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4-Wheel Drive: Permanent Vs. Temporary

27 September 2016

4-Wheel Drive

Updated over the course of the 1970s by Subaru, which was the very first manufacturer to incorporate them in mass-produced sedans, 4-wheel drive transmissions are now increasingly common in the automobile industry. Consisting of a vehicle operation on 4 driving wheels, this type of transmission is now available in two main variations: permanent and semi-permanent (or temporary) 4-wheel drive. But what do each of these variations really involve, and what should you focus on when choosing your vehicle?
Permanent 4-wheel drive: the traction solution
This type of transmission is increasingly common on recent 4-wheel drive (4WD) models. Most often used on SUVs and luxury sedans, it involves – as its name suggests – a constant torque distribution between the 4 wheels of the vehicle. Here, as opposed to traditional models where only two wheels are driven, all four tires supply the necessary power to handle low-traction situations (snow or ice, for example) where the vehicle loses its directional ability and its traction. In this permanent 4-wheel drive system, a central differential (or inter-axle differential) is integrated to allow the front axle to turn more quickly than the rear axle when turning.
A permanent 4-wheel drive transmission has some interesting advantages, especially with regard to vehicle traction, which is optimized on all surfaces and in all weather conditions. This additional driveability and traction also guarantees added safety for the driver. However, this additional and virtually unconditional safety comes with a relatively high weight and cost. Indeed, the permanent 4-wheel drive system is quite heavy and bulky, thus adding to the weight of the vehicle and leading to higher fuel consumption on the road. In concrete terms, we’re talking about an extra 0.4 to 1 L per 100 km.
Semi-permanent 4-wheel drive: the flexible solution
The temporary 4-wheel drive system is a sort of hybrid between 4-wheel drive transmissions and 2-wheel drive models. We also start off here in normal 2-wheel drive vehicle operation. Then, when one of the driving wheels loses its traction and the vehicle detects a significant difference in speed between the front and rear wheels, a viscous drive system (involving a multi-disc clutch and a coupler) helps send some of the torque to the “free” drivetrain, thus gradually shifting the car into 4-wheel drive. Today, this type of transmission is mainly used in SUVs, sedans with transversely mounted engines, and several traditional 4×4 models.
Unlike permanent 4-wheel drive systems, temporary 4-wheel drive systems are less bulky and consume less fuel. On the other hand, as you might expect, they prove less reactive in certain circumstances.
And where do Subaru models fit in with all this?
A real trailblazer in this field, Subaru is considered one of the specialized leaders in the design of 4-wheel drive models. The brand also has the most extensive range of 4-wheel drive vehicles in the world. Now boasting a proven track record, Subaru has developed several variations that meet various specific requirements. But practically speaking, the Subaru Legacy, Outback, Forester, and Impreza with manual transmissions, as well as the Subaru WRX, are the models that let you enjoy the benefits of permanent 4-wheel drive. If you’d rather take advantage of the flexibility of temporary 4-wheel drive, we recommend the Tribeca, the Outback 3.6R, and the Legacy 3.6R, among other models from the brand.