Haldex and the earlier vicous coupling AWD systems discussion

The First Generation V70XC had a Viscous Coupling developed by GKN to send power to the rear wheels when the front were slipping and TRACS traction control which can brake a spinning wheel.

Second Gen 2002-2003 used Haldex First Generation until MY2003 - 2005 when it got Second Generation. Third Generation XC70 2006+ uses Third Generation Haldex.

So what does this mean? Below is a good explanation by Ian a user on a VW board, but still relevant to Vovlo as the tecnhology is the same:


Ok, so people are constantly confused about what 100% torque transfer means.
Torque is about applied force. If you're not actually applying it (putting it to the ground), then
it's irrelevant. An engine can't produce 300 lb-ft of torque without a load. A differential can't
send torque anywhere without a load, and where the load is, is what defines your torque transfer

So let's take a simple example. A 4x4 truck with locking diffs front, rear and center.
Let's say all 3 diffs are locked.
- Front tires on ice, rear tires on pavement. Hit the gas. 100% torque transfer to the rear, because those
are the only tires with any grip (load). You can't send 50% of the torque to the front, because they have zero traction.
Yes, they turn, but no significant torque is required to turn them.
- Rear tires on ice, front tires on pavement. 100% torque transfer for the front.
- Ok.. unlock the center diff (regular open diff, no lock). No matter what the traction situation is,
you'll have a "perfect" 50/50 torque split. BUT we'll see that's not so desirable.
- Front tires on ice, rear tires on pavement, you'll go nowhere because it requires virtually zero torque to spin the fronts, and thus the rears will get the same zero torque. That the definition of a true 50/50 split.

An open diff always sends the same amount of torque to both sides and the actual amount that
reaches the ground is 2x the applied torque at the lowest traction side. So your total grip
is always heavily limited by the lowest traction side.

What you usually want in an AWD system is a a 50/50 static distribution (just meaning
you're applying power to both sides of the diff when traction is the same all around)
and the ability to provide a total lockup instantly should one end/side lose
all of its traction. Fully locked allows for a 100% torque transfer to whichever end or
corner can use it.

Ok.. so where do Viscous Coupling systems and Haldex fall here?

There are two general types of Viscous Couplings..
1. Just a vicsous coupling between the front and rear axles with no center diff.
One axle is driven all the time, the other does little on the other side of the VC.
Eventually one axle spins faster than the other, indicating a loss of traction,
the speed difference causes the viscous fluid to heat up, and become well.. more viscous
and that transfers torque across to the other axle. A viscous system generally can
never lock fully (it needs that speed difference to maintain the heat), so it may only be able to
transfer 80% of the torque to the axle that can use it the most. But even here,
a FWD with pure viscous coupling center, could send 80% of the torque to the rear
if the front had no traction and the rear all the available traction.
Often VC systems overdrive the input shaft a little bit so there is always a slight
speed differential which keeps the fluid hot and provides quicker lockup when
the speed difference increases further. A lot of lower end AWD vehicles (minivans for instance)
use this setup, and ironically some of the highest end cars, like Porsche and Lamborghini have used
it, because they only need to drive the front tires during launch, but otherwise want
a RWD car.
2. Open center diff, with VC between output shafts. This is an open diff
at the center which provides a static 50/50 torque split, but when one side
starts to spin faster than the other indicating a traction imbalance, it reacts
just like the above pure VC. A 50/50 split is perfectly acceptible as long
as one end or the other doesn't lose so much grip that 50% of the torque will
cause it to lose traction and spin, and only after it does spin will the VC kick
in and send, again.. say 80% of the torque to the axle that can use it the most.
WRX works this way, as do many others.
The VC alone weighs very little but adds drag, especially when the input shaft
is overdriven. You can almost always recognize a car with VC only because it'll have
a listed 90/10 static torque split as it drags the rear around a bit to keep the fluid hot.
The open center diff plus VC has the added drivetrain loss of the differential itself
and the fact that it's applying torque to every gear in the drivetrain at both ends
of the car all the time. I don't know if they typically overdrive the VC to keep it hot or not.
If not, then it would have slower activation than the usual pure VC. If so, then you have to
add in that drivetrain loss as well.

Finally. Haldex. Well, Haldex is a part time, automatically activating AWD system.
The reason people consider it full time AWD is because it activates quicker than
most other full time AWD systems when conditions call for it.
It provides a 100/0 static torque split. That gives it an efficiency advantage
because it doesn't have to drive the gears in the rear drivetrain. They just freewheel
which does cause some drag, but not as much as loaded gear upon gear so it's more
efficient than a system with a center diff and 50/50 split. It has an advantage here
over VC too because it doesn't have to keep the VC hot by dragging it around
when not activated. It probably does overdrive the input shaft a bit, but this just
keeps a small amount of hydraulic fluid pumping around freely. The activation is software
controlled and consists of closing valves which cause the hydraulics to engage
the clutchpack. If they overdrive the input shaft they can provide *any* static
torque split they want with partial activation of the valves. Word is, it has already
been done.
But when it's really needed, it activates in less than 15 degrees of wheel rotation (typically
less than 100ms), provides a near 100% lockup for 100% torque transfer to the rear (if
the front is totally without traction). This means it activates quicker than both pure VC
or a diff plus VC. The reason it gets dissed as an AWD system is that for just putzing around
it normally stays in the 100/0 f/r distribution and it won't activate until there some
need for it. You don't feel it working until it's really needed. Reviews have said as much.
An open center diff changes the handling balance of the car even when you don't really need the
added traction but it also means that the handling characteristics are always consistent.
A Torsen center diff does the same, but has the advantage that it works in 95% of low
traction situations even where the open center diff has to fall back on the VC because even
though the Torsen is just a torque multiplier, 3-4 times the lowest applicable torque is often more than
enough to power the car where you want. Particularly true of road cars.
Anyway, what system is "best" depends on what your priorities are.

A system with a center diff, VC, or Torsen may handle best, but you may pay for it
in efficiency and that hurts the amount of power you can put to the ground at high
speeds, as well as your fuel efficiency. Haldex can do everything another system can do
in nasty conditions and then some, but has a noticably different handling balance in the dry.
From what I've heard of people who drive it, it's just different, and you need to know that to make
the most of it, you simply need to push it hard. Kinda have to trust that it's there and
go ahead and push the car over the edge, knowing that it'll catch you and pull you through.
Of the road tests I've seen of the 4motion, it's happiest when flung around with great
abandon, and I've never seen one spin.