page_title: Limited-slip differential
section_title: Viscous (VLSD)
hierarchical_section_title: Limited-slip differential / Types / Speed sensitivity / Viscous (VLSD)
caption_attribution_description: S13 HICAS differential, could be confused with an S15 differential, because the S15 differential has the speed sensor built into the differential.
context_page_description: A limited-slip differential is a type of differential that allows its two output shafts to rotate at different speeds but limits the maximum difference between the two shafts.
  In an automobile, such limited-slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.
context_section_description: The viscous type is generally simpler because it relies on hydrodynamic friction from fluids with high viscosity. Silicone-based oils are often used. Here, a cylindrical chamber of fluid filled with a stack of perforated discs rotates with the normal motion of the output shafts. The inside surface of the chamber is coupled to one of the driveshafts, and the outside coupled to the differential carrier. Half of the discs are connected to the inner, the other half to the outer, alternating inner/outer in the stack. Differential motion forces the interleaved discs to move through the fluid against each other. In some viscous couplings when speed is maintained the fluid will accumulate heat due to friction. This heat will cause the fluid to expand, and expand the coupler causing the discs to be pulled together resulting in a non-viscous plate to plate friction and a dramatic drop in speed difference. This is known as the hump phenomenon and it allows the side of the coupler to gently lock. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, and so is easier to cope with for the average driver. New Process Gear used a viscous coupling of the Ferguson style in several of their transfer cases including those used in the AMC Eagle.
  Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. In particular, any sustained load which overheats the silicone results in sudden permanent loss of the differential effect. They do have the virtue of failing gracefully, reverting to semi-open differential behavior. Typically a visco-differential that has covered 60,000 miles (97,000 km) or more will be functioning largely as an open differential. The silicone oil is factory sealed in a separate chamber from the gear oil surrounding the rest of the differential. This is not serviceable; when the differential's behavior deteriorates, the VLSD center must be replaced.
caption: Nissan 240SX Viscous LSD
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