 Eaton G80 locking differential. Click image to enlarge |
By Jim Kerr; photos by Chris Chase
Drive axle, rear axle, differential: they are all terms we use when describing the unit that directs the engine’s power from the transmission to the tires. Differential may be the best term to use, because the drive wheels have to turn at different speeds when the vehicle goes around a corner. The differential allows this. It also splits the torque to the drive wheels, which is great when the tires are on good traction surfaces, but put one wheel on ice or loose dirt and you can easily become stuck. That’s where Eaton’s locking differential comes in useful.
Locking differentials lock both wheels together when additional traction is needed. This is usually done on rear axles only, because locking the front axle makes it very difficult to steer. Mercedes G500’s have offered an electric locking front axle, and combined with the locking rear axle, it is perfect for getting the vehicle over fallen logs and out of deep mud holes, but switch it off if you want to turn.
Dana and Eaton are two manufacturers of locking axles. Both have been around for decades. You can find Dana locking axles in a variety of larger trucks and delivery vans. They are designed for heavy-duty use and they have been used in everything from Canada Post delivery vans to one-ton Dodge trucks. Eaton has been making locking axles for both light duty and heavy-duty vehicles since 1973, and you only find them in GM vehicles.
Eaton recently demonstrated the advantages of a locking differential in a rear-wheel drive pickup over even the best four-wheel drive system and four wheel traction control. The vehicles were positioned with one wheel on dry pavement and the other rear wheel on a set of rollers to simulate a start with one wheel on ice. An object placed in front of one front wheel simulated driving up a curb or out of a rut. Then the driver attempted to drive each vehicle away.
Without a locking axle, the wheel on the rollers would spin but the vehicle wouldn’t move. Even with traction control, which brakes the spinning wheel to cause torque to transfer to the other side, there wasn’t enough traction to cause the vehicle to climb over the obstacle.
 A GMC Sierra with an Eaton G80 locking differential traverses the ditch simulation. Click image to enlarge |
With the locking axle, the vehicle was driven slowly on the roller and the wheel spun freely. By accelerating slightly so the spinning wheel is turning about 100 rpm or about 13 km/h, centrifugal weights in the differential latch to a locking mechanism that forces the differential gears to spread apart and engage clutches in the sides of the differential. Within one turn of the wheel, the clutches lock both sides together and the truck drives over the obstacle with ease. I have pulled out of icy parking spots with a GM vehicle equipped with an Eaton locking axle many times when I would have been stuck without one.
Another demonstration simulated driving the trucks over a ditch at an angle. When the truck was in the middle of the transition, one rear wheel and one front wheel had almost no load on them. With the four-wheel drive vehicles, both the one front and one rear wheel without load would spin and the truck would move no further. It was stuck. Even with four-wheel traction control, the vehicle remained stuck because the computer would reduce power at the same time it braked the spinning wheels. There still wasn’t enough torque transfer to the wheels with traction to get the truck moving.
As the two-wheel drive truck with the Eaton locking axle reached the transition, the unloaded rear wheel began to spin but within one turn of the spinning wheel the axle had locked and the truck drove through. It demonstrated that a locking axle in a two-wheel drive vehicle can perform better than four-wheel drive.
 Eaton G80 locking differential. Click image to enlarge |
Eaton’s locking differential is used for low speed traction. At speeds above about 30 km/h, a centrifugal weight disables the locking mechanism so that the wheels can turn independently. Unlike limited slip or Positrac differentials which have spring loaded clutches that are engaged at all speeds, the locking differential now allows one wheel to spin. This ensures vehicle stability if you encounter black ice on the highway. If both wheels were locked, they would both spin and the rear of the vehicle would have no stability. Without being locked, only one wheel spins and the other maintains some traction.
Traction at low speeds with the stability of an open differential at higher speeds: Eaton offers this for only about $300 as an option on GM trucks. It’s definitely worth the money.
