Tech Talk: OS Giken Limited-Slip Differentials
When you need predictable control, you need a limited-slip differential.
The OS Giken LSD is one of the most prevalent brands of limited slip differentials in racing today. They don’t just make a LSD because a car came out, it’s because a race team requested it. So, what makes the OS Giken unit so desirable that even US domestic race cars demand the Japanese brand? We take a closer look in this tech article.
What makes an OS Giken LSD different from a traditional LSD unit is that it’s not a Posi-unit. An OS Giken unit keeps its clutches disengaged until there is a demand for it. Jonathan Lugod, Product Manager for OS Giken USA, explained it this way, “Our units don’t lock up until there is a demand from the driver’s input. This not only improves predictability but also the longevity of our units, especially since we don’t require the use of a friction modifier.” That’s especially good news for transverse transmission users whose gearbox oil is shared by the differential unit.
You’ll also notice that the spider gears are far smaller than most LSD units, too. This means that there is more surface area on the clutches for better lockup capabilities. You’ll also see grooves on the clutches, which are there to allow for oil to reach between them and aid in keeping them separated until demand. Finally, you’ll see that on both the gears and the clutches are several grooves rather than four or six bumps. This creates a stronger connection, much like the splines on an axle: the more splines you have, the more the load is spread and the more power and torque can be applied.
An OS Giken unit does not allow for user servicing or tunability. If you want the lock up character to change or it needs service, you have to contact them and send the unit to them. “It’s sort of like shock tuning is done,” says Jonathan, “You have to know your product because of how our units work. We don’t have an external preload adjustment like most aftermarket units do.” The preloading is done two ways, a pair of sprung bevel washers on the outside of the clutch packs and a set of springs that reside between each spider gear pinion.
The springs actively push on the housing, keeping the clutches separated and disengaged until the spider pinions push on the channels of the housing to put force on the clutches. This means as you are off the gas, the LSD acts more like an open differential until you start to accelerate or decelerate, making it far more predictable when driving. You can request that the lock up action happens differently than how they normally set a unit up and this adjustment is done by those little springs.
Each spring has a certain rate, which will make the clutches lock up sooner or later depending on their combined rate. While you see in these pictures that there are three springs between each pinion on each housing, that doesn’t always mean that is always the case. Sometimes there are only two and the central one is removed. A special OS Giken specific tool holds on to the nuts and only allows it to be tightened so far.
By doing this, it allows the unit to stay perfectly balanced no matter how many springs or what rates are used. It’s also the number one problem that occurs when a user tries to service their own unit. However, they do feature markings to ensure that, when serviced, that they go back together as balanced. Also, each housing is not only balanced but they are fully forged units that are heat and chemically treated to increase longevity. That grey coloring is not a paint or powder coat, but is the chemical treatment that’s like gun bluing.
There is a no-so-secret secret to their one-way units: they aren’t true one-ways but “1.1-ways,” as Jonathan describes it, “there is a small ramp on the deceleration side of the unit. This makes braking more predictable as it keeps the wheels moving at the same rate under hard braking.” Then there are the different ramps on the two-way units. The steeper, more drastic ramp is made for quick lockup and is usually seen in drift cars while the shallower ramps feature slower and more precise lockup for road racers.
The units are designed to fit and work in a racing environment. They go so far as to include the bolts on B-series transmissions but the unit might not allow for the use of the original cable-driven speedometer. “These are racing LSDs,” says Jonathan, “While we guarantee fitment within the differential housing, they are racing parts. We don’t guarantee against wear, just fitment and for what works for a racing environment. If you’re looking for something for your street car, we have it and it will fit, but you might not be able to use your OE speedometer cable.”
The OS Giken LSD is trusted by racers the world over thanks to its long service life and predictable nature of the unit. The fact that it is not a posi-unit, rather it’s a reactive unit, is what makes it perfect for road racing and drifting applications. While street cars can use them, OS Giken units are racing parts and that’s where they are intended to work. However, if you’re willing to get around the speedometer issues on some cars, you’ll find that the unit works very well and is very quiet because of its reactive design.