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  Project NSX: Part 2 - Tuning a Limited Slip Differential; Tricks of the Pros

By Mike Kojima and Billy Johnson

In part one of Project NSX we went through the transmission of our NSX to address some of its weak points - following the guidelines in our "Building the Bulletproof Transaxle" article. Before putting our transaxle together we installed and tuned an OS Giken Super Lock limited slip differential to replace the factory Honda unit.

Differentials are used in a car so that the driven wheels can rotate at different speeds through a turn. This is important because while in a turn, the outside wheels have a longer path to travel (due to a larger radius) and make more revolutions than the inside wheel. A differential allows this to happen without resistance using engine driven orbiting spider gears that are in mesh with bevel gears which are attached to the axles.

In vehicles without a differential such as a kart, the axle is solid and locked (aka: spool) with both driving wheels forced to rotate at exactly the same speed, pushing the car straight. By nature a differential wants to send all of the engine's power to the wheels with the least amount of traction; the path of least resistance. The side effect of this is unwanted inside wheel-spin out of corners, and even single wheel spin on straight-aways (aka: peg-legging, one-tire-fire, etc…) which greatly affects acceleration since the power is not able to be put to the ground efficiently and is lost through the inside unloaded tire.

When accelerating out of a corner, the inside tire represents less than half of the available grip compared to the outside loaded tire. Under aggressive driving conditions, this wheel spin also makes a car difficult to steer with the throttle because the inside tire will spin and the car will not rotate under acceleration, or slide/drift. Because of the shortcomings of conventional ‘open’ differentials, they are not desirable for either fast lap times or ability to rotate and throttle-steer cars.

For a really good, easy to understand video to explain this check this out!

Stock ring gear left and OS Giken right show the interior splines.

 A limited slip differential fixes this problem by resisting the differentials tendency to deliver all of the power to the free spinning wheel. It accomplishes this by reducing the difference in speed between the two driven wheels and thus delivering the engines torque to both wheels. 1991-1994 NSX’s like our project NSX have conventional clutch-type limited slips. These limited slips use driven and drive clutch plates preloaded with a cone-spring (Bellville Washer) to provide a resistance to the bevel gears spinning against each other at different speeds. This resistance helps keep the driven wheels spinning at closer speeds to each other rather than the big discrepancies between the two wheels like in an open differential. The amount of limited slip resistance is controlled solely by the preload of the cone-spring which in the case of the stock NSX is pretty light. This is fine for the output of the stock engine with soft stock suspension and small tires but as you increase the power and grip, the locking ability is not strong enough and inside wheel spin rears its ugly head again. While the stock LSD unit can be preloaded to be ‘tighter’ and more aggressive, it still has shortcomings in locking ability, design, and tune-ability. For our turbocharged engine, and planned bigger and stickier tires, this was not going to cut it.

The main parts of the OS Giken Super Lock diff are shown here minus the clutch plates.  The pressure ring and the spider gears are driven by the engine torque through tangs that engage the diff case that the ring gear is bolted to.  Since the NSX uses an unusual ring gear with spline on the ID, you have to use an OS Giken ring gear with this diff.  The spider gears drive the bevel gears which are coupled to the axles via splines on their ID. The thin skinny gear is the speedo drive gear.

The OS Giken "Super Lock" LSD is a more advanced design of a limited slip called a Salisbury differential. The Salisbury diff is one of the most common designs in racing cars and uses cone-spring preloaded clutches to provide the limited slip action just like a conventional limited slip but with a twist. The cross shaft of the spider gears is actually a cam that rides in a hole between two half’s of a split carrier that is driven by the ring gear of the differential.

As the ring gear applies torque to the differential case and the carrier, the cross shaft cam which is coupled to the wheels is wedged into the carrier’s "ramps", applying force that wants to split the two halves apart. This force squeezes the differential’s clutch packs. As more torque applied to the differential, the more the clutch plates lock together and prevent differences in wheel speed.

The parts that define a Salisbury differential, the pressure ring which contains the spider gears on the cross shafts and the bevel gears which the spider gears drive that attach to the axles.
The knee bone is connected to the... The pressure ring, spider gears and bevel gears fit inside the differential case.  The slots inside the diff case drive the pressure ring through tangs on its OD which then drive the spider gears and the bevel gears that are attached to the axles.  The ring gear bolts to the case and is driven by the pinion gear which is attached to the drive shaft.  The drive clutch discs are also driven by tangs that fit into the slots in the case.  The driven clutch discs couple to the bevel gears through teeth the couple to the teeth on the ID of the bevel gears.
This image shows how all of the various parts are oriented in the differential case, the pressure ring and spider gears are in the middle, the bevel gears are on either side and the  clutch packs are also on either side of the pressure ring.

 

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Comments
Drew
Drewlink
Friday, September 11, 2009 5:44 AM
Amazing article Mike!
BenFenner
BenFennerlink
Friday, September 11, 2009 10:30 AM
I learned a bunch!
JDMized
JDMizedlink
Friday, September 11, 2009 3:56 PM
A+.
I already knew this stuff, but it's great that someone in the business finally shed some light on one of the most important part of a car.
Keep the tech articles coming!
M-P
M-Plink
Sunday, September 13, 2009 5:57 PM
Great article...picked up a few new bits of knowledge along the way. I am very happy to hear about the WPC treatment that gets rid of the low speed chatter. The Cusco RS (80% lock) that I had in my 350Z worked great, but was hell at parking lot speeds. I tried tons of diff oils and the only one that seemed to make it go away at all was Motul 90PA, but didn't eliminate it completely. I'm going to have to give WPC a shot next time.
Mike Kojima
Mike Kojimalink
Sunday, September 13, 2009 6:21 PM
The Cusco RS has 12 clutches per side like the OS Giken. It also has negative pressure ring springs like the OS as well. Unless you use the diff for drifting, I would suggest switching to the stiffest pressure ring springs, WPC treating the clutches and deactivating one or two sets of clutches per side, while keeping the cone spring that came with the diff.

With our time attack car experience we have gained 1.5 seconds a lap in a turbo 350Z by tuning like this.

I am going to experiment with increasing pressure ring spring tension in drift cars as well to help with forward traction which is getting to be problematic as cars get more power and drift speeds get higher.
Mike Kojima
Mike Kojimalink
Thursday, September 24, 2009 10:50 AM
Added a really simple viedo to show how a diff works!
Profusion
Profusionlink
Wednesday, January 06, 2010 6:43 PM
Now do one on a clutch type? (Article)
BillyJ
BillyJlink
Monday, March 01, 2010 5:08 PM
revengemotorsports - a Salisbury differential IS a "clutch-type" limited slip differential.

The article is 4 pages of in depth background, function, and ways to tune a salisbury/clutch-type LSD.


Billy
mikoto
mikotolink
Wednesday, October 13, 2010 10:03 PM
nice. good info. great!
EB Turbo
EB Turbolink
Thursday, March 24, 2011 5:44 PM
The cusco type MZ(don't hold me to it) has options in the spider case exchange between 1.5 and 2 way. Giving you more options with less parts. I have taken a used axle parted off the spines and welded a 19mm nut to the end. I put another used axle in the vise with both axle pieces and a digital torque wrench I can verify the proper Broadway torque i am looking for.
eimarshall
eimarshalllink
Friday, September 30, 2011 5:27 PM
did you have the entire LSD WPC treated? or just the plates? I'd imagine you really just need the plates treated because they are subject to the most friction but i can see how it would help with the rest of the gears and such.
BillyJ
BillyJlink
Saturday, March 10, 2012 8:44 AM
EB Turbo: Most Cusco (RS and MZ) have pressure rings with two ramp angle combinations which is a very unique and cool design characteristic. The OS Giken has more tuning options that can fine-tune the handling of the car, but for big changes without buying more parts, the Cusco fits that bill.

eimarshall: The spider gears, plates, conical preload springs, ring and pinion gears were all treated. At a minimum, the ring and pinion gears and clutch plates should be treated.

-Billy
kyoo
kyoolink
Sunday, February 03, 2013 5:46 PM
awesome article.. you guys should run one of these in the project ix and see how it does compared to the X's AYC!
Anonymous User
Anonymous Userlink
Saturday, July 02, 2016 3:54 PM
Tarik Laaraj
Tarik Laarajlink
Friday, July 07, 2017 9:39 AM
does OS Giken's LSD provide torque when one wheel is off the ground?
Mike Kojima
Mike Kojimalink
Friday, July 07, 2017 3:43 PM
yes a lot
Tarik Laaraj
Tarik Laarajlink
Monday, July 10, 2017 7:15 AM
Thank you Mike.
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