Well interesting to geeks the likes of me anyway. A Haldex development engineer talks about the system and dispels some of the rumours. Nicked some bits from tech articles too.
Tophers' Haldex share options must be up for renewal!
I am employed by Haldex Traction in Landskrona.
So you have a vested interest in "bigging it up" for your own company!
It has some revolutionary new properties, the most evident being the intelligent way of dynamically controlling how much force goes to the front axle and how much goes to the rear axle.
Bollox - the Haldex does NOT control how much "force" goes to the front axle. That has long been decided by the engineers at VW (if we are talking about Dubs), and is determined soley by the transmission and final drive ratios. Haldex can NOT change those!
The Haldex AWD system is used in the 1001 hp Bugatti Veyron!
Ohhh yes - so it is. But what you fail to state is that the Veyron needs TWO Haldex units, because one Haldex can NOT do the job which just one Torsen can do!
My work requires me to travel a lot. In the winter test season, a lot of time is spent in northern Sweden, in the towns of Arjeplog, Jokkmokk or Arvidsjaur.
Thats nice for you. Would you like a medal? Do you have the chest of a man to pin it on to?
Since the late 1998, Volkswagen replaced the viscous-coupling Syncro system with a new system called "4motion". First shown in Audi TT and Golf 4motion, the new system uses a multi-plate clutch center differential developed by a Swedish company, Haldex, and computer software from the Austria 4WD specialist Steyr-Daimler-Puch. At this moment, it is only offered for the transverse-engined Golf IV platform, but there is no technical reason prevents it from applying to Audi's longitudinal-engined models.
OK, why then, if the Haldex is so good, has Audi not used it on their longitudinal cars. Few would argue with the competencies of the 4wd system of the longitudinal Audi S and RS cars - and do so admirably without the need for Haldex.
The Haldex center differential is similar to Porsche 959's PSK system mentioned in the above, it is only smaller, simpler and cheaper thus making mass production feasible. The center differential is mounted near the rear axle and just in front of the rear differential. As shown in the picture below, its clutch consists of 6 discs ....
LIAR, LIAR, LIAR, LIAR!!!!
The Haldex is NOT a "differential"! It is merely a clutch-based coupling - in exactly the same way that an engine is connected to a gearbox. When the clutch is "open", the crankshaft and the gearbox first motion shaft are entirely independent, and are free to rotate at different speeds, and even in different directions. When the clutch is fully "closed", then the crankshaft and gearbox shaft rotate in unison, at IDENTCAL speeds. A "differential" works completely different, using very differing engineering principles, which the Haldex does NOT, and can NOT use.
They are immersed in oil bath to reduce friction. Actuation is made by hydraulic pressure. Normally the input and output shafts rotate with a speed difference (could be implemented by different final drive ratio), therefore the discs are rotating relative to each other. When no pressure is applied, the clutch is not engaged thus torque will not be transferred to the rear axle. Increase the pressure on the multiplate clutch, the latter will be partially engaged, thus sending torque to the rear axle. The more the clutch engages, the more torque transfers to the rear axle.Computer determines how much torque to be sent to the rear wheels. Normally it is 50:50, but in tight corners when wheels on one of the axles is slipping, the driver can easily feel the torque is transffering from one to another axle. Volkswagen claimed 100% torque could be sent to either axle.
Where?
VW have NEVER claimed a torque distrbution to either axle! VW (correctly) only claimed that 100% torque can be sent to the front axle (in a front engined car). More lies!
Compare with the Porsche 959's unit, Haldex's unit has 7 fewer discs in the clutch. This makes the Haldex unit more compact and cheaper. The down side is not capable to handle as much torque (959 had 369 lbft, Audi TT has 206 lbft). Besides, 959's discs were organised as 6 pairs of independent clutches, each actuated by individual hydraulic actuator. The Haldex has just one actuator acting on all six discs, again, this saves weight and cost. However, I suspect if it could vary the amount of torque split as precise as independent clutches.
Which is EXACTLY one of the reasons why the Veyron needs TWO Haldex units!
Based on the journalists comment about the handling of Audi TT and Golf 4motion, it seems that the 4motion system performs even better than the traditional Torsen-differential Quattro. The age of Torsen Quattro is passing away.
What - those "Swedish" journos who have a vested interest in Swedish companies? Or was it muppets from the likes of MaxPower and the like, who you have fed and watered, and given a free "holiday" in some admittedly remarkable location.
Don't forget to tell everyone that "handling" and "traction" are two very different aspects of vehicle dynamics!
And don't forget, it is NOT Haldex who determine the "handling" of a car - but I have a sneaky suspicion that the R&D and engineering and testing departments of VW and Audi will determine that! Or are you saying that Haldex also determine spring rates, damper rates, roll centres, weight distribution, suspension types, suspension geometery, wheel/tyre offsets, tyre types - oh, and the most important item of all - the structural rigidity of the chassis/monocoque!
And where - exactly, is it stated that "The age of Torsen Quattro is passing away"?
Note: "4motion" is not equal to Haldex system. Volkswagen also use "4motion" to represent the Torsen-LSD system used by Passat. Therefore, "4motion" is actually a marketing nameplate instead of indicating the mechanical design.
Oh boy. I think most VAG people know that already!