Demystifying the Differential, a Crucial Drivetrain Component
It’s part of a car’s drivetrain, the system of components that sends power to the tires.
A differential plays an essential role in pushing or pulling a car in a straight line and efficiently and comfortably moving it through a corner. Basically, it's a device that allows two sides of an axle to operate at different speeds. Depending upon which direction the car is turning and its front, rear, or all-wheel-drive configuration, it divides up power from the drivetrain to the wheels.
Without a differential, turning would be quite annoying. While a car is turning, the outside wheel and inside wheel are spinning at different speeds. The outside wheel is spinning faster as it has a slightly longer path to take than the inside wheel. Without a differential to sort out the power between the two tires, as well as just account for the difference in speed when power isn't applied, it'd create a lot of tire noise as the inside tire would spin faster than it needs to. The lack of a differential would also massively expand the vehicle's turning radius, making something as simple as driving through a tight parking lot a major pain in the neck.
Thankfully, more than 100 years of automotive research and development has helped continually refine and modernize differentials to make driving easier and even boost traction in certain scenarios, such as on track in motorsports. They help off-road, too, by ensuring that trucks can roll over large obstacles with ease. Let's discuss not only what a differential is and how it works, but the different common kinds that are out there.
The Basics of Differential Configurations
A differential takes power coming from the engine through the drivetrain and splits it up between the wheels that it's assigned to control. In a front-wheel drive car, the differential and transmission are one unit, referred to as a transaxle. In rear-wheel drive cars, the most common configuration has the engine and transmission up front with the differential out back to split the power between the two rear wheels.
In an all-wheel-drive or four-wheel-drive car, the most common method for power to be divided up is via a transfer case attached to the transmission, which sends a certain amount of power to the rear wheels and the rest to the front wheels. Some have settings to change up how much power goes where, but that's a blog for another day.
Transaxles aren't just for front-wheel drive, though, as they exist in rear-wheel drive cars, too. It all depends on the manufacturer's designed weight balance and packaging. The latest Aston Martin Vantage has its twin-turbo 4.0-liter V8 up front, but the transmission and differential out back encased in a transaxle. As does the Lexus RC F GT3 race car. The Porsche 911's engine is behind the rear wheels, and the transaxle sits in front of it between the rear wheels. Wild, right?
How Does a Differential Work?
The simplest method for explaining this is looking at a rear-wheel-drive car. The spinning motion of the driveshaft makes its way to the input pinion inside the differential housing. This turns a ring gear that the differential itself is attached to, which then, through a series of gears, turns the rear driveshafts. The ratio between the input pinion and ring gear is the final drive ratio, also known as the rear axle ratio. If the final drive ratio is a performance-minded 3.73 (at least when it comes to early 2010s BMWs), that means the ring gear has 3.73 times more teeth than the input pinion.
With everything else being equal, such as the same 2011 BMW 128i's 6-speed manual transmission, the revs will sit higher in each of the transmission gears than they would with a 3.23 final drive ratio. Though, there are other factors at play to determine these, such as changing the transmission gearing.
What Are the Different Types of Differentials?
There are three common types of differentials: open, limited-slip, and locked.
An open differential is the most common type and the least expensive to produce. Essentially, it divides up power between the drive wheels, though in an instance of slip more power is sent to the slipping wheel that has less traction. It's sort of like electricity, as in it always takes the path of least resistance. This is often remedied with electronic traction control, though it's not always the most effective way to improve traction.
On the other hand, a limited-slip differential (LSD) biases power to the tire with more traction, or resistance, thus doing a better job getting out of a low-traction scenario, like splashing through a puddle or driving over a patch of ice.
A locked differential starts out as an open differential, but when it is actuated via an electric or pneumatic system, it splits the power equally, 50/50, between the two drive wheels. This is good for maintaining drive to the tire with traction, or even if one wheel is suspended off the ground. The wheel that's in contact with the ground will still propel the vehicle forward.
However, while locked, the vehicle's turning radius is greatly expanded, and tires attached to the locked differential skip and chirp, as they're spinning at the same speed. When unlocked, the differential will allow for the different wheel speeds that happen while turning and revert back to the properties of an open differential.
A welded differential, which is really only common in drifting, is like a locked differential that cannot be unlocked. The drive wheels always have a 50/50 split of power, which is good for creating burnouts and willing the car into a drift, but again, that can become annoying when turning in non-drifting scenarios happens.
Types of Limited-Slip Differentials
There are two common types of limited-slip differentials to discuss: the clutch-type and mechanical-type (also known as a helical limited slip differential). There's also the viscous-type, but we'll save that for a future blog.
The clutch-type is typically meant more for track use only and comes in the form of one-way, one-and-a-half-way, or two-way variants. By clutch-type, we mean that there are clutch packs inside the differential to lock together and send equal amounts of power to each of the rear wheels. The rate at which the clutch discs lock together and send equal amounts of power to each wheel is called the lock-up rate. Typically, the harsher the lock-up rate, the more precise and skillful one's inputs must be.
A one-way differential locks the rear wheels under acceleration. A two-way differential locks the rear wheels under acceleration and deceleration. A one-point-five-way differential is a middle ground with a less-harsh lock-up rate.
A mechanical limited-slip differential uses gears instead of clutches to transfer power to the wheel with more traction, and then backs it off as traction increases, resistance reduces, and slip dissolves. Common types of helical differentials are the Torsen (short for torque-sensing) and Wavetrac, each of which uses its own unique, patented ways of limiting slip.
Which Is Best for Me?
There are so many varieties and nuances to choose from, so it all depends on the application. The beauty of a locker (a common term for a locking differential) is the driver actuates when it locks. However, not all lockers are the same, as some will unlock if the vehicle rolls backwards, while some will stay engaged.
When it comes to limited-slip differentials, the mechanical-type is most common as far as what consumers can buy off the showroom floor. However, they start to fall short performance-wise when it comes to getting the power to the tarmac as efficiently as possible, which is where clutch-type differential comes into play. Clutch-type differentials, however, require maintenance involving replacing clutches, whereas typically the only maintenance that mechanical differentials require is a fluid change.
But then, some limited-slip differentials lock electronically, meaning if someone wants to put down a smokey burnout while exiting a corner in an on-track, performance-driving scenario, the electronically locking diff will do so more effectively than a conventional mechanical diff. A good example of this is BMW's Active M differential, which uses various inputs like throttle input, yaw rate, wheel speed, and overall speed to determine when to best lock the differential for optimum performance.
Again, regardless of each differential's design, the idea is to send power to either of the drive wheels attached to the front or rear axle. Some are more geared toward (pun intended) comfort and cost, whereas others are geared toward optimal performance. Then, there's a large middle ground that possesses a certain balance of the two.
If all of this is still a bit unclear, never fear, our Recurrent Media colleagues over at Donut Media put together an excellent video that's an awesome visual demonstration of how a differential works.
Finally, check out this video by Engineering Explained to gain a bit more visual knowledge regarding the differences between a clutch-type and helical limited slip differential: