How Does Haldex All-Wheel Drive Work?
This popular technology provides an innovative way to deliver power to all four wheels.
Volkswagen
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Haldex is a part-time all-wheel-drive (AWD) system that automatically powers all four wheels only when necessary. It's mostly used on vehicles that operate primarily in front-wheel-drive (FWD) mode. Less frequently, it's employed on rear-wheel-drive vehicles to send power to the front wheels.
The name is derived from the Swedish Haldex Traction company, which originally designed the technology. In 2011, the company finalized the sale of its Haldex Traction division to Tier 1 automotive supplier BorgWarner.
Haldex All-wheel Drive Engages Only When Wheel Slip Is Detected
In a Haldex system, the driveshaft to the vehicle's rear is always engaged, but the differential it connects to remains inactive under normal circumstances. The technical explanation is that the Haldex system is an electronically controlled hydraulic-mechanical part-time AWD system.
A pump on the differential pressurizes hydraulic fluid, and when electronic sensors detect wheel slip, a valve opens. This allows fluid to flow to the Haldex clutch and provide power to the wheels equipped with the Haldex differential. The system is designed to operate without the driver needing to take any additional action to engage it.
Since 1998, there have been five generations of Haldex. The first generation appeared on FWD-based Volkswagen and Audi cars. While these first Haldex systems reacted after wheel slippage was detected, later generations became more proactive.
The latest Haldex-equipped vehicles, such as the VW Golf R, use an active electronic differential lock that can apply brake pressure to a single rear wheel. Sensors monitoring steering angle, throttle position, engine rpm, and vehicle yaw are also used to determine when the Haldex differential should be engaged.
Fuel Economy and Compact Packaging Are Key Haldex Attributes
From a buyer's perspective, the primary benefit of Haldex is its fuel-economy boost compared with full-time AWD. Due to friction and other internal losses, driving all four wheels takes more energy than only two.
Powering the rear wheels only when additional traction is needed means that, under normal circumstances, the vehicle uses less fuel. Vehicle designers also appreciate Haldex's compact size, which makes it easier to package in the densely engineered space under a vehicle.
Service Intervals, Torque Split, and a Reliance on Electronics Are Haldex Drawbacks
The inherent design means Haldex differentials can only send up to 50% of the engine's power to the non-dominant drive axle. For example, a Volkswagen Golf R, which is based on the front-wheel-drive GTI, can only send half of its available power to its rear wheels.
A Haldex differential might also require more frequent maintenance than other AWD systems. This usually entails a fluid and filter change. Additionally, unlike purely mechanical AWD designs, Haldex relies on electronics to know when to engage, meaning something such as a blown fuse could cause it to malfunction.
Haldex All-Wheel Drive Is Used on Everything From Crossovers to Exotics
Haldex is generally associated with vehicles from the Volkswagen Group. Though most vehicles that use Haldex AWD are mainstream machines, the automaker has also used the technology on exotic cars under its corporate umbrella.
For example, the Bugatti EB 16.4 Veyron uses a Haldex unit to drive the front wheels, as does its successor, the Chiron. Lamborghini also employs Haldex systems to power the front wheels on the Huracán and Aventador sports cars.
Volkswagen uses Haldex AWD on its Tiguan and Golf R, and corporate cousin Audi fits it to some of its smaller cars, including the Q3 and the TT. Other brands that have used Haldex AWD include Cadillac, Ford, Saab, and Volvo.
