressure. This page about the Golf 4-motion is very interesting and says that 10% is always put to the back. HERE

Whether Audi have set the A3 and TT up the same or different I don't know.

[Viraje]

The Haldex clutch is simply a wet clutch that is controlled by the ESP system. It does not react to differences in torque or speed itself, as in some other 4wd systems.

There will always be a degree of spin at the wheels when a change of speed is occuring. For example, in order to achieve the fastest accelaration, the wheels should be spinning around 15% fatser than the road iirc. The ESP ECU detects this, along with other sensor inputs, (throttle position, ABS sensors, steering angle sensor, accelarometer sensor etc), and determines how much power should be applied to the rear wheels, by controlling the HALDEX clutch.

On a straight road at constant speed, there will be no difference in speed between front and rear wheels, and so the Haldex clutch is probably all but disengaged. But as soon as you floor it, the front wheels will spin relative to the back, (albeit unnoticably to the driver), and the ESP system will get the rears to help out.

Constant speed in a straight line is not a common situation in normal everyday driving, and as a result the Haldex is engaged to some extent pretty much constantly. It is the amount that it is engaged that will depend on the sensor inputs.

Does that make sense?

[StephV6]

Scotty, there is almost always a difference because the load on (or wieght of) the car is constantly moving from back to front axle, ie the tyres are under different loads then the rolling radius is different thus the speed of the axles.

Add to that tyre pressures are always different front to rear, same goes with suspension setup.

Then when cornering the front inside tyre lift sligthly so speed is increased as well because of rolling radius again is increased.

All these info are from an answer from the marketing president of haldex to Mark's email. see below

it's a bit long but very very interresting:

"

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.

-----------

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"

[StephV6]

Viraje,

From this it seems like the ESP doesnot affect/interfere with the haldex operation.

Seems normal to me too as I think that you always want a clear separation between different systems. They can use the same sensors but the haldex operation should not rely on the ESP.

That's a base philsophy I've always been told about engineering.

Stephane

[Viraje]

Interesting email, thanks for that. Just to clarify, when I refered to the ESP system controling the Haldex, I meant in terms of the ESP being generic acronym that describes all the various electronic aids, ie the EDL, EBD etc, including the Haldex.

I agree, switching the 'ESP' button off shouldn't effect the Haldex operation, in so much that it just turns off the other systems, for exactly the reasons you state.

[StephV6]

[ QUOTE ]

Interesting email, thanks for that. Just to clarify, when I refered to the ESP system controling the Haldex, I meant in terms of the ESP being generic acronym that describes all the various electronic aids, ie the EDL, EBD etc, including the Haldex.

I agree, switching the 'ESP' button off shouldn't effect the Haldex operation, in so much that it just turns off the other systems, for exactly the reasons you state.

[/ QUOTE ]

cool

[Scotty]

Very interesting. I'll have another read tomorrow when I'm sober and not so tired.

Thanks for publishing.

[Daemon]

Plenty of excellent information in this thread, but I want to

add something that doesn't appear to have been addressed yet.

There's plenty of talk about how differential rotation between

input and output shafts is required to get the mechanical pump in

the Haldex unit to work, and that gives the general impression

the front tires need to be "spinning" to maintain torque

transfer. Not true. Because it is a hydraulic pump,

and hydraulic fluid is incompressible, once full lock is acheived it

takes very little differential rotation to maintain maximum pressure

on the clutch. Somewhere buried in the Haldex technical documentation

on the haldex traction website, it states that it takes only 1-2 rpms.

That rotational difference is easily obtained through normal tire scrub

during accelleration, the above mentioned effective tire diameter

differences, and because the rear tires normally follow a shorter

path through a corner than the fronts, they usually turn slower thus

providing all the rotation Haldex needs to maintain as much pressure

as desired. In fact at 30mph, 2 rpms of differential rotation can be

maintained with only 0.5% difference in rotation between the front and rear tires. That's practically nothing.

The "as desired" part is regulated by the Haldex ECU

operating the master valve which controls fluid pressure to the clutch

at any pressure it wants. Here's a photo of the ECU housing and master valve.

The other trick it has up its sleeve is that it has an electric

hydraulic pump it normally uses for priming the main mechanical

pump. The electric pump has the capability of supplying enough

hydraulic pressure to achieve a high degree of clutch engagement if

so instructed. Between that, the mechanical pump, and the valve, it

can pretty much supply any amount of engagement desired at *any* time.

The only thing a Haldex equipped system can't do is allow the rear

tires to turn faster than the fronts. Power always goes through

the front diff first, then to the Haldex unit and then to the rear.

The rear tires can be made to turn the same speed as the fronts,

but never faster. However, since the rear tires do normally follow

a shorter path than the fronts through corners, if you force them

to turn the same speed as the fronts you can increase their

slip angle, thus giving you a very neutral handling feel.

Only time you'll see full on oversteer is during a 4 wheel

drift when all 4 tires are slipping.

BTW, one thing that is obvious if you read between the lines of the

the email reply from the Haldex rep above is that Haldex programming

is ever evolving. And this is true. The older S3 and TT225's Haldex

system is less pro-active and more re-active than the newer R32 and

TT3.2 programming. The difference can be felt while driving, and

seen directly on an AWD rolling road dyno. Someone earlier in the

thread mentioned how they noticed it pulsing the speed of the rear

tires with the TT on the dyno. TheR32 on the same dyno will not do

that because it maintains pressure as long as the engine load is high,

instead of just reacting to wheelspin. Haldex has made further updates

with something called PreX originally developed for the Volvo XC90,

that allows it to engage even before the car moves or there is any

wheelspin at all by watching the actual rate of change of the throttle

position and by driving the priming pump more. It's pro-active.

Requires nothing more than replacement of the above pictured ECU and

master valve with the new PreX version.

ian

[StephV6]

Thanks Daemon, good stuff!

what I also understand between the lines is that the VAG software of the haldex control is not the full-on version and that they compromise the performance to the life of the parts.

understandable on mass produced cars like ours but it would be interesting to know what the performance orientated version would do.

[fingermouse]

wow wot an answer

[Daemon]

[ QUOTE ]

I also understand between the lines is that the VAG software of the haldex control is not the full-on version and that they compromise the performance to the life of the parts.

[/ QUOTE ]

Ya, that's true. Since Haldex is fully software programmable

and can see any input on the CAN bus, they can choose to

give it more advanced features, but often do not, because

not every model has all the inputs it needs available to it.

Some they work around.. like the steering angle trick he

talks about. Calculate the effective steering angle by

looking at the difference in speed of the 4 wheels speed

sensors instead of relying on there being a steering angle

input (only available on cars with ESP).

[ QUOTE ]

understandable on mass produced cars like ours but it would be interesting to know what the performance orientated version would do.

[/ QUOTE ]

Well, they are already selling the PreX version of the Haldex ECU

to aftermarket distributors. Here's the European distributor

http://ace-competition.com/index.php?page=haldex〈=eng

and I believe http://www.eiptuning.com/ will be the exclusive

US distributor for Haldex products.

I happen to have one of these in my R32. Its effects are

noticeable but not an enormous difference on dry pavement, but

in the wet and in snow and ice, it allows for much more

active control of the rear end of the car with the throttle.

See the last link in the videos thread at the top of the R32 forum

to see it in action.

ian

[StephV6]

i've seen them and wondered at the time if the version on my A3 was different from the R32 as there is no way mine would do donuts like that i think even in the snow

[Chav]

oops sorry wrong forum!.... this is obviously the university challenge one

there's some serious grey-matter usage in this thread tho

[Daemon]

[ QUOTE ]

i've seen them and wondered at the time if the version on my A3 was different from the R32 as there is no way mine would do donuts like that i think even in the snow

[/ QUOTE ]

Well, my stock R32 could do donuts like that, but it's easier

to get them initiated with PreX as it's a bit more

aggressive about getting torque to the rear. I suspect

you could pull it off with even the old system (I've heard

stories), but it takes more work with the handbrake.

ian

[StephV6]

well i've never driven my car on snow but i cant see it do something like that

certainly me the problem and not the ca

anyway would you recommend that upgrade. it looks like it's about £700 +tax