"Haldex" in this context refers to recent all-wheel drive (AWD) systems used by Volkswagen's 4Motion and Audi quattro transverse drivetrains. (Longitudinal drivetrain VWs and Audis employ a Torsen center differential AWD system.) Some have referred to Haldex as "fake" AWD. I'll explain as best I can and leave you to decide on your own.
Differentials in Review
Differentials in Review
When a vehicle turns a corner, the outside wheel travels a further distance than the inside wheel. Thus the two wheels rotate at different speeds — they experience a differential speed relative to one another. As such, a solid coupling between the wheels would result in a torsional binding through that coupling (axle) between the wheels relieved when one or both tires slide against the driving surface. On low traction surfaces like dirt this works well enough, but on high traction surfaces like asphalt and concrete it's a problem resulting in understeer and critical stresses on driveline components.
Automotive drivetrain differentials split engine torque (twisting force) between two output shafts connected to a wheel on the left side and another on the right. This allows wheels powered by the engine to rotate at different speeds.
An open differential allows all of its input engine torque to follow the path of least resistance. In other words, if both wheels have the same amount of traction — force resisting wheel rotation — 50% of the torque travels to each wheel. However, if one tire has zero traction (e.g., loose sand, ice or even suspended in midair) as much as 100% engine torque flows to this wheel which spins uselessly while the other tire which does have traction receives as little as 0% of the torque. This problem makes it easy for a vehicle to get stuck in mud, snow, wet grass, etc. and is the reason some cars can only do a "one-wheel burnout."
I once met a person trying to back her new Lexus SUV out of a space in a gravel parking lot. She had pulled a little to far forward when she parked resulting in one of the front wheels dropping into a washed out channel. The forward weight balance of this truck meant the opposite side rear wheel went high, though still touching the ground. Unfortunately, the owner not only chose to buy the 2WD version of this luxury barge with off-road pretensions, she also opted for an open differential. So when she put the truck into reverse gear and tried to back out, the rear wheel that had very little vehicle weight on it spun uselessly, the truck unmoving. Her allegedly off-road truck got stuck in a gravel parking lot. After a friendly laugh, some buddies and I jumped on the rear bumper to get enough weight over the spinning wheel that it could create enough friction to escape. Folks, don't buy 1WD off-roaders.
A locker differential, often found on part-time 4WD vehicles, provides two modes of operation: open or coupled. When open, a locker behaves exactly like any other open differential. When coupled, both wheels are locked together: They rotate at the same speed with the torque applied to each wheel in proportion to its traction. A spool (used on some drag racing cars) is like a locker that is always locked. (I'm avoiding the term, "solid axle," which typically implies a suspension component involving a specific differential and axle configuration.)
A limited-slip differential (LSD) limits the amount of torque that can be applied to one wheel. Different types of LSD can direct as much as 100% of their input torque to the wheel with traction. Some ways to implement a LSD are with a viscous coupling (e.g. the old VW Syncro system), geared LSDs (e.g., Torsen, Quaife), mechanical clutches, electronically controlled clutches or systems like VW/Audi's EDL/XDS that emulate clutches with an open diff and the car's brakes. https://en.wikipedia.org/wiki/Limited-slip_differential
You can tell you have an open differential if you lift both wheels off the ground and rotate one of them. If the other wheel turns in the opposite direction, you have an open diff. Systems that use the brakes to effect limited slip and some diffs that use electronically controlled clutches on an otherwise open diff behave this way too. If they turn in the same direction, you have some kind of mechanical limited slip.
The front and rear differentials on the Mk7 Golf R are simple open types with VW's Cross Differential Lock (XDS) which uses force applied to independent brakes to provide the limited slip capability.
Why is FWD "Bad?"
Why is FWD "Bad?"
The front-wheel drive powertrain layout suffers the disadvantage that, during acceleration, the weight of the car transfers from the front axle to the rear axle. More weight pressing down the rear tires increases their friction (force of static friction = the coefficient of friction of the surfaces involved multiplied by the force (including gravity) pressing them together (known as normal force) or Fs = µN). This is great for rear-wheel drive vehicles that get better traction on the drive wheels under hard acceleration. Conversely, less weight over the front axle of a FWD vehicle means less traction for the drive wheels leading to wheelspin at lower levels of acceleration. Thus when you're powering through the apex of a curve or drag-racing an AWD car, you want most of your torque going to the wheels with traction — the rear wheels.
FWD also suffers increased understeer as wheel torque overtakes traction. You're already putting a load on the front tires with the slip angle produced by steering. Acceleration adds to that until a tire loses grip, switching from static friction to lower dynamic friction. The only ways to recover from power-on understeer is to reduce torque or straighten your steering (tricky as you approach the edge of the roadway). Power-on oversteer of a car slipping its rear wheels, on the other hand, may help you rotate the vehicle through the turn without backing off (judiciously) applied torque.
That isn't to say FWD is bad, it's just a different set of engineering compromises. There are many advantages like compact packaging allowing more passenger and cargo space, lower weight and lower cost. In low traction situations, the natural forward weight bias of FWD puts more weight and, hence, more traction on both the drive and steering wheels. Plus their inherent understeer is easily corrected by the natural reaction of average drivers to lift off the throttle or even apply the brakes. But those advantages don't particularly increase performance.
How Does Haldex Gen V Work?
The Haldex all-wheel drive (AWD) system in Mk7 Golfs serves the same function as the center differential in conventional permanent AWD and the transfer case of a part-time four-wheel drive (4WD) system. The engine transmits torque through the transmission to the center diff/transfer case which splits the torque, distributing it to the differentials driving the front and rear wheels.
Where VW's Haldex differs from most AWD systems is that it does not employ a center differential between the front and rear axles. Instead it uses a wet clutch pack to engage/disengage the rear axle. Engine torque is always directly applied to the front differential like the front-wheel drive (FWD) GTI. The Golf R transaxle also sends torque through an output shaft to the Haldex rear differential. This output shaft rotates at the same speed as the input to the front diff. Clutches in the rear diff couple it to the output shaft.
In this way, theoretically between 0% (clutches fully disengaged) and 50% (fully engaged) of engine torque can be applied to the rear axle. Similarly, between 100% and 50% (respectively) of engine torque is applied to the front axle. Why no more than 50% split? Haldex's directly-coupled clutch arrangement works like a conventional FWD when disengaged with zero torque applied to the rear axle. When it's fully engaged, the system mechanically behaves like a solid shaft between the front and rear diffs with no center diff between them. In other words, because the inputs of both diffs rotate at the same speed, 50% of the engine torque is applied to each just like a locked differential. (It's unclear to me if the Haldex system operates at partial clutch engagement when a small amount of slip is detected during an extended maneuver.)
Looked at another way, most AWD systems send engine torque from the transmission output shaft to the center diff. The center diff splits torque between the front and rear diffs. A Haldex 4Motion system sends engine torque from the transmission output shaft directly to the front diff and the Haldex clutches.
Caveat #1: Much of the literature refers to the 50% torque split I regurgitate above. That's a literal-minded assumption of similar traction between the front and rear wheels. (Certain LSDs can send greater than 50% torque to either axle in equal traction conditions.) Conceivably a situation is possible in which the front wheels have little — let's say zero — traction, while the rear wheels have plenty of traction. (Insert your wild drag strip wheelie fantasy here.) In this case, with the Haldex clutch pack fully engaged both front and rear diffs are spinning at the same speed. But because the front wheels offer almost no resistance (just frictional and inertial losses) to engine torque, 100% of that torque works against the rear wheels. But this would seem a bizarre and highly temporary situation indeed, particularly if you wanted to turn. (Reduced traction of the front wheels results in understeer.) It's far more common that one would have low traction on one side and high traction on the other rather than a front-to-rear difference. (E.g., one side on an icy road and the other on a gravel shoulder.) In other words, since we're talking about road cars here and not bog crawlers, whatever your left front wheel rolls over, your left rear wheel will encounter shortly thereafter. So we're sticking with the 50/50 ratio quoted in the literature.
Caveat #2: Additionally, I wrote above that the rear receives 0% of engine torque when the Haldex clutch pack is fully disengaged. Most of my research refers to a static, straight ahead driving torque split of 90%/10% front/rear. Some torque is applied to the rear when both axles are rotating at the same speed, possibly in part to run the hydraulic feeder pump for the clutch pack. I might also theorize some slip is built into the system to smooth rear axle engagement but I don't know. The opportunity for tuning this dynamic handling system as a whole would seem broad and will, no doubt, be improved over the course of future generations.
Haldex Generation V uses an electronic control design that can proactively engage the clutch before any actual slip occurs. It monitors steering angle, throttle position, engine torque, engine RPM, individual wheel speeds, vehicle yaw and other sensors. These feed the anti-lock brake system (ABS), anti-slip regulation (ASR, a.k.a. traction control), Cross Differential Lock (XDS: an advanced descendent of VW/Audi's Electronic Differential Lock, EDL), the Electronic Stabilization Program (ESP) as well as the Haldex working together in the integrated 4Motion system. These data combined with those systems act in concert to vector torque to the wheels in proportion to where the driver is directing travel.
Not Real AWD
Front-wheel biased? No center differential? That's not real AWD. That sucks!
Well…maybe. VW's Haldex system, in fact, behaves like FWD most of the time. That means when the fronts slip — and even the lowliest of FWD cars suffer wheel slip — there is a transition time before torque is transmitted to the wheels with grip. But Haldex is fast, the clutches engaging in milliseconds. Granted, in a dynamic handling situation, this can be a problem better solved by an AWD system that's already smoothly applying 50% or more torque to the rear wheels. But the advantages of Haldex over, say, a Torsen center diff are less fuel-consuming friction, lighter weight, arguably lower complexity and lower cost. It's an engineering trade-off. And, unlike older, purely mechanical AWD systems, the Mk7 Golf R's 4Motion can predict ahead of the onset of slip when the Haldex torque bias needs to be changed like other sophisticated torque-vectoring AWD systems.
Not all Haldex Is the Same
Previous generations of Haldex were, needless to say, less sophisticated. The earliest were purely mechanical, reactive systems engaged by a speed difference between the two sides of the Haldex clutch pack after slip had already started. As such, they were late to engage and offered little in the way of sophistication in the application of rear axle torque. A friend of mine who tracked his first generation Audi TT staunchly defended its first generation Haldex until he drove an A4 with its Torsen-based quattro. Then the harsh engagement mid-corner that unsettled his TT so badly was made readily apparent.
"If Haldex is good enough for exotics like the Lamborghini Aventador and Bugatti Chiron, it's good enough for me!" Not exactly. Both those cars are mid-engined with the output shaft connected directly to the rear differential. Think of them as basically RWD cars that occasionally send torque through the front wheels.
The gen. IV Haldex (as equipped on the above exotics) benefits from the first electronic control in an integrated system. Gen V reduces some complexity of the previous system by removing the clutch pack's hydraulic pressure accumulator and its control valve that allowed the gen. III and IV Haldex to react in milliseconds. Gen V's feeder pump bypass allows it a similar pressure ramp with less weight and space. Presumably the addition of XDS and general 4Motion software improvements have also improved the perceived performance of the Mk7 Haldex unit within the overall system.
My own impression of the Mk7 Golf R 4Motion system is that it just works. Mostly. I can launch hard from stop lights on slippery, rain-soaked streets without drama. With the power tuned up to almost 400 bhp, I can feel the Haldex clutch pack engage hard but near instantly on full zoot upshifts. I'm concerned that shock could upset handling in a turn. I haven't taken it to the track so I can't say I know how it would behave coming on power before apexing a long sweeper. It's certainly never lurched unexpectedly punching through any public road curve I've tried, but I can't seriously exceed more than 3/10ths the car's capabilities on a public road; a race track environment is completely different. I haven't been able to induce power-on oversteer like the Evo or RWD cars, but that may have more to do with VW's tuning for understeer. (To be fixed with upcoming chassis and suspension mods.) Despite the added power it has zero torque steer. And it returns excellent fuel economy for a performance-oriented car — I typically get 30-32 mpg on my morning commute with a few forays up to 34.
But the Stage II tune exposes the hard engagement of the Haldex clutch with several loud, harsh pulses as the front wheels alternate between slip and grip on various surfaces. In the end, when you reach maximum power blasting through the apex of a turn, the best possible 50% torque distribution to the rear will always result in understeer. (Much greater than 50% rear torque split is part of how the Focus RS achieves its famous "drift mode.") The only solution is to reduce power during understeer. That's simply the nature of the beast.
Haldex is a major component of how the Mk7 Golf R performs, so a comparison to similar cars with sophisticated mechanical and torque vectoring systems is apropos. Personally, I feel the car is more in line with the mission of the Subaru WRX: exciting AWD dynamics in a reasonably tame and happily livable street car. However, the Golf R's steep $40K price tag invites comparison to the loftier, hard-core WRX STI and the indomitable Ford Focus RS. Head-to-head comparisons can be found linked above. In them the Mk7 Golf R manages an entirely respectable showing against its track-ready competitors. Ultimately it cannot beat the RS on its own turf. My own biased perception — and the primary reason I chose this car — is that the Volkswagen is tuned for more than raw performance. The hatchback is eminently practical (Subaru, I'm looking at you), the interior materials, fitment and style is top notch in its class, it's quiet and composed when you want it to be and its subtle, handsome styling doesn't scream, "Look at me!" I wasn't looking for another Evo which is more the mission of the STI and the RS.
So, is the Mk7 Golf R's Haldex system real AWD or not? I'll readily admit the simple mechanical bits seem dubious. Indeed, early generations of Haldex lacking in electronic sophistication exhibit troubling compromises. But taken with the Mk7's predictive 4Motion system as a whole, practical comparisons with some very capable contemporary mechanical/real AWD competitors yield convincingly comparable results. Whatever you decide, I am pleased with how this car puts the power to the pavement.
More recently Deutsche Auto Parts produced a great video, Quattro vs Quattro, which does a much better job explaining the differences between a traditional mechanical AWD and Haldex than I ever could. The side-by-side low traction launch demos are very instructive. Bear in mind, however, that Paul is comparing a current generation Quattro to a purely reactive first generation Haldex.