Limited Slip Differentials (legal LSD)
|Limited-Slip (or Traction Lok - the Ford name, or
Posi-Traction, GM name) is a function of the differential allowing one wheel to spin only
a percentage faster than the other. This has its advantages in allowing more traction,
especially during hard cornering, because a normal or "open" diff will allow the
inside wheel to lose traction and spin as fast as it wants, while the outer tire (with
most of the traction anyway) just freewheels. This is generally opposite of what you want
to have happen. A Posi unit will allow the inside wheel to spin only so much faster than
the outside, at which time it transfers torque to the outer wheel, increasing power that's
transmitted to the ground. This is achieved by several methods. The most common is with
clutch packs in the differential that are set to a "breakaway" threshold.
Another semi-common way is using viscosity (Viscous LSD's), which use a special
differential fluid to control the amount of slip. A third method is a Torsen type, where
there's a worm gear and pinion (the theory being the worm gear can rotate the pinion, but
not vice versa). In effect, you can think of cars without Posi to be "one wheel
drive" and cars with Posi to be two-wheel drive.
Picture a car rounding a corner;
the inside and outside wheels have to carve differently sized circles.
Because of this, they must travel at different speeds or independent of
eachother. The non-drive wheels are no problem, but the drive wheels are
connected to eachother via halfshafts, ring gear and driveshaft. This is
where a normal OPEN differential comes into play. It allows the drive
wheels to spin at different speeds when rounding a corner (the tires and
drivetrain would bind and not corner as well without it). The problem with
totally open differentials is that you cannot control how much and when they
slip. Trying to accelerate on ice or gravel will usually result in only 1
tire spinning, producing plenty of smoke, but no go.
Technical Dept Home
Dave Lum 4/98