Chevrolet introduced Positraction in 1957, and then a funny thing happened: It soon became the generic name for limited-slip differentials on every brand of American car.

When the Motor City’s carmakers began to offer limited-slip differentials in the 1950s, they weren’t really new. For example, Auto Union employed a ZF limited-slip axle in the early 1930s. During WWII, Ray Thornton developed the NoSpin differential for tandem-axle military trucks—the unit we know as the Detroit Locker. So when Studebaker and Packard introduced a limited-slip axle in 1956, manufactured by Dana and marketed as Twin Traction, it was mainly new to American car buyers.

In late 1956, Chevrolet introduced a limited-slip differential, also produced by Dana, as a $48 option on 1957 Corvettes and passenger cars. And Chevy gave the unit a very catchy trademark name, as things turned out: Positraction. With one important exception, Positraction functioned like any conventional drive axle. (For a great explanation of standard differential operation, see our fine old Jam Handy film here). The exception was a pair of small, multiple-plate clutches, one behind each side gear, as shown at the red arrow above.

In normal, straight-line operation, the clutches held the side gears firmly to the differential case, providing equal torque to both drive wheels for better traction. If you’ve ever driven a car with a conventional differential in winter, with one free wheel spinning helplessly on one small patch of ice, you can see the obvious benefit. But in cornering, the clutches allowed a limited amount of slip between the side gears and case, so the rear wheels could turn at different speeds to provide almost normal differential operation.

These exploded views provide a closer look at the components. At top is the Studebaker-Packard Twin Traction, while the Chevrolet Postitraction is at the bottom. While both units were developed and manufactured by Dana, Twin Traction originally used cone clutches while Positraction used multiple-disc clutch packs (arrows). Otherwise, the units functioned the same. However, multi-disc clutches soon proved to be the way to go in this style of limited slip differential—though there are many other types, too.

In 1958 Pontiac adopted the Dana unit but with its own copyrighted name, Safe-T-Track. Soon all the automakers joined in with their own limited-slip options from various suppliers: Oldsmobile was Anti-Spin; Chrysler was Sure Grip; Ford was Equa-Lock or Traction-Lok, and so on. But then a funny thing happened. Positraction somehow achieved the magical Kleenex effect and became the common name for all brands of limited-slip axles, used by most everyone on the automotive scene—for example, in Marisa Tomei’s beloved scene in My Cousin Vinny. The terminology may not be technically correct, but everyone will know what you’re talking about.