Rebuilt Diff

So, after driving the car around town for a while, I figured it was time to finish the rear end work.

Previously, I had replaced some of the bushings in the rear end with polyurethane bushings. However, I did not do any work on the differential. Also, the bushings in the rear sway arm end links had not been replaced. These bushings were in bad shape - they were somewhat compressed and hardened, and provided some free-play to the sway bars. This is not good - you enter a turn, and the car leans some before the sway arm does anything to prevent it.

Again, off to Group 2 I went. Group 2 has a technician (Noah? I think - I must confess, I forget some of their names) who's an expert at rebuilding Alfa diffs (among other things.) During my visit to the shop, he even showed me an extra diff they had at the shop, opened it up, and showed me exactly what he would do on my car's differential. Very cool.

The GTV diff is a limited-slip type differential (LSD for short), although it looks very much like a conventional open-type differential. However, the diff has friction plates on either end of the internal gears. When one axle moves more than the other side, plates press against the friction plates, preventing (to some extent) the differential motion between both end. Shims can be used to preload the friction plates, and to adjust how much friction the plates provide (from almost none, to almost locked.)

Why are LSDs a good thing? Open diffs will send torque to the wheel that moves the most. This helps when the car turns well within the limits of cohesion. For example, as you turn (say, to the left), the outer driven wheel (the right wheel, in this case) would move a bit faster than the inside wheel. This makes the diff send a bit more torque to the outer wheel, which helps the car turn in the direction it is turning already (left.)

For most typical situations, this is a good thing.

However, when one of the driven wheels looses traction (say, ice on one side, dry on the other), it will spin faster than the wheel on the other side. The differential then transmits more torque to the spinning wheel - the one with no traction. If this occurs in an icy road (ice under one wheel, dry pavement under the other), most of the torque is sent ot the wheel with no traction - the one over ice. The result: one wheel spins, the other does nothing, and you go nowhere.

By providing friction between both sides, the LSD ends up "connecting" both wheels (to some extent.) When one wheel spins, the friction between both axles will force the other (non-spining) wheel to also spin. In essence, the LSD will help even out the torque sent to both wheels, allowing some torque to be sent to the higher-traction (slower moving) wheel even as the other side spins.

Marisa Tomei explains this very well in the movie "My Cousin Vinny". It's a good movie, but if you are impatient, fast-forward all the way towards the end of the film to listen to her much-better explanation.


So, back to the GTV, the guys at Group2 replaced the friction plates (small little disks, made of very exotic materials, machined to perfection, and hence, somewhat expensive.) They also cleaned things up in there, replaced one of the rear bearings, and put it all together. They also finished all the bushings work I had pending.

The end result: the car rides much quieter now. Before the service, the rear end was very noisy. I suspect replacing the bearings and the worn-out clutch plates made things much quieter. The new bushings tightened the rear end much more. So now the car rides much better - the tail does not feel sloppy, and remains truer, with very little side-to-side motion. This helps with the car's steering - you point the car where you want it to go, and since the tail is tighter, the car follows your inputs better.

Admittedly, I've not pushed the car too much. As I get comfortable with it, I might start pushing the car more around some corners to see how it behaves (understeer vs. oversteer, predictability vs. twitchiness, ease of corrections, etc.)

At the end of the day, the car feels much better - especially when compared with how it originally ran when I first bought it, one year ago.

Next: Heating and cooling issues...