Differential & Locking Differential

There are mainly two aspects to judge how powerful a 4WD system is. One is inter-shaft power distribution system; the other, is inter-wheel power distribution systems. The former decides front and rear axles power distribution; the later decides coaxial wheel power distribution. These two contribute the SUV through capability. There are four common shaft power distribution systems: viscous couplings, multiple disk clutch, mechanical differential and transfer case.

Viscous coupling is the simplest and cheapest. Its structure is a sealed container filled with silicone oil, which has two groups slotted metal blades one connected to the front axle, one connected to the rear axle. Two groups of metal blades are separately immersed in silicone oil. It uses characteristic of silicone oil heating expansion becoming sticky for power transmission. If front wheels slip, the axles speed difference increases. The relative movement of two metal blades makes silicone oil heating, expanded, and sticky to prevent relative movement between the inner and outer panels.

Multiple disk clutch has quick response but low reliability. The differential transmits power by friction torque. There are two sets of friction discs, a group of active disks connected to the front axle, and the other group of driven disks connected to the rear axle. It is controlled by the electronic control system.

Torsen differential is agile and durable but expensive. Torsen differential, also known as torque-sensing differential, is a fully automatic purely mechanical differential, to achieve locking function fully without the aid of human or electronic systems. It has the structure of double worm turbine. Use one-way transmission of torque from the worm to the turbine to achieve differential "self-locking" feature.

Locking differential makes the differential useless. The left and right side of the wheels are connected by stiff shaft. The wheels speed is same. Even if one side of the wheels slip, the other side of the wheels can also make the car move forward, but this state cannot run in a paved road.