What the Heck Is a Differential? And Why Is It So Important?

Open, locking, limited-slip? All the differential types explained.

Published June 22, 2023

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In the automotive world, especially when it comes to off-road trucks and SUVs, differentials come up all the time. Is your differential locking, limited slip or open? Do you have a transfer case? One or two speeds? What’s your final drive ratio? And that’s before we head into various brand loyalties, “Got to have a Ford 9-inch,” or “the 12-Bolt Chevy is the one to beat.”

The conversation can get a bit confusing, especially coming to it as a novice. Here’s a quick rundown of what a differential is, the different types of differentials and why they are important.

What is a differential?

A differential serves a single, important purpose: to send power to two or all four of a vehicle’s wheels while simultaneously allowing the vehicle to turn. When you turn, the car changes direction at a certain angle, following a radius around a corner. For that to happen, each of the four wheels needs to turn at a slightly different radius. And that means each wheel travels at a different speed.

For the non-driven wheels, this is no problem. Nothing binds them. But you only have one engine, which only spins at one speed at any given moment. To allow two or more wheels to rotate at different speeds, all while receiving power from one source, each drive wheel connects to a shaft with a gear on the end, which in turn connects to a gear inside the differential.

chevy silverado gear patrol full lead — The differential performs a basic vehicle function: allowing the vehicle to turn.

Those gears meet more gears in the center, which attach at 90-degree angles in the center and to the other wheel’s shaft on the other side. All told, four gears lie in the center, one from each wheel shaft, and two sit at a 90-degree angle. These four gears (usually bevel gears, by the way) live inside an assembly.

Separately, another gear attaches to the assembly, which meets the driveshaft (which transfers power from the engine) with another gear on the end. The gear at the end of the driveshaft spins the assembly gear, which in turn rotates the assembly and, as a result, the two wheels connected to it.

When a vehicle moves in a straight line, all the drive wheels spin at the same speed, as fast as the assembly spins them. And that means none of the four gears inside the assembly need to spin at all. But once a vehicle starts turning, each drive wheel must spin at a different rate. To compensate for the difference, the four gears get to work, with the faster-moving wheel’s gear effectively spinning around the slower-moving wheel’s gear, as the whole assembly also spins, moving the vehicle around the corner. Lovely!

What is an open differential?

All of the above also explains an open differential. A spherical-shaped lump in the center of your axle that houses six gears, connected in such a way that each individual wheel spins at whatever speed it needs. This is a pretty standard setup for vehicles that are not really aimed at going off road.

Here’s the rub with an open differential: If one drive wheel has less traction than the other, there’s nothing to stop it from spinning up while the other drive wheel(s) stand idly by. You know, when the surface gets lumpy, shifting vehicle weight around. Or uneven, with patches of mud, or rocks, or loose gravel, or sand, or moss, or whatever. That’s where limited-slip and locking differentials come in.

lexus lx 600 — The Lexus LX 600 F Sport Handling uses a Torsen limited-slip differential.

What is a limited-slip differential?

A limited-slip differential is an open differential with additional parts to assist both drive wheels to spin at, or closer to, the same speed. The traditional, mechanical way to make a differential limited-slip is to take an open differential design and install a viscous coupling. A viscous coupling does not physically connect the two wheels together but “connects” them by attaching a set of clutches to the end of each wheel and filling the space between them with a thick, viscous fluid.

Basically, if one wheel starts spinning at a different speed than the other, it’s not just those gears inside the housing that start spinning, but those two clutch plates start spinning next to each other, stirring up that thick, viscous fluid in the process. That added resistance makes it harder for the two wheels to spin at different speeds, which forces more torque to head to the wheel with more traction. Good thing!

Modern cars mimic this behavior by applying brake pressure on the spinning up wheel, the one with less traction. Commonly called a brake-based limit-slip differential, it’s actually just an open differential with help from software and the braking system.

There are, in fact, multiple ways to limit slip in a differential; these are just a couple of the most common.

mercedes g class — The Mercedes G-Class is one of the most capable off-roaders on the planet and features front, rear and center locking differentials.

What is a locking differential?

A locking differential is a differential that locks the two wheels to each other mechanically and the engine, effectively forcing everything to spin at the same speed. And they will, whether you’re bending around a corner or on two different surfaces or whether one wheel is even on the ground at all.

This differential is simple and effective to pull yourself through difficult terrain. Just know that something will have to slip if you ask your vehicle to take a corner. However, note that you can have a locking, rear, front, or center differential. Each serves a slightly different purpose.

Locking Center Differential

A locking center differential forces the front and rear axles to spin at the same speed. Still, each wheel may be connected by an open or limited-slip differential, which means the individual wheels may spin at different speeds.

Locking Rear Differential

A locking rear differential is the most common type because it forces both rear wheels to spin at the same speed for traction benefits but still allows the front wheels to spin at different speeds to maintain some steering capability.

Locking Front Differential

A locking front differential does exist and can help a truck or SUV in particularly treacherous terrain. You almost always see front-locking differentials with rear-locking differentials, not by themselves. They are used when you want all four wheels of a four-wheel-drive vehicle to spin at the same speed and claw through a slippery surface in a straight line. Turning capability with a locked front differential is quite limited.

2021 jeep® wrangler rubicon 392 — While many crossovers offer all-wheel-drive systems, the Jeep Wrangler uses a 2-speed transfer case to shift between two-wheel-drive and four-wheel-drive.

What is a transfer case?

Four-wheel-drive trucks often include another part, transfer cases. These send power to all four wheels instead of just two of them. Two-speed transfer cases do the same but have an extra set of gears built in to function as a torque multiplier, meaning the engine will rev a lot higher to go the same speed. This allows vehicles to climb steep, slippery surfaces, among other things.

What is a final-drive ratio?

The gearing that connects your engine to your wheels in the differential is called the final-drive ratio. Changing that number also changes how fast your engine spins compared to the vehicle speed. Generally speaking, the higher the final-drive ratio, the higher the torque multiplication and the lower the top speed.

There you have it. Differentials allow vehicles to turn, but can also completely change how power gets distributed and put to the pavement… or gravel, dirt, sand, rock or, well, you get the idea.

How-To’s & Explainers, SUVs & Trucks