Tech: Limited Slip Differential Basics/AE86 Cusco LSD and Install
The term LSD is often used within the auto performance world. What exactly does LSD mean, and why is it so important? A Limited Slip Differential splits the power coming through the driveline and distributes even power to the drive tires, regardless of a vehicle’s driveline configuration. In a two-wheel-drive vehicle, distributing torque to both drive wheels equally is a must for putting the power to the ground while also maintaining control of the vehicle’s ability to be steered in the desired direction.
Certain vehicles are equipped with limited slip differentials from the factory – especially when optioned by the owner at the time of purchase – but the majority of vehicles sold today are equipped with an open differential.
An open rear end allows both wheels to maintain the same amount of torque. Even when the two wheels are rotating at different speed, their average speed is always the same. When a vehicle with an open differential encounters a low traction surface, it directs power to the wheel with the least amount of traction. This results in the wheel on the low traction side spinning, while the opposite wheel with high traction receives little to no power. The low traction wheel doesn’t provide proper resistance to the differential’s internal side gear to spin the carrier and transfer power to the opposing wheel. The carrier pinion gears and the carrier simply rotate around the side gear with the most traction, which causes the spinning tire to continue on its trajectory.
Conversely, a mechanical clutch-type limited slip differential is made to lock when it senses uneven wheel rotation on either side of the axle. The uneven torque feedback compresses the internal clutch plates together for a lock-up reaction, effectively putting the power down to the road surface.
When equipped with a limited slip differential, a car can start accelerating earlier and harder out of corners in comparison to an open differential, as all of the available torque is transferred to the ground effectively.
These days, there are plenty of LSD’s offered on the market that can be adjusted to the specific driving style of the vehicle owner. This 1986 Toyota Corolla GT-S (AE86) is the recipient of one of Cusco’s limited slip differentials; follow along as we install and set up a Cusco LSD.
Cusco’s Type-MZ LSD is a cone spring-type limited slip differential. This system has a stack of thin clutch discs. Half of the discs are coupled to one of the drive shafts, and the other half are coupled to the spider gear carrier. There are a total of 20 clutch plates inside the unit.
Within a cone type unit, the end cone plates give additional pressure to the pressure plates to initializing torque. The Cusco MZ LSD comes enabled for two different profiles in terms of effect. This is done by incorporating two different cam angles on the pressure ring. For example, this particular unit has 1.5 and 2 way capabilities; this design is suitable for drifting and road racing, since vehicle control greatly depends on the throttle operation. A 1.5-way setup allows equal force on both wheels on throttle and partial lock on deceleration, while a 2-way locks on throttle and deceleration.
The initial torque (also known as breakaway torque) refers to how much torque it will require for the LSD to reach full lock. The Type-MZ LSD has the highest rated initial torque rating out of any of Cusco’s limited slip differentials. We should also note how Cusco’s MZ plate grooves are cut radially. This enables instant oil movement, which has the end result of improving efficiency and durability.
Disassembly, Inspection and Cleaning
The first and foremost task during the install process is to inspect each of the bearings and races for pitting or uneven wear. The inner carrier bearing races should not spin on the carrier journals. The carrier races should have a snug fit in the housing. Inspect the carrier race bores for grooves from spinning races. Carriers and housings where the races have spun and damaged the mounting surface can be machined to accept slightly larger races and bearings.
The side gear bores inside the carrier should not have any abnormal wear. All gear teeth should be smooth but not excessively shiny. Inspect gear teeth for pitting, chips, breaks, and signs of uneven wear or overheating. Inspect for worn axle splines. All questionable parts should be replaced. Proceed by thoroughly cleaning all new and used parts with solvent cleaner or a parts washer.
WPC Surface Treatment
In order to minimize friction and maintain clutch plate lifespan, every component of the LSD including the ring and pinion was sent to WPC Treatment prior to assembly.
What is WPC, you ask? A metal surface treatment designed to improve the material’s surface; the end goal is to reduce friction and strengthen parts. The WPC treatment is executed by firing ultrafine particles at the part’s surface at very high speeds. The media, when projected at high velocity toward the product surface, creates compressive stress (the compressive load per unit area of an original cross-section) at the impact points. At the same time, a micro-thermal reaction takes place.
It effectively seals minor surface fractures, allowing a condensed surface to form, which improves surface density due to the material compaction. This altered and highly compact surface condition overcomes the problem of brittleness that is usually encountered when metals are hardened. Treatable metals include steel, aluminum, titanium, copper, and brass as well as soft-surfaced bearings. WPC can basically treat any metallic item in an unassembled state, from rotary apex seals to rear differentials.
Left: WPC Treated Right: Non-treated
Many of the top motorsports manufacturers – such as HKS, Suzuki Sports, TRD, and Tomei Powered – rely on WPC to treat their high-performance products. The WPC process uses a special ceramic bead composition ranging from 20 microns or larger depending on the application and area being treated.
Shot-peening can’t duplicate the results of the process. In comparison to shot-peening, the media being used for the WPC process is 50 times smaller and is applied at a higher velocity, pressure, and impact. The numerous impacts during the WPC process not only increase the residual compressive stress but also cause it to reside a lot closer to the surface. The result is a product that looks dull in appearance (as shown in the image to the right), but has durability unrivaled by conventional peening.
LSD Clutch Pack Assembly
The owner of the vehicle requested to set the 1.5-way Cusco LSD for this article to 80 percent lockup, since his AE86 delivered close to factory horsepower levels and is destined to drive on the street from time to time.
From the Cusco factory, all LSDs are set at 100 percent lock-up, however dropping it to 80 percent lockup (20 percent slip) can be easily achieved by setting the clutch plate combinations and their order. Traditionally, these clutch plates are made to grip when the outer tab and inner tab clutch plates are combined. Conversely, if the outer tab and outer tab plates (or inner tab and inner tab) are put together, they slip. The inner cross shaft and how the pressure rings are cut into the diamond shape is what regulates the LSD direction. Notice that there are two different shapes of the diamond shaped lobe (cam lobe). This particular unit is set at 1.5 way. The unoccupied lobe is the 2 way profile.
The 1.5-way profile of the cam lobe is at 55 degrees when accelerating and 20 degrees when coasting. The 2-way version features 55 degrees on acceleration and 55 degrees on deceleration. The cross shaft and its lobe must face a certain way in relation to its driveline in order to work properly.
Setting pinion bearing preload and ring and pinion backlash.
The Weir Performance 4.778 ratio ring and pinion requires special installation procedures compared to a factory Toyota unit. If the ring gear will not fit flush against the seat of the LSD casing, it must be heated to 100 degrees Celsius (212 degrees F) in an oil bath. Much like flywheel bolts, new ring gear bolts are always recommended to be installed with Loctite. Torque down these bolts uniformly to the specified rating listed in the factory service manual.
Setting the depth of the pinion shaft.
The factory Toyota pinion shaft sleeve is a crush type, meaning that the pinion bearing preload rate is adjusted by incrementally crushing this sleeve. This factory unit should be measured and used as reference. The Weir Performance solid pinion spacer is robust in comparison to the factory unit; shims of varying thickness were used to adjust the bearing preload and pinion shaft depth.
Setting the ring gear backlash.
Loosely reinstall the bearing caps along with the side adjusting nut. These side adjusting nuts were marked left and right and its position during disassembly. By turning these threaded side nuts, the LSD assembly basically moves left or right. By using a special service tool (SST) and a dial indicator, we adjusted the backlash until it reached the specified number listed in the factory service manual.
Inspect the tooth contact between the ring and pinion gear by coating the ring gear with gear marking compound. Rotate the ring gear past the pinion gear three or four times and then back so the pattern can be seen. The proper contact pattern can be found in the factory service manual. Setting gear backlash is something that is not beyond the shade tree mechanic’s ability – if and only if they are attentive to details and methodical during the installation process. Improper backlash settings can cause binding of the gears or complete failure.
When the final assembled product has been calibrated and torqued properly, it will yield great results. You can expect many years and many miles of dependable use from your differential if it is set up properly.