Marius

... the Haldex will probably have built up partial pressure and some power transfer already. And as lateral forces are introduced, imperceptible slip will increase, providing even more rotational speed difference to increase the Haldex pressure. For practical purposes, the Haldex doesn't wait until the front wheels break traction.

FastFast

The Haldex is clearly inferior
to more advanced AWD systems. In fact
it was designed to provide traction
and a low cost 4WD system.

First of all cars using haldex drive
like FWD or RWD cars as it is impossible to
split the torque according to a preset
level for normal driving conditions.
ie. 50% front 50% rear or 43% front 57% rear
etc.

Thus a car using Haldex will behave or
like a FWD or like a RWD car depending on
how it was designed.

While in a full time 4WD car the preset
split reduces the chance of wheelspin from
the start, haldex will only fight it by
transfering torque to the (secondary*) driven wheels. *rear wheels on the TT

This is where the trouble starts, while ie. An Lancia Delta Integralle Evo3 will transfer 47 front and 53 rear to give it a RWD feel and
to combat understeer, a TT will under normal
conditions transfer 100% of the power to the font
wheels and in extreme cases 100% to the rear wheels. This makes a TT very tricky and nervous
on snow or gravel as the Haldex keeps working
and changing the attitude of the car between
FWD style and RWD style. As I said it was
intended to make sure that you keep moving,
not intended as a sporty AWD system.

Example: TT v/s Evo3

TT: If one front wheel looses grip torque
is send to the rear wheels up to 100%, should
the rear wheels lose traction as well the
Haldex system will be in an unstable state
trying to figure out where to send the power too,
the idea that it would settle down for 50/50
or something like that is optimistic as in fact
haldex will not settle down for anything.

Evo3: Front wheel loses traction, more torque
is send to the rear wheels, an additional torsen diff
now sends the power to the rear wheel with the
most traction, should both rear wheels lose
traction, the system reduces torque to the rear
wheels and send more to the front. If all wheels
slip it comes back to 47/53 and keeps it this way
so that the handling is predictable under severe
drifts.

Anyways if you believe that haldex is supperior
to the AWD systems used on ie. a Subaru Impreza WRX or an elderly Evo3, you're kidding yourself LOL

This is why smart AWD fanatics stay away from S3s and TTs. LOL

Regards,
Fastwheels

Marius

Somehow I get the impression you have no real experience with any of these systems.

RogerL

"very tricky and nervous on snow" ? I don't think so!!!

smallTTs

I think you tagged it, Marius.

strawtarget

First, I like Haldex and I trust that it works. I'm not bashing it, but I <i>am</i> curious.

But I have a couple questions about things that have been said in this thread that I find... "interesting".

#1. Someone said that the Haldex gets info from the ECU, and that it changes the torque split based on throttle movements and engine braking. I've never heard this before... I was under the impression that it ONLY activates if there is a difference between the speeds of the shaft coming from the front wheels and the shafts going to the rear wheels. What's the straight scoop on this?

#2. Let's assume that your car is on a very low traction surface. Imagine that you apply a gradually increasing amount of power via the gas pedal. If the front wheels slip and the Haldex engages and the rears bite, does the haldex immediately disengage once the fronts and rears are moving in unison? If so, what's to keep the fronts from immediately slipping again now that they have 100% of the torque? Couldn't this be a problem? I understand that in most circumstances, once you're moving it's a lot harder for the fronts to lose their grip. But in my example, I'm applying gradually increasing power with the gas pedal so that there is always enough torque to cause the fronts to slip, but not enough torque for 4 wheel spin. In summary, "When does Haldex <i>dis</i>engage?"

RogerL

The acceleration on ice graph may help explain #2. Click the graph for a bigger version...although it's still a bit hard to read.<ul><li><a href="http://www.haldex-traction.com/technical_information/performance_info/performance_graphs.htm#">Haldex performance graphs</a></li></ul>

Marius

1.) The Haldex needs actual s minute rotational difference front/rear to build the hydraulic pressure, but both the build-up, the retention or the release of the pressure is governed by ECU, via electronically controlled valves.

2.) HOW long the pressure can be retained in locked-up state is one of the things the manufacturer won't tell us. In real life, what happens is that the rear appear to stay locked up for as long as you want to keep up that 4-wheel spin.

Marius

When you have continuous 4-wheel traction, and no apperent rotational difference front/rear, I imagine the Haldex -- if it needs a "topping up" of pressure -- will just let itself slip a tiny, imperceptible amount, to let the pump work enough to keep up the pressure.

FastFast

I have driven pretty much all the
full time 4WD drive cars that are out there.
I have owned a bunch and I still drive
one of them.

And honestly if you have never driven a
Delta EVO3 and then a S3 you will never know
why the EVO3 is so much supperior to the
S3 and any other full time 4WD car.

So... I talk from first hand experience
I have driven most of them LOL

Lanica, Celica, Imprezza, Mazda, Audi etc...

And in my opinion the Delta EVO3 and the Imprezza
still have the best overall 4WD balance... but
I prefer the EVO3... as to the S3, it drives
like a FWD car.

Regards,
Fastwheels

BTW. my EVO3 still has 260HP on the wheels not
flywheel and hairpins are a minimum 60Mph thing
with it. But to understand that you need to
drive a EVO3... then you will understand LOL