What Is Posi Track?

by Bob Brown

Posi-Track is a shortened generic term for the GM-branded name "Positraction." It is a type of limited slip differential. To understand how limited slip differentials or LSD works, you must understand how a differential works, and how it relates to torque. In basic terms, a differential takes the power from the output drive of your engine and transfers it to your wheels.

What is a Differential

The differential on an automobile is located in the middle of the axle. It is found on the rear axle of a rear-wheel drive car, the front axle of a front-wheel drive car and on both axles for a 4-wheel or all-wheel drive car. As defined by Merriam-Webster Dictionary, a differential is "a drive-train gear assembly connecting two collinear shafts or axles (as those of the rear wheels of an automobile) and permitting one shaft to revolve faster than the other."

Spool Differential

The spool differential locks both sides of the axle so the torque is distributed equally to both wheels at all times. This is useful in off-road and drag racing applications, but can cause the rear end to slide uncontrollably in high-speed turns on pavement.

Open Differential

An open differential allows the two wheels to spin at different speeds from each other. When cornering or turning the inside wheel has less of a distance to travel than the outside wheel. An open differential allows the outside wheel to make more revolutions keeping it from dragging or sliding. The disadvantage to an open differential is it will always transfer torque equally, thus the wheel with the least resistance will slip and no more torque will be applied. This means if one wheel is in ice or snow and the other on dry pavement, the wheel that is slipping will get the torque, or "power," and the wheel on dry pavement will not get enough torque to propel it forward. In other words, "You're stuck!"

Limited Slip Differential

The limited slip differential is perhaps the most versatile, as it performs well in a wide range of uses. A limited slip system uses clutches or springs between the two halves of the axle to apply torque equally but still allows a "limited" amount of slip when cornering, turning or when accelerating in loose gravel, sand, mud or snow. This type of differential system was branded by the major car companies. Some of the more well-known are GM's PosiTraction, Ford's Track-Loc and American Motors Corporation's Twin-Grip.

Types of Limited Slip Differentials

There are two types of limited slip differentials: mechanical and viscous type. Mechanical limited slip differentials work by using either a clutch system as discussed above or the more modern torque sensitive system. This type of system uses clutches instead of gears to adjust slip. Viscous type limited slip differentials work by using a thick, viscous fluid to provide resistance to achieve the same type of properties of the clutch system. This is also a popular application for all-wheel drive vehicles as it allows more "slippage" on dry pavement in high-speed turns.

About the Author

Bob Brown has attended Johnson & Wales University in Charleston, S.C. for baking and pastry arts and DelTech Community College in Dover, Del., for construction management. He has been a general contractor and carpenter since 2001. Previous to that he has been a baker and pastry chef.

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