Haldex Traction AB
is a division of the Swedish
manufacturing company Haldex AB
. It specialises in the development and manufacture of All-Wheel Drive systems. Haldex is currently producing their fourth generation AWD system called, by saab, XWD (Cross-Wheel Drive). The first generation of Haldex’s four-wheel drive system was introduced in 1998. Through continued product development, the system characteristics have been refined and improved for each new generation. Haldex is one of the leading suppliers of four-wheel drive systems.
First Gen - 1998
The Haldex Coupling made its first appearance in the Audi TT and VW Golf with a patented hydraulic-mechanical All-Wheel-Drive concept that is electronically controlled and allows AWD performance that had never been seen before. This system attempts to engage the rear wheels when the front wheels start to slip.
Second Gen - 2002
The second generation of Haldex coupling is an electronically controlled permanent 4x4 system with a Haldex differential calculating how much drive should be directed to the rear wheels. The Haldex system automatically distributes power between the front and rear wheels depending on slippage, but normally sends 95% of the power to the front wheels. It can react in as little as one seventh of a revolution of any wheel. Power transfer in the second generation is much more effective. Electronic sensors and controls can modify the system's response characteristics, for better operation in conditions ranging from dry pavement to loose gravel, snow, ice, or mud. A pre-charge pump charges the hydraulic system to allow the coupling to engage so quickly. This pump is improved again for the third generation to allow for greater speed.
Third Gen - 2006
The third generation of Haldex coupling made its appearance on the newly re-designed Land Rover Freelander 2
in the United States). With enhanced capabilities, It allows more immediate off road response. The Freelander's intelligent 4x4 system has been developed in conjunction with Haldex, whose acclaimed centre-coupling technology continuously alters the front-rear torque split, normally through a hydraulically operated multi-plate wet clutch. However, Land Rover wanted an electronically controlled centre coupling – linking the propshaft to the rear differential – that could pre-engage at rest to reduce wheelspin from standing starts, engage quickly when traction loss was detected and disengage quickly without compromising stability control systems. The system also had to transmit the necessary torque to achieve Freelander 2's off-road traction demands. The result is used exclusively on Freelander 2
, and proactively engages full-time 4x4 rapidly and completely. An improved high-pressure pre-charge pump charges the hydraulic system as soon as the engine is started, allowing for full-time 4x4 from rest. It also reduces the time taken to achieve full torque once wheel-slip has been detected – within 15 degrees of wheel-slip rotation (compared with over 60 degrees of wheel-slip rotation with more conventional units). The Freelander 2's Haldex unit is designed to allow up to 1500 Nm of torque transmission. An accumulator also speeds up the unit's response. Full torque transmission can be achieved in just 150 milliseconds. In effect, the new Haldex coupling gives the proactive engagement benefits of full-time 4x4 and the efficiency and fuel economy of an on-demand system.
This generation of Haldex coupling is later shared with Volvo's XC70 and XC60.
Fourth Gen - 2007
introduces a unique combination of Haldex Couplings on its 9-3 Turbo-X
in late 2007. Called XWD (Cross-Wheel Drive), it allows enhanced traction, safer driving, and better control.
The main components of the new Haldex system are the Power Take-Off Unit (PTU), Limited Slip Coupling (LSC) and eLSD (Electronic Limited Slip Differential). The PTU is the final drive unit at the front of the vehicle that transmits power to the front wheels and sends power down the driveshaft to the rear wheels. It is not a Haldex design, but is required to adapt the system to a front-wheel drive vehicle. The LSC sits at the rear of the vehicle in-line with the driveshaft. It controls the torque split between the front and rear wheels of the vehicle. The LSC sends torque to the eLSD that sits between the rear wheels. The eLSD transfers torque to the two rear wheels. Like with XWD, previous generation Haldex systems also included an LSC and an LSD. However, with the new system Haldex significantly redesigned the workings of their LSC. The LSC is still a clutch pack that adjusts torque split depending on hydraulic pressure. It is the method of fluid flow through the device that has changed.
A large complaint about the old system was its lagging response time. LSC versions 3.0 and earlier used a built-in pump to create hydraulic pressure on the clutch pack to increase the torque drive to the rear wheels. While efforts were made on Haldex's part to create pre-emptive torque by adding a check valve and feeder pump to provide some instant pressure when triggered by wheel slippage, it was still limited in capacity. That is why for version 4.0 Haldex made an effort to improve response time by eliminating the hydraulic pump built in to the LSC, which also reduced its overall packaging size. Instead they have added a proportional pressure release valve with an accumulator that is kept filled by a detached feeder pump. This provides more instant response by holding the valve open to limit the torque drive to the rear wheels and keeping the hydraulic fluid flowing through the system. That way when rear torque is demanded, the valve closes and hydraulic pressure is already there.
The LSD used by Haldex 4.0 is also not the mechanical limited-slip differential of old. They swapped the old system LSD for an electronic unit. The eLSD works in much the same way as the LSC, a feeder pump and pressure relief valve are used to control hydraulic pressure on the differential clutch pack. This allows for complete control of the rear differential lock-up without the need to wait for wheel slippage to occur. The system has its own control unit contained in the LSC. This control unit communicates between the vehicle systems to get sensor input for data such as wheel speed, rpms, throttle position, steering wheel input, etc. It also works with anti-lock brake and traction control systems.
The XWD system can transmit 100 percent of available torque to either the front or rear wheels. However, for those conditions to occur one end of the vehicle would have to lose all traction, like driving on ice for instance. During a standing start the rear wheels are put to use, without the need for any slip to occur. Then under straight-line cruising conditions, to conserve fuel and driveline wear, the torque split to the rear wheels is reduced to a level between 5 and 10 percent. Also up to 85 percent of torque can be transferred by the eLSD between to any single rear wheel if necessary. The system can adjust torque splits based on calculated conditions, such as those that indicate an aggressive lane change maneuver, to effectively reduce overstreer or understeer without any wheel slip occurring. In the event that some wheel slip does actually get to occur, the system can react more timely and efficiently than in the past.
Thanks to this technology, the Saab 9-3 Turbo-X, with only 280 BHP can run a slalom faster than many high performance sports cars, beating German and Japanese rivals, including the Porsche 911 Turbo. Saab has an agreement with Haldex for exclusive first year access to the technology. Haldex is currently developing a XWD system for Hyundai Applications.
NOTE: The XWD moniker is a trademark owned by Saab Automobile AB, and thus cannot be used by any other make.
Haldex In Use
"Haldex LSC" refers to the coupling system — its most prominent uses are in all transverse engine
4x4 models of the Volvo Cars
and the Volkswagen Group (Volkswagen AG) marques
, and Volkswagen
), making them non-permanent 4x4s, unlike longitudinal engine
models (Audi A4
, previous-Volkswagen Passat), which use a Torsen
centre differential. However, its short reaction time (only a seventh of a wheel spin) relative to systems like a viscous coupling, and ability to fully lock mitigate its theoretical disadvantage to a full-time 4WD (like Torsen) setup.
The Haldex LSC has been used in 4WD models of Volkswagen by the name "4motion", and Audi cars under the name of "quattro". The same labels have also been applied to Torsen-based, permanent full-time 4WD systems from both Volkswagen and Audi, which has led to confusion, and some heated debate as to the relative merits of each system. This is due to confusion over nomenclature; based on the fact that the original Audi Quattro coupé did use a torque-sensing centre differential (not Torsen, however), many fans believe that "quattro" refers to said type of 4WD system, not realising that it is simply a trademark. (It is interesting to note that "quattro" is also used on the Audi Q7 SUV, which uses neither Torsen nor Haldex; instead, it uses a BorgWarner system).
As can be seen, the Haldex LSC is used in conventional road-going cars, crossovers, and full-on SUVs.
Vehicles Equipped with Haldex AWD
The Haldex 4WD system is currently used in the following vehicle models: