Introduction to Differentials
How Diffs/LSD Work
Many people assume that at least two wheels receive power from the powertrain. Their logic assumes both front wheels pull a front-wheel-drive vehicle or both rear wheels push a rear-wheel-drive vehicle. In about 95 percent of all vehicles, this is an incorrect assumption. In actuality, almost all vehicles on the road use an “open” differential; a mechanism that dates back to the third century in China. As a result, there are situations where the open differential only directs power to a single wheel. When putting the power to a single wheel, it tends to be the wheel with the least amount of traction. By understanding the purpose that a differential serves, we will explore the function of the open differentials, limited-slip differentials and the spool.
When a vehicle is going in a straight line, all wheels are traveling at the same rotational speed. This, of course, assumes that you have the same size tires on the left and right sides of the vehicle and that both the front and rear tires are of the same height. If different size tires are being run from the front to the rear, the rotational speed of the front wheels will not match the rotational speed of the rear tires. However, as long as the tires match from the left to the right side of the vehicle, both front tires will share the same rotational speed while both rear tires will share the same rotational speed. While traveling in a straight-line doesn’t require a differential, the need for a differential becomes apparent when you begin to understand the dynamics that occur when a vehicle makes a turn.
During a turn, the outside wheel must travel at a higher rotational speed than the inside wheel. If that sounds weird, consider this situation for clarification. If a vehicle makes a 90-degree turn on a radius of 25-feet to the inside tire, the outside tire travels on a radius of 25-feet plus the track width of the vehicle (distance between the center points of the two tires). For this example, let’s say that the track width is 60 inches or 5 feet. This would mean that the outside tire must travel on a radius of 25 plus 5 feet or 30 feet. In this example the total distance traveled during this turn by the tires would be:
Inside tire: (25 feet x 2 x pi) / 4 = 39.25 feet
Outside tire: (30 feet x 2 x pi) / 4 = 47.10 feet
To solve this equation, we use the formula that the circumference of a circle is equal to pi times the diameter (where the diameter equals the radius times two). We also know that there are 360 degrees in a full circle and that 90 degree, the angle of our turn, only accounts for a quarter of the entire circumference of the circle. This is why we divide our circumference product by four in this example to establish the distance traveled by each wheel.
The “Open” Differential
As the name implies, the differential is the mechanical device that allows the inside and outside wheel to rotate at different speeds in a turn. An “open” differential is the simplest type of differentials utilized by vehicle manufacturers. In an open differential, the axles go into the side gears contained with the carrier. These side gears (a.k.a. sun gears) mate to spider gears (a.k.a. planet gears) mounted on a cross-shaft or planet trunnion. While you don’t need to know all the parts, you do need to understand the function. The open differential allows the inside and outside wheels to turn at different speeds. This difference in speed occurs in the center of the carrier between the side and spider gears. That’s the good news about an open differential. The downside to an open differential is that when the pedal meets the metal, power gets distributed to the wheel with the least amount of traction.
During straight-line acceleration, an open differential will put the power to the wheel with less traction. As a result, a “peg-leg” type burnout is generally the result. This diminishes straight-line performance and the result is slower 60-foot, 0 to 60 mph or quarter-mile elapsed times. During aggressive cornering under power, an open differential will tend to spin the inside tire (the tire that has less traction since the turn has caused the body and weight to transfer to the outside wheel). This diminishes the ability of a vehicle to traverse a road course or curve with ultimate speed. As you could imagine, there is a significant advantage to having both wheels pushing or pulling a vehicle. Hence, the limited-slip differential was born.
Limited-Slip Differential (LSD)
What if a differential could be designed to work in both an “open” configuration and a “locked” configuration? This was the purpose behind the development of the limited-slip differentials. Before we explore the inner workings of the various types of limited-slip differentials in existence, let’s first understand the basic function and operation of the LSD in both a straight-line and cornering situation.
As the name implies, the purpose of the limited-slip differential is to limit or control the amount of slip allowed in the differential. For maximum straight-line acceleration, an “ideal” LSD would allow zero slip between the left and right drive wheels. This would cause both drive wheels to equally propel the vehicle forward. Instead of just a single contact patch from an “open” differential, a properly-functioning LSD would be able to use both tire contact patches to maximize vehicle traction and acceleration. The result is better acceleration times.
In a corner, a well-designed LSD would also prove to be beneficial. In this situation, the LSD would neither provide a full-lock or a full-open situation. Instead, the LSD would seek to bias additional power to the outside wheel. This, in turn, would reduce inner wheel spin and allow the driver to begin accelerating out of the turn sooner.
Different Types of Diffs (Open vs Factory LSD vs Aftermarket)
Let’s say you ended up with a vehicle that has an open differential, but the factory offered a limited-slip differential as an upgrade. Not everyone has deep pockets, so it’s only natural to consider locating a factory LSD for your vehicle. While you may find a “good” deal that makes sense for this upgrade, saving up for an aftermarket LSD is almost always the better value. While OEM LSDs have the same function as aftermarket LSDs, they are designed with stock power outputs in mind. As a result, the construction of OEM differentials is in many cases significantly weaker than aftermarket models. These differences can often be seen in the number of spider gears or the number and size of the clutch plates used within the OEM LSD. The bottom line is that if you are planning to make more than stock-level horsepower, the additional cost of a KAAZ LSD is more than justified.
Clutch, Gear, Cone and Viscous
The LSD was born from motorsports in the 1930s. Currently, there are four classes of design for an automotive limited-slip or torque-biasing differential. To simplify the discussion, we can eliminate cone and viscous LSDs as they are not available from the performance aftermarket for import performance vehicles (cone-type from Auburn are available for some domestic applications, but no one to our knowledge offers a viscous LSD in the aftermarket). That leaves clutch-type LSDs and gear-type automatic-torque-biasing (ATB) differentials.
Not all clutch-type LSDs are created equal. The OEM clutch-type LSD that came in your 1984 RX-7 is likely to have a significantly different design and performance profile than a KAAZ LSD. Still, all clutch-type LSDs will share some common traits.
First, a clutch-type LSD will always send power to the wheel with more traction or grip even if one of the drive wheels loses contact with the ground. Second, KAAZ clutch LSDs will almost always have larger diameter clutches and a higher clutch count than the OEM. This increases the torque capacity of the locking mechanism and generally increases the responsiveness of the LSD. Third, most clutch-type LSDs can be “tuned” to vary the responsiveness of the locking effect. This can usually be done by changing the clutch capacity (rearrangement of the clutch discs), the initial torque (changing the cone and/or coil springs and preload) and the cam ramp angles (gradual or aggressive engagement). Finally, a clutch-type LSD may also be available in a number of different configurations for a particular application. This will allow a selection of a 1.0-, 1.5- or 2.0-way clutch-type differential to deliver the locking effect only on acceleration (1.0-way), on both acceleration and deceleration (2.0-way) or a combination of full locking effect on acceleration and a reduced effect on coast, deceleration and braking (1.5-way). Depending on the powertrain layout of the vehicle (FF, FR, AWD), the placement of a 1.0-, 1.5- or 2.0-way differential will deliver different results.
The Bottom Line
If you are a “set-it-and-forget-it” type, a gear-type LSD may be your best bet, while if you like to tune and tweak setups for maximum performance, the right clutch-type LSD set up properly will deliver the quickest lap times. In many forms of motorsports (especially those with restricted power), the car is set up around the LSD. With the exception of a massive increase in power or grip, nothing will deliver the performance increase possible like an effective LSD. Although we are a little late to making this revelation, there is a reason why LSD upgrades are among the first upgrades made to performance vehicles destined for the track in Japan, the UK and Europe.