September 2021 - Part 2On to the limited slip differential (LSD). As shown in May, I bought an upgrade/overhaul kit from Racing Diffs in Serbia, designed as a direct swap-out. The LSD in transaxles is a mechanical clutch-type.
The internal clutch discs naturally wear out, so from all reports any LSD of this age would probably offer no actual function unless previously overhauled. I had no idea of the status of my diff, and no ready means to make measurements, so presumed it was all original and worn. The factory setup utilises two clutch discs, offering around '25% lockup'. A common upgrade is changing to four clutch discs, providing around 50% lockup.
The Racing Diffs kit not only provides four clutch discs, but adds preload shims that create constant pressure to keep the discs ‘active'. I've read pros and cons for this; some people are against having any preload because it causes faster wear and generates more heat. But even though the discs might wear faster, their function is effective for a longer lifespan because everything is kept firmly compressed. Whether due to the materials used, the disc design (in particular the addition of oil holes), or the preload shims, Racing Diffs claim longevity of twice normal. The factory setup provides no preload
Traction improves with an LSD, but also promotes oversteer in wet conditions, and understeer in the dry. Increasing the lockup percentage would only exaggerate this. I'm not really qualified to determine exactly what suits anyone best, but I interpret having good LSD function as a 'performance upgrade'.
Does anyone here have the Racing Diffs setup? I'm interested to hear feedback about any good and bad aspects. I'm also interested to hear about people who have upgraded to four 'standard' clutch discs, and particularly how they've found it on the street, in wet and dry.
Time to crack open this diff!
The workshop manual recommended marking the diff casing and cover, to ensure they were refitted in the same orientation. I used a 'punch'. The diff was secured in a vice to hold it still and remove the bolts on the cover. It's not recommended to remove the large ring gear or bearings unless strictly necessary, because minor changes can upset alignment with the pinion gear, and backlash and preload between diff unit and gearbox casing.
Cover off, and first half of the LSD internals removed, comprising the clutch disc and plates, thrust ring and crown gear.
Inside the cover was a tabbed 'shim ring', featuring a 'wrought' surface. A similar shim ring existed on the other side of the diff. The ends of the crown gears spun against these.
Then, three 'external blade plates' and a single 'internal clutch disc'. The bladed plates locked externally into the surrounding diff casing; they thus rotated with the casing, but were free to move side-to-side when the parts were compressed during lockup. The clutch disc had internal splines to lock into the axles, but could also move sideways.
The blade plates had smooth surfaces, and the clutch disc had a rough friction surface. Without a new disc for comparison, I didn't know how worn it was.
Everything removed from the casing.
The other side had a similar setup of three blade plates and single clutch disc, a tabbed shim ring, but also a spacer ring (smooth, no blades or splines).
When everything is assembled, the factory specification suggests clearance behind the cover (ie: a gap, without preload) of 0.1-0.2mm. This can be achieved by using blade plates of different thicknesses. Each are about 2mm thick, but are available in three thicknesses: 1.90 / 2.00 / 2.10mm.
The central 'spider' comprised four small side-pinion gears and their interlocking spider shafts.
On either side of the spider were crown gears: the splines secured the clutch discs. The thrust rings (only one shown here) fitted over each crown gear, holding everything in position, and had protruding external 'blades' to lock into the diff casing like the blade plates.
Note the V-shaped cut-outs for the spider shafts. These particular factory cut-outs were at 45 degrees (green lines).
Here, I held the pieces together with clamps to show the assembly in its resting state. The spider shaft is centrally located within the 'ramps' on each thrust ring. Thus, the thrust rings are positioned closely together (red arrows), and exert no force on the clutch discs that normally sit either side.
During hard acceleration, the thrust rings push against the spider shafts and are forced apart (red arrows); the same scenario occurs if one wheel starts to slip. The amount the rings spread apart is exaggerated in my photo, but they compress the clutch discs and blade plates together, and friction locks the components to limit slip.
With the factory setup, up to 25% of the torque feeding to the 'fast spinning' (ie: slipping) wheel is transferred to the other (gripping) wheel. In a modified LSD with around 50% lockup, up to 50% torque would feed to the gripping wheel. Without an LSD (ie: 0% lockup or 'open' diff), around 0% of torque is transferred to the gripping wheel, which means all the torque goes to the slipping wheel and the car has no grip, and no movement.
The 'ramp angle' significantly affects performance of the LSD. At one extreme, a 90 degree cut would create a square box, and the spider shafts would have no effect on pushing the thrust rings apart. A shallow cut, like 30 degrees, would be much easier for the spider shafts to slide against, making for a more immediate and forceful effect. 45 degrees - like the Alfa Romeo thrust rings - operates somewhere inbetween.
The Alfa LSD also works equally in both directions - during acceleration and lift-off/deceleration - for a '2-way' action. Racing Diffs also manufacture aftermarket thrust rings with different ramp angles cut into each ring to choose from, for either more aggressive or less aggressive action, but I decided against fitting these at this stage. The choice of angles are 30/30 degrees (aggressive, 2-way), 30/60 degrees (aggressive in one direction, less aggressive the other, '1.5-way'), or 50/65 degrees (less aggressive each direction, 1.5-way). A combination of differing angles for each direction creates a 1.5-way LSD.
If I find the new upgrade too aggressive, I could try the Racing Diffs rings with the 50/65 setting: the combination of less aggressive ramps with extra clutch discs might be a nice compromise that still offers more grip than the factory setup (I'll have to research how to calculate the percentage lockup).
The diff casing naturally had four 'splines' (channels) to secure the bladed plates and thrust rings. This prevented the parts rotating against each other, but could still slide axially along the channels, for lock-up and release.
Showing the new Racing Diffs parts alongside the old originals. The new parts replaced only the blade plates and clutch discs, meaning the original spacer was retained. Note the old setup comprised four parts each side, each about 2mm thick, for a total stack thickness of 8mm. The new setup had five parts, and not surprisingly each was 1.6mm thick, creating the same total stack thickness.
The new clutch discs featured holes, to capture oil and facilitate lubrication to reduce heat and wear. There were two discs for each side instead of the old single disc design.
The new preload shims were conical shaped, like a Belleville spring washer.
Reassembly with the new parts, each liberally coated with Redline oil. The second clutch disc on view in the photo will rotate directly against the thrust ring, but from all reports this does not cause concern for damaging wear.
One thrust ring, crown gear, and half the spider fitted.
This was the new setup: crown gear inside the thrust ring, and new stack. The splined clutch discs locked over the crown gear, and rotated together. They were free to rotate independently of the thrust ring and blade plates (red arrows).
All internal components now fitted into the diff casing, with preload shim on top.
Presence of the preload shims created a total internal component thickness about 4mm greater than the original. The photo shows the cover resting in position, and the gap to overcome to clamp everything together.
I used the hydraulic press to hold the cover closed whilst torquing the bolts, and completing the upgraded LSD.
