Project NSX Part 2, Tuning a Limited Slip Differential

Here are the cone springs used by OS Giken for adjusting the initial preload on the clutches.  OS has three optional spring rates.  Most other reputable brands of LSD have different weights of cone springs that you can use for tuning as well.

The OS Giken diff uses a cone-spring that preloads the clutch plates like a conventional old fashion clutch-type limited slip for setting the initial breakaway force. This is the resistance to differential action with no additional load from the cross shaft cam. OS Giken has 3 different cone springs available; 1.4mm, 1.6mm and 1.8mm. The thicker the spring, the more preload is on the clutch stack. The stiffer the cone spring, the higher initial breakaway torque will be, the more the car will want to understeer off throttle or part-throttle. We chose the lightest spring for the least amount of preload which should allow the car to roll freely on entry to mid-corner until throttle is applied. You could say this will behave towards the direction of a 1-way LSD under deceleration. Most Japanese LSDs have several optional cone springs available for tuning initial breakaway.  The Cusco RS differential is unique in this aspect as it uses coil springs instead of the cone spring.  This is because a coil spring can have more preload travel and thus is less sensitive to disc wear changing the initial breakaway preload compared to a short stroke cone spring.  The Cusco diff can be tuned by changing the coil springs.

Billy cleans the threads on a wire wheel of each ring gear bolt, another step to help ensure proper torque.

A 1.5-way behaves somewhere in the middle of the two differentials. Its main advantage is a stability and grip under deceleration when compared to the 2-way. For street use and most road racing/grip driving situations, the 1.5 way diff is often the most popular for RWD applications. Surprisingly as the sport of drifting becomes more advanced and drift speeds get higher, more and more drifters prefer the feel of a 1.5 way differential due to the added grip during deceleration which allows for higher entry speeds in drifting.

Deactivating clutch discs is one of the most common tricks to tune a diff.  Just about all diffs come optimized for drifting or very powerful cars with very sticky and wide racing slicks with pretty solid lock up.  For grip driving it is common to deactivate some clutches to allow for some differential action on the exit of a corner.  This helps a grip car get every bit of forward traction possible.  In the top picture, Billy has placed two drive and two driven clutches together.  This takes four discs and effectively turns them into two.  This is how we are going to run Project NSX.  In the bottom picture, three drive and three driven discs have been placed together making 6 discs two.  This will soften the locking action even more if needed.  This trick will work on all clutch type LSDs.

Another trick is to deactivate clutches. Most Japanese LSDs have a very aggressive locking ability which locks the rear tires together like a spool under throttle. In most cases this will generate understeer until traction is lost. With aggressive throttle inputs it is great for drifting. Gas it, the diff locks and you go to oversteer. For grip driving, this isn’t so great because you are looking to maximize forward traction and exit speed. Some differential action is needed to maximize forward grip by allowing the rear tires to maximize their cornering and acceleration grip, which they need to travel at different but close to the same speeds. A common trick is to deactivate some clutches to reduce the total locking power of the diff. This is done by flipping the drive plates so they don’t have a driven plate in-between them, effectively taking them out of action.

Some diffs like OS Giken Super Lock and Kaaz have 12 clutches per side but most diffs have 8-10 sets of clutches on each side of the diff. Grip drivers in the know normally will deactivate anywhere from 1-3 clutches per side depending on the diff, track, and type of tires being used. Depending on the number of plates, deactivating each set of plates is often referred to as 80%, 60%, 40% lock and so on. Be aware that the more clutch plates are deactivated, the less the diff is able to lock.

For higher horsepower applications, you don’t want to deactivate as many plates as a low horsepower application. Since project NSX will eventually be turbocharged with high HP, we chose to deactivate 1 set of clutch plate per side. For the FXMD NSX Time Attack Racecar that has super wide slicks and even more power, we chose not to deactivate any plates to get the full locking ability of the LSD.

This tuning trick is applicable to the OS Giken diff only.  The OS Giken super diff has another facet of tuning, the pressure ring spring.  These springs counter the wedging action of the cross shaft cams by pushing the halfs of the pressure ring together.  The springs can effect the quickness of onset of the locking action. They are the small springs shown in the yellow slots of this cutaway diff.  This feature gives OS Giken diffs are tremendous amount of tuneabilty and smoothness.

OS Giken pressure ring tuning springs are available in three weights.  The stiffest springs give the smoothest locking action while the softest spring gives the most aggressive lock.  For project NSX we opted for the stiffest spring.