Automotive differentials have been around for nearly 200 years. During that time, they’ve done an important job without getting much attention from any drivers except enthusiasts. That’s starting to change, though. Even mainstream rides now offer the kind of performance that needs a specialized differential. As a result, you may have heard about vehicles with limited-slip differentials, locking differentials, Torsen differentials and more. Still, you may not have heard exactly what these different components do.
What Is a Differential?
If you’ve watched any sort of race on an oval track, you may have noticed a certain strategy. Whether they’re Olympic runners, NASCAR drivers or jockeys, the competitors usually fight for the inside lane. This is because if you’re in an outside lane, you’re essentially running around a bigger, longer track. It’s the same with the path your car’s tires take when you’re turning. The outside wheel has to travel farther than the inside wheel. So, for proper handling, it also has to move faster than the inner wheel to cover a longer distance in the same amount of time.
In a car, a differential adjusts the speed of the driven wheels to make sure that happens.
How Does a Differential Work?
You can think of the differential as one of the pieces of your vehicle’s drivetrain. Within the engine, the up-and-down motion of the pistons spins the crankshaft. The spinning force is then transferred to the driveshaft. In a simple rear-drive example, the driveshaft then makes a “T” with the axle shafts that are attached to the rear wheels. The differential is where all the shafts meet. There the driveshaft spins gears that transfer power to the axle shafts to move the wheels. The trick is that the gears allow the axle shafts, and therefore the wheels, to rotate at different speeds.
What Is an Open Differential?
An “open differential” is typical for most cars. It’s the least expensive and least complicated type of differential, and it lets the wheels rotate pretty much independently of each other. This allows each wheel to turn at its own speed during turns. Thanks to the differential, the engine simply drives the wheels at different speeds until the road straightens out.
The problem with open differentials is that they get this done by sending more power to the wheel that’s easiest to rotate. That’s fine when you’re turning on a smooth, dry road, but it can create problems on slippery roadways. If one wheel hits a puddle or patch of ice and starts to lose grip, the open differential sends the power in that direction, because that wheel is already spinning so easily. Needless to say, this won’t necessarily help with traction. The wheel may just spin faster since it can’t get a grip on the road. In these situations, you’d rather have a differential that sends more power to the wheel that already has traction. You can rely on a limited-slip differential for that.
What Is a Limited-Slip Differential?
A limited-slip differential minimizes the difference in speed between the driving wheels. If one tire loses grip and begins spinning more quickly than the other, the differential locks the two axle shafts so they spin together. The effect is to send more torque to the wheel with the most traction. With this in mind, a limited-slip differential also is a great choice for performance vehicles.
At the dragstrip, a locking differential helps the driving wheels spin in concert for both controlled, powerful acceleration and smoky two-wheel burnouts. It also leaves room for some differences in wheel speed. That gives it the flexibility to boost performance in the turns as well.
As for the actual locking process, companies use a variety of different systems. Popular alternatives include clutches, special helical gears and viscous couplings that take advantage of hydrodynamic friction. And you can also find unique approaches within those categories. For example, a Torsen limited-slip differential acts much like a helical version, but it has proprietary gear sets. (“Torsen” is a brand name and refers to the unit’s “torque sensing” ability.) Additionally, some vehicles currently offer an electronic limited-slip differential. It’s like a clutch-style setup where the power delivery is managed by electronic sensors.
What Is a Locking Differential?
If you’re into hardcore off-roading, you should consider a vehicle with a locking differential. It can literally lock the axle shafts together so that all of your wheels have to turn at the same speed at all times. This is perfect for when you want maximum traction at all four corners. If only a single tire has grip, it can receive up to 100 percent of an engine’s torque.
The downside to a locked differential is that it’s like having no differential at all in daily driving. When you make a turn, your vehicle’s wheels will slip and skid as they try to compensate for traveling different distances at the same time. That’s a perfect recipe for worn tires and rough handling. That’s why we’ll emphasize that new SUVs and pickups have lockable, not permanently locked, differentials. With one push of a button, you can have the best of both worlds.
Bonus Question: What Is Torque Vectoring?
There are similarities between torque vectoring and an electronic limited-slip differential. In each case, automotive electronics and sensors fine-tune how much torque is delivered to each wheel. Torque vectoring takes things to the next level with more advanced computer technology. This allows for even more precise control of the individual wheels. Another notable difference is that, with torque vectoring, it’s not always the differential itself being controlled. With systems like the new Active Torque Vectoring from Subaru, the brakes get involved. Subaru’s system automatically applies braking power to the inside front wheel when you’re turning. By applying that force very precisely, the setup ensures the difference in speed between the two wheels is just right to get the best grip in your current conditions.
Krome, C. (2019) What Does a Differential Do in a Car? Retrieved from https://www.carfax.com/blog/what-does-a-differential-do-in-a-car