Mechanical LSD Differentials

The Basics

When turning, the outer wheels travel a wider arc than the inner wheels. There is therefore a difference in turning speed between the inner and outer wheels. The front wheels can spin independently and turn at different speeds. The rear wheels, however, are linked to the engine through the gearbox and tailshaft, with the job of transferring power from the engine and driving the car. The job of a differential is to transfer this power but allow the rear wheels (or front wheels in a FWD car) to spin at different rates. The word differential refers to this difference in rotation speed.

This most basic type of differential is known as an open differential.

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Image from Wikipedia

 

The Performance Problem

The issue comes when there is not enough traction to transfer power from the tyre to the road. This may occur from the road surface being wet or slippery, or from the engine being highly powered. In this scenario, the wheel with the least traction gets all of the power, and the other wheel with potentially more traction free wheels. If trying to drive up a slippery hill, the car will be struggling to move forward, with one wheel spinning. When trying to drive fast on a track, the car will also struggle to move forward, with excessive wheel spin. This is what is known as ‘single spinning’.

Locked Diffs

It is possible to weld the centre of the diff gears so that the left and right axles are ‘locked’ at the same speed. When turning, however, this gives very undesirable characteristics. Best case scenario is the car will understeer dramatically, as the wheels want to travel at the same speed which is only possible in a straight line. More likely is that the car has some degree of oversteer. At low speeds, this will lead to chirping and clunking from the chassis as the tyres skip across the road. Under power, the car will be very tail happy, breaking out the rear in lurid slides. Great for a burnout car and good for a drag car, but illegal and close to undriveable anywhere else.

Limited Slip Differentials

An LSD aims to provide the best of both worlds. It will act like an open diff most of the time, but when one wheel starts to spin, the diff will lock to some extent to transfer power to both wheels. Driving up that slippery hill, now the car has more of a chance because two wheels are delivering power, and on the track, forward acceleration is enhanced for the same reason. Two wheel drive instead of one wheel drive helps a great deal.

Many turbocharged sports cars, including the S13-15 series from Nissan, come with factory LSDs. Nissans generally have viscous LSDs, where a thick fluid inside the diff prevents the two sides from turning at vastly different speeds. Explaining the various types of LSDs is beyond the scope of this article, and plenty can be found online to explain them.

What is important is that the transition from open to locked in a factory LSD will be smoother and somewhat unpredictable. Giving the car throttle exiting a corner might have an unexpected transition to oversteer. If you’re trying to provoke it for drift, it can be hard to know when it will happen. Factory LSDs are also criticised for not completely locking.

Mechanical LSDs

Most serious track and drift cars have mechanical LSDs. The function of these are explained in the following video:

 

The essence is that on each side (let and right axles), some parts are directly connected to the driveshaft, and other parts directly connected to the centre gears. When these parts (which are clutch plates and metal discs) are pushed against each other, the driveshafts must rotate at the same speed and the diff is locked. Mechanical diffs are a lot more aggressive but ultimately more predictable in their locking characteristics. The driver can apply power and know when the rear will lock and provide oversteer. Mechanical LSDs also offer 100% locking and some can be tuned to change how aggressively the diff locks. They are perfect for circuit and drift.

The image below depicts an exploded view of a KAAZ mechanical diff. The alternating clutch plates and metal discs can be clearly seen (labelled as plates A & B). The important thing to note is that when pressure is applies outwards (from the diff centre towards the axles), the diff will lock.

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1 vs 1.5 vs 2 way

When shopping for a mechanical LSD, the above terms are frequently mentioned and refer to the lockup characteristics of the diff. A 1 way will only lock when accelerating and are commonly used in FWD cars. A 1.5 way diff will lock under acceleration, and potentially lock under deceleration, although not as aggressively. a 2 way diff will lock equally aggressively under both acceleration and deceleration.

What type of operation the diff has depends on the cam and ramp setup. Remember that when the pressure is applied outwards from the centre, the diff locks. This outward pressure comes from the cam pushing against the ramps.

Imagine two rectangular pieces of timber lying on the ground with the ends touching. If a round pipe was pushed against their join, it is very unlikely the pipe will be able to separate the timber blocks and push them apart.

Now cut the ends of the the timber blocks on an angle where they touch so that now form a V groove. Pushing the round pipe wedges in the V shape and forces the timber blocks apart. The steeper the angle of the V, the more easily the pipe will be pushed apart.

The right hand side section of the image below depicts 1, 1.5 and 2 way cam/ramp arrangements. Essentially, the ramps around the cam in a 1 way will form a triangle, a 2 way forms a equally proportioned diamond, and a 1.5 way a diamond with one pointy end and one stubby end.

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Identifying My 2nd Hand Diff

I managed to purchase a used mechanical diff from an S13 for $770 delivered from eBay. It was said to be a 1.5 way Cusco in the original 4.08 final drive ratio (seen as 49:12 on the ring gear). It came with the casing and stub axles pictured. When buying a new mechanical diff, you just buy the centre, which means installation includes disassembling the factory LSD to swap the centres. Purchasing one like I did would cut down greatly on installation time and complexity.

Although it’s hard to tell from the images, the cam and ramp arrangement seen in every other opening has a 1.5 way shape. That is, a diamond with one pointy side and one stubby side.

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Cam in the middle, pointy diamond ramps for acceleration (pictured to the left of the cam), and stubby diamond for deceleration (pictured to the right of the cam).

Determining the brand required further research. There is variation in the appearance of various brands depending on the exact model. I figured that if I could find an image that closely matched my diff, I could make a safe and educated identification. The images below show the three most probable brands for my diff.

Nismo (left) – Casing is cast with some parts machined smooth.  Openings alternate between round and rectangular with rounded edges.

Cusco (centre) – Fully smooth casing with every opening being an elongated oval.

KAAZ (right) – Fully smooth with every opening round. Cams/ramps and plates alternate in each opening. Match!

Lubrication

The diff came without oil, so therefore I could research and select whichever oil I liked. KAAZ websites warned not to use synthetic oil, with KAAZ offering their own transmission oil, for which the best price I could find was $70 a 2L tin. Considering the diff oil is recommended to be changed every 2nd engine oil change, with 1.8L needed in an S13/14, the long term expense is significant.

After much forum trawling, others were happy with Castrol LSX 90, which is around $45 for 4L and mineral based. A friction modifier can be added which makes the oil slipperier and therefore makes the clutches inside the diff engage less aggressively. The more friction modifier, the tamer the locking operation of the diff.

S13/14/15 R200 diffs have a drain hole and fill hole. When servicing the diff oil, you drive the car for a while to get it warm and runny, before taking out each bung so that the oil can drain. The drain hole is then plugged with fresh oil going into the fill hole until it starts to spill out. When the diff is in the car, there is no room for a bottle, so the oil must be pumped into the fill hole.

Outside of the car, I had the luxury of turning the diff on its side, putting in approximately 2L of oil with a funnel, then turning it back the right way to let the excess 0.2L run out. After the filling bung is back in, you should degrease and wipe down the diff casing, so you know that any fresh oil on the outside means a leak.

 

Installation

Swapping an S13 diff is fairly straightforward. All of the nuts and bolts are either 14mm or 17mm. A jack is required to lower and raise the diff from and to the subframe, which should be available once the car is on stands for safe under car operation. The process is as follows:

  1. Undo five 14mm drive shaft bolts and nuts each side. The handbrake needs to be when cracking them, then taken off to rotate the wheels to access the other side of the drive shaft.
  2. Undo four 14mm tailshaft bolts. Putting the car in either 1st or neutral will control whether the tail shaft can spin freely or hold still for cracking bolts.
  3. Undo four 17mm nuts from the rear diff housing that hold it to the subframe.
  4. Undo two 17mm bolts holding the front of the diff the chassis at the tailshaft end. At this stage you must have a jack under the diff to catch and lower it down safely.

I did the job without removing the rear sway bar or exhaust. I found the sway bar handy for helping support the diff where the jack alone might make it hard to balance. Either way some grunt will be needed to angle it out and down.

Installation of the new diff is exactly the same but in reverse. Once again, the trickiest part is angling the diff to align the rear studs before the two 17mm bolts can go in to secure it in place. Some strength and coordination required. Possible alone but easier with a helper.

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Comparing the new diff to the old pre-installation. 6 bolt drive shafts also exist so check for this before starting. The red sway bar can hold the front of the diff nicely to assist in getting it in and out.
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New on the left and old on the right. Note the variation in rear covers for the same R200 diff. The new housing has much less pronounced cooling fins. On the new diff, you can see the five 14mm bolts and nuts on each driveshaft (left and right), the four 14mm nuts and bolts on the propshaft attachment (top), the four studs for the 17mm subframe nuts (bottom), and the holes for the 17mm bolts (holes near the top, either side of pink texta).

 

Driving Impressions

Please note: a new mechanical LSD centre needs to be broken in. Refer to the manufacturer’s instructions. This usually involves driving in a figure eight pattern for at least 30 minutes and then changing the oil.

The grey 180 from 10 years ago had a 1.5 way LSD and was pretty drivable. It only skipped and clunked like a locker on tight, slow turns, and engaged fairly predictably on track. This diff is the opposite. It is stupidly aggressive and at this stage much closer to a fully locked diff. Under any meaningful throttle, it locks up and the rear inside wheel can be heard skipping on the road. I’ve yet to give a test under boost because I’ve grown old and timid, but it will be a drift machine if left like this. I believe it would be near undrivable in wet weather as it stands.

The solution is to add some friction modifier to the diff oil to try and tame it. Since the diff is now installed, I’ll have to crawl underneath and pump in the addictive to the fill hole. small update to come.

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