Locked In

Don’t Get Left Hanging, What You Should Know About Traction Control

- Jonathan Hanson

Many years ago I was exploring a trail in the mountains east of Tucson, Arizona, in my FJ40, and came upon a couple in a shiny new 4x4 Toyota pickup. The driver had managed to high-center the transmission skid plate on a rock ledge so that the right front and left rear tires of the truck hung just an inch or two in the air and spun uselessly when the fellow applied throttle.

The couple, new to backcountry driving, was bewildered that their “four-wheel-drive” vehicle had been rendered completely immobile by such a minor obstacle and were convinced something was wrong with the truck. I picked up a fist-sized rock from the side of the trail and kicked it solidly under the hanging rear tire. “Try it now,” I said—and the Toyota bumped free. They were astonished and even more bewildered—until I explained how a differential works.

Open Differential

When any four-wheeled vehicle makes a turn, each wheel needs to rotate at a different speed because it travels a different line than the others, and thus a different distance. If, for example, the two rear wheels were locked together with a solid axle, the tires would scrub horribly every time the vehicle turned, and handling would be affected dangerously.

Thus, we divide that axle in two and connect them with a differential—a system of gears that transfers power from the driveshaft to the wheels while allowing them to rotate at different speeds in a turn. In a four-wheel-drive vehicle we add another driveshaft and differential at the front, so those wheels can be driven as well.