What is Positraction in a Car?

What is Positraction?

Positraction, which is short for positive traction, is the brand name for a specific type of limited-slip differential (LSD) that was made popular in General Motors and Chevrolet vehicles in the ‘50s, ‘60s, and ‘70s. While other automakers like Ford, Pontiac, Chrysler, and Oldsmobile had their own versions of the limited-slip differential, the Posi rear end, as it came to be called, was only installed in cars by GMC or Chevy.

A limited-slip differential like the Positraction distributes power to specific wheels to give you the best possible handling and traction in any given driving situation. This is done by pushing power to the wheel with more grip, which prevents wheel spinning and burnouts, especially on icy or wet roads.

If the power is completely even on both wheels in a rear-wheel drive car, you could end up spinning out or fishtailing since the wheels will have different amounts of pressure and traction on specific areas of the road. Knowing what a Posi rear end is and how it works will help you decide if it is the right choice for you and your vehicle.

What is a Limited-Slip Differential?

A limited-slip differential is a device that allows two wheels to spin at different speeds. This is necessary because when you are turning or accelerating, the wheels will have different amounts of pressure and traction in a given situation. Specifically, when you are turning the vehicle, the wheel that is on the outside of the turn has to travel a farther distance than the one on the inside. This means that the wheels have to turn at different speeds if the vehicle is going to make the turn without any handling problems or spinouts.

A limited-slip differential works by utilizing pinions and spider gears to direct power to the wheel that has the most traction in a turn or acceleration. By adjusting the gear ratios between the side gears with a spline, the differential allows the outer wheel in a turn to spin at a different rate and prevents wheel slippage. This is different from a fixed axle, which was common in early automobiles, which connected the two axle shafts and made them spin at the same rate no matter what. This caused serious wheel spin in turns and as vehicles became capable of higher speeds, especially with muscle cars and race cars like the Buick Skylark or Dodge Charger, it became necessary to create a device that allowed the wheels to spin at different rates.

What Makes a Positraction Differential Unique?

For the most part, a Positraction differential is no different than other limited slip differentials that are on the market. It is simply the brand name that was given to the LSD that was installed on popular high-performance cars by General Motors, which gave it its prestige and made the Positraction unit sought-after by car lovers. Other automakers in the USA created their own versions like Posi-Trac, Sure-Trac, Tru-Trac, and Trac-Lok, all of which applied the same principles as the Posi rear end.

There have also been several aftermarket versions that you can buy if you want to install one in your vehicle for better handling both on- and off-road, such as the Yukon Dura Grip Positraction and the Detroit Truetrac. Essentially, all of these diffs accomplish the same thing by distributing more power from the driveshaft to either the left or right rear tires.

Benefits of Positraction:

Positraction differentials have become highly sought after over the years, even if their prestige comes from their name only. Limited-slip differentials are common on many new production cars except for some 4WD vehicles designed specifically for off-roading. This is due to the many benefits that an LSD offers and how the design can improve the handling and overall performance of a vehicle.

Increased Traction:
More than almost anything else, a limited-slip differential offers increased traction for your vehicle. By applying more force to the wheel that has the most traction, an LSD will give your vehicle the best possible turning ratio for your wheels in any given situation. If you want to make sure that your wheels aren’t spinning out or that you aren’t losing out on valuable traction when you need it most, like on icy or wet roads, an LSD is going to be your best bet.

Better Acceleration:
A limited-slip differential will also help you get better acceleration, which is one of the reasons why they are so popular with drag racers and other high-performance vehicle drivers. As you step on the gas, the tires are going to be on two different points on the road, and, depending on the weight distribution of the vehicle, they might spin at two different rates, which can cause burnout. With an LSD, the proper amount of torque will be applied to the wheels as they spin in real time, which will give you a better grip on the road when starting from a dead stop.

Tighter Cornering:
Cornering is also vastly improved when a vehicle has a limited-slip differential. The increased cornering performance is the main benefit of an LSD and if you have ever driven a vehicle with a different type of differential, you have probably noticed a difference right away. Because an LSD allows the wheels to spin at different rates, you don’t have to worry about the outer wheel trying to turn the vehicle at a different angle from the inner wheel. The differential will automatically compensate for any differences in the wheel speed and you will get tight, responsive cornering.

Disadvantages of Positraction:

Even though LSDs are incredibly common in today’s automobiles, there are some downsides to them. For the most part, though, the downsides are more than made up for by the advantages you get in handling and safety. However, knowing what these disadvantages are will be helpful if you are deciding between different differential types for a project car or restoration.

Higher Repair Costs:
If you opt specifically for a Positraction differential, it is important to note that repair costs could be expensive. Replacement parts like a new spool or differential carrier can be hard to find unless you know the specific part number and have a source for Positraction parts. However, if you are already a vintage car enthusiast and are set on using a Positraction rear end, you probably are familiar with the availability of parts so this may not be an issue for you.

Increased Maintenance Needs:
With a limited-slip differential, and specifically an older Positraction model, you may find that you have some increased maintenance needs to keep it running smoothly. Because the parts inside the differential are constantly moving and grinding together, you will need to keep it lubricated with gear oil and sometimes an additive like a friction modifier to keep it working properly. With most modern LSDs, though, there will not be as much upkeep needed as there will be with a Positraction-brand unit.

Uneven Tire Wear:
Because of how limited-slip differentials work, you may also notice that your tires are wearing unevenly. Any time one tire has more traction than the other, you can end up with premature wear on one or two different tires. This can be solved by getting regular tire rotations and making sure that no one tire is favored by the LSD for too long.

Types of Limited-Slip Differentials:

Mechanical Clutch-Type:
Most limited-slip differentials are going to be a mechanical clutch type. The mechanical clutch is the mechanism that analyzes the speed of the wheels and the specific amount of traction that is needed for the ideal spin rate of the wheels. This is the most common type of LSD and the mechanical element is what allows the wheels to spin at different rates when you are cornering or accelerating.

Viscous-Type:
Viscous-type limited slip differentials use liquid to create resistance in one of the wheels and increase the torque in the one that has the most traction. Unlike the mechanical clutch type, this type of LSD has fewer moving mechanical parts, which means there are fewer things that can break down or need to be replaced. However, the liquid will eventually lose its viscosity and it will need to be replaced periodically. This can result in more maintenance, which can end up being more expensive in the long run.

Other Types of Differentials in a Car:

Aside from limited-slip differentials, there are some other types that you may find on a vehicle. Depending on the type of driving you are likely to do, one might be better for you than the other. Knowing what options you have will help you make the right decision.

Open differential:
An open differential is fairly common in family sedans and economy cars. This type of differential splits the power of the engine into two independent drive axles. It allows the two wheels to spin at different rates without the specific mechanics of the limited-slip design.
__
Locking differential:__
A locking differential is also known as a welded differential and it is common in large trucks and vehicles meant for off-roading. This type allows the two wheels to spin at the same rate at all times. While this can make cornering more difficult, it is better for driving on uneven surfaces where the traction of each wheel will change from moment to moment.

Many off-road vehicles will allow you to switch between differential types so you can drive on roads and have better handling when necessary.

Torque-Vectoring:
For super high-performance vehicles, a torque-vectoring differential is often the default type. This has an additional gear train which allows the torque to be fine-tuned for each specific wheel.

This can result in ultra-tight cornering and acceleration. Most production cars will not have this type.

Final Thoughts:

The Positraction limited slip differential was incredibly popular with muscle car enthusiasts across the country and has become synonymous with the culture. Even if you are simply using your car for commuting or driving around town, an LSD is a great choice, no matter what brand it is. Knowing what a Posi rear end does and what your other options are will help you choose wisely for your specific needs and driving habits.