Torque-Sensing Differentials

Trail savvy: Off-road driving tips, skills & practices

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As four-wheel drive systems have evolved over the years, many different variants have surfaced to do essentially the same job: distribute power to all four wheels. How this is achieved through various systems affects vehicle dynamics on- and off-road. Last month we explained how electronically-controlled clutches automatically distribute power, a system typically used by All Wheel Drive (AWD) soft roaders, which are not equipped with low-range gearing.

In this article we explore how the mechanical Torsen (TORque SENsing) differential works, what its benefits are and its application – typically on large SUV-type 4x4s with longitudinal drivetrains, low-range gearing and permanent four-wheel drive (4WD).

The history

Having a fail-safe centre differential is at the core of permanent 4WD technology; one which allows the wheels on the front and rear axles to follow a different path to prevent transmission wind-up, but which can also lock front and rear when traction is variable.

An invention patented by American gear boffin, Vernon Gleasman, in 1958 provided a solution. His unique combination of worm wheels and worm gears allowed for axle speed differences just like an open differential, but locked up automatically when there was inter-axle slip. It was a purely mechanical system, acted immediately, and was very robust.