“Positraction” is the trade name General Motors gave its limited slip differential units in GM and Chevrolet vehicles in the late 1950s. The term Positraction describes the mechanism that provides drivers with more traction control of their cars than they get with an open differential. People say Positraction when they talk about limited slip differentials generally. Think of it as saying Kleenex when you mean tissues or Xerox when you mean making copies.
What is a Differential?
A differential is a device within your vehicle's axle assembly. Differentials do three things: deliver power to each axle shaft or wheel, let the outside wheel spin faster or slower than the inside wheel, and serve as the last gear reduction drive system for the drivetrain. Variable wheel spins are essential when driving. When you're turning a corner, your car's outside wheel needs to travel further than the inside wheel, so the outer wheel needs to rotate faster than the inside wheel. The differential makes the variation possible. Automakers install different kinds of differentials on their vehicles, depending on the vehicle type.
Most passenger cars have open differentials on their rear axles. Sometimes called a standard or conventional differential, this mechanism features two side gears in the differential carrier, with each gear splinted to accommodate an axle shaft. Gears inside the case, called spider gears, on a shaft in the case, transmit power. A ring gear bolted to the differential carrier drives the case.
The spider gears enable the wheels to spin at independent speeds, so drivers can turn and maneuver safely. If you hit a patch of loose gravel while driving an open differential car, the differential delivers more power to the wheel experiencing the least resistance. You get wheel spin on the loose surface side, but the side in contact with solid ground hardly moves, which keeps the vehicle under control.
An open or conventional differential system works well for paved roads and highway driving and helps tires wear evenly. However, this differential can cause problems for off-road, snow, or slippery-surface driving. Open differentials direct all the power to the wheel with less traction when one wheel has much more traction than the other. This total power transfer can cause the wheel with less traction to slip and spin.
Limited Slip Differentials
Limited-slip differentials (LSDs) don't work like open differentials. A limited-slip differential senses when a wheel is spinning, so it sends more power automatically to the wheel with less traction, but it doesn't send all the power to that wheel. If all the power goes to one wheel, the driven wheel can lose traction, and the other wheel won't drive. Because a limited-slip differential prevents sending too much power to one wheel, it keeps both wheels powered, rotating, and maintaining traction, even in off-road, uneven, and slippery environments.
Why Limited-Slip Differentials Are Different
When Ferdinand Porsche designed a Grand Prix racing car in 1932, the vehicle experienced excessive wheel spin at speeds up to 100 miles per hour. To solve the problem, Porsche hired the engineering firm ZF to develop a limited-slip differential. Today's automakers minimize tire slippage by producing limited slip differentials in several types of designs, using friction plates and cones or gears. Unlike conventional differentials, limited-slip differentials feature two power paths between the differential case and the axle shafts. Spider gears transmit some power to the side gears as in open differentials. Friction between the differential case, clutch plates, and side gears delivers the rest of the power to the wheels.
How Positraction Differentials are Designed
Positraction differentials are designed with clutch plates that enable independent axle rotation when the vehicle is turning or when a wheel is slipping. Positraction units prevent loss of traction by sending power to both driven wheels. At the same time, each wheel rotates independently when necessary, preventing the non-slipping wheel from losing power. A slipping wheel loses power with the conventional rear end design. Positraction units benefit drivers on wet pavement and slick or icy roads, as well as racers launching off the starting line or taking a turn on a race track. The Positraction limited-slip differential design offers better vehicle handling and acceleration, and drivers get more forward power because the driven wheel slips less.
The Popularity of Positraction Units
GM has offered Positraction rear ends since 1957, and its 8.875-inch 12-bolt Positraction was most popular in Chevrolet passenger cars from 1965 to 1972. Pontiac offered a version of a Positraction unit, called Safe-T-Track, as an option on vehicles including the 1963 Pontiac Tempest.
Owners of muscle cars in the 1960s and 70s took advantage of the benefits Positraction offered. Rear-wheel-drive automobiles, including Chevy and Ford model muscle cars, were equipped with a live axle, instead of independent rear tire suspension. When lots of torque is applied through the differential on a vehicle with a live axle, there is less traction on the right rear because the axle wants to turn with the torsion of the drive shaft. However, the axle is mounted to the vehicle frame, so it can't move. Cars without Positraction units would spin one wheel, but those with Positraction rear ends didn't have that problem. Some owners of muscle cars today are retrofitting their cars with equipment including the Duragrip Posi Differential. This equipment enables drivers to keep both wheels on the ground, achieve better traction, and avoid one-wheel burnouts.
How to Check for Positraction
If you're shopping for a car with an original Positraction rear end, look for a tag on the right lower cover bolt. There is a two- or three-digit axle code or a production date if the car has Positraction. If the rear end has an open differential, the tag is on the left lower cover-mounting bolt.
Another way to check for Positraction is by jacking up the rear end, blocking the front wheels, and putting the car in neutral. Ask another person to hold one rear wheel while you try turning the other wheel. If you can turn the wheel on your side while the other wheel is held still, the rear differential is not a Positraction unit. If you cannot turn the wheel on your side while the other person is holding their wheel in place, then the axle is a Positraction. Original gear ratios for GM 10-bolt 8.5-inch Positraction differentials were 2.41:1, 2.56:1, 2.73:1, 3.08:1, 3.43:1, 3.73:1, and 4.10:1.
Another type of differential, called a locking differential, uses dog clutches instead of friction plates to send power to each wheel. A dog clutch transmits power by pressing the tooth of one side of the clutch to the tooth of the other clutch. Generally, this type of clutch is used in manual transmissions. When one wheel turns faster than the other, the differential disengages the appropriate clutch automatically and delivers power to the wheel that has greater traction. Instead of sending power in a smooth, progressive motion as with Positraction, the locking differential operates with a ratchet-like movement.
Some differentials combine an open differential with a mechanical locking device drivers operate and lock manually. When drivers lock their devices, both axles turn at the same speed no matter what road conditions exist. To allow their vehicles to function with open differentials, drivers unlock the devices.
The development of Positraction decades ago enables drivers to operate their vehicles faster and safer. Driving on slick or icy roads, and off-road driving is more predictable for motorists due to the creativity of automotive engineers. However, whether you drive a vehicle with or without Positraction, you are always responsible for operating it safely