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What is Positraction?

By Michael O'Connor | May 4, 2022

Positraction, or positive-traction rear end, is a brand name for a limited-slip differential (LSD), which helps distribute power to the rear wheels and ensures that they are spinning at the same speed.

The Positraction name was used on various General Motors vehicles in the 1950s, '60s, and '70s; other brands at the time had their own names for limited-slip differentials.

When one rear wheel is slipping or sliding, a Positraction differential -- and indeed all LSDs -- will push more power to the wheel with more grip. This gives you more traction control with your vehicle and allows both of the rear wheels on a rear-wheel-drive car to grip the road evenly.

Understanding how Positraction works and why it is important can help you make the right decision when buying a new or used car.

How Does Positraction Work?

Positraction works by directing torque to the wheel that needs it the most at any given time. If you are driving under normal conditions on a road or highway, the differential will provide an equal amount of torque to both of the wheels. This helps provide a smooth ride and is needed in most driving situations without any tight corners or slippery surfaces.

When you drive through a tight corner or accelerate harshly, your wheels will want to spin at different speeds, which can decrease the traction and cause handling issues.

A limited-slip differential will adjust the speed at which the rear tires spin and allow the axles to spin at different rates. By distributing torque to the wheel that has the most traction on the ground, the vehicle will be much more stable and you will have better control due to less power going to the slipping wheel.

What is a Differential?

A differential is a component on your vehicle's axle that transforms the power from the engine into movement in the wheels. The differential, or diff, splits the torque from the engine two ways and allows each wheel to spin at two different speeds. This keeps the wheels from locking together, which can cause issues when turning and greatly decrease your ability to control the car, even in normal driving conditions.

Differentials work by having pinions that turn gears inside the differential case using the driveshaft, which, in turn, spins the axle shafts using different gear ratios.

In an open differential, which was a common type of differential in early automobiles, the same amount of torque is always being applied to each wheel. This can cause slippage when turning in tight corners or in off-road situations when the amount of torque being created by the engine needs to be distributed unevenly to the different wheels.

This is where a limited-slip differential, also known as Positraction, comes in handy.

History of Positraction

Positraction was first introduced by GM in 1957 with its 8.875-inch 12-bolt Positraction unit, which was common in Chevrolet passenger cars from 1965 to 1972 and Pontiac countered with their version of the Posi rear end, called the Safe-T-Track in the 1963 Tempest.

Since these vehicles were rear-wheel-drive, they benefited from the ability to transfer torque to different wheels based on the driving conditions. Muscle cars and race cars by Chevy and Ford in the 1960s and 70s took advantage of limited-slip differentials to stop fishtailing when accelerating, which was a problem since they were relatively light with massive V8 engines.

Limited slip differentials were produced by different automakers under different brand names throughout the 50s, and 60s. Chrysler introduced its Sure-Grip differential while Lincoln had a unit called a Directed Power system. Oldsmobile produced a limited-slip diff called the Anti Spin and Ford’s was called the Traction-Lok.

When you purchase an aftermarket version of this differential today, it will usually be the Eaton Corporation’s version, which is called the Truetrac. All of these units had essentially the same function but the General Motors Positraction became the most recognizable name and is often used as a catch-all for the LSD.

Advantages of Positraction

Positraction rear ends offer a number of benefits to drivers and specifically to drivers who often find themselves in less than ideal road conditions where wheel spin can be an issue.

In fact, cars that don’t have a limited-slip differential are required to have some kind of electronic traction control unit that does a similar thing in regards to controlling power in one or more wheels.

Understanding the benefits of the Positraction system is important and can help you choose a car when you are in the market for one.

Better Grip

Perhaps the most important benefit of an LSD is the fact that you can get a better grip and traction on the road. If your wheels are spinning, you are not in complete control of the car and this can be a big problem, especially in snowy or icy conditions.

A Posi rear end will help keep your wheels on the ground and spinning at the same rate no matter what kind of road you are on. This translates into a better driving experience as well as better cornering at high speeds or on windy roads.

Fewer Burnouts

The traction offered by a Posi differential is also beneficial when accelerating from a dead stop.

When you mash on the gas pedal in a car with an open differential, the right rear tire will spin much more than the left. This is because the axle tries to spin with the torque being produced by the drive shaft. When this happens, your vehicle ends up in burnout or fishtail, which can decrease its acceleration time.

This is why many drag racers install a Posi rear end to get better take-off times at the light.

Wheel Lock Detection

Cars with a Positraction rear differential don’t have issues with wheel lock like vehicles with an open differential do. When wheel lock happens on the rear axle of a rear-wheel-drive car, turning becomes very difficult because both wheels are spinning at the exact same rate.

Since the inside wheel needs to turn slightly faster than the outside one in a turn, when the wheels are locked, the car won’t be able to turn as efficiently. This can cause problems with handling and result in understeer since the rear wheels will want to go straight, even though you are turning the car.

Downsides of Positraction

While having a Positraction rear end is usually beneficial, there can be a few downsides to them.

Not everyone is going to need one of these differentials and for the most part, modern vehicles will simply have electronic traction control that does a similar thing without all the moving parts, side gears, spools, and other components that need to be in play with one of these rear ends.

Knowing what some of the downsides are can help you decide if one of these diffs is right for you and your driving habits.

Uneven Tire Wear

Because different amounts of torque are being applied to the wheels with an LSD, this can result in uneven tire wear. While the wheels are going to be spinning at the same rate, they will have different levels of power that are being applied to them.

Because of this, you will need to rotate your tires more often on a vehicle with a Posi rear end than you would with one that has an open differential.

Cost of Repair

Positraction differentials have many moving parts that need to be in good working order for the component to do its job effectively. When one of these parts breaks, the rear end needs to be taken apart and rebuilt, which can result in some hefty repair costs.

Higher performance almost always translates to bigger bills, so it is important to keep this in mind when deciding if an LSD is right for you and your car.

Types of Limited Slip Differentials

While they may do the same thing, there are different types of limited-slip differentials. The difference, however, has nothing to do with the result of transferring power to different wheels but instead has to do with how it is done.

Understanding the different kinds of LSDs is important and will help you choose the right type.

Clutch-Type

A clutch-type LSD, such as the Yukon Dura Grip, uses clutch packs in the differential carrier to transfer torque to one wheel or another. These units have springs that engage side gears and put pressure on the clutch packs.

If both wheels have equal traction, the clutch packs distribute the power equally but as soon as one wheel starts to slip, they will transfer the power to the wheel with the most traction.

Gear-Type

Gear-type differentials don’t use friction surfaces and clutch packs to transfer torque but instead use gears to move power to one wheel. These differentials use worm gears that mesh together depending on the traction of the different wheels.

Axial and radial thrust is transferred to the gear pinions and they move power to the wheel that has the most traction and removes it from the one that is spinning.

Final Thoughts

When choosing a car to buy, it can be overwhelming to deal with all the different options available for the drivetrain and other components. If you know what kind of driving you generally do and what the road conditions are like in your area, you can narrow down your search and get the car that will work best for your specific needs.

Knowing what a Positraction differential is can help you get the right rear end for your car and ensure that you get the best possible traction when you are out on the road so you can drive with confidence.