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...

Alignments, Rear-End Work, More Driving Impressions

Last time, we had just finished installing the front end. The thing is, parts of the suspension are adjustable to make sure the wheels point in the right direction, and to have them sit at the right angle with respect to the road. To do this, I needed a specialized alignment shopt to do this - this could not be done at home.

In addition the rear end needed similar repairs as the front end: replace bushings, springs, dampers, clean things up, and check bearings. In addition, since the rear wheels are the ones driven by the engine, there is a differential back there to deal with. I figured I'd have the differential inspected and serviced as needed.

Before these repairs, the car seemed to float laterally over the actual suspension and wheels while turning and during transition. So rather than "pointing" in a fixed manner during a turn, the car pointed left and right of your intended target during turns. This made for scary-interesting driving, since every time you steer the car, you needed to wait to see what the rear end did, then correct your steering inputs, then see how these corrections affected the car, and so on.

Bushings are one of those things that tend to wear out over time on solid axle, trailing arm suspensions such as the ones found on the GTV. Replacing bushings and tightening up things would help minimize the lateral play on the suspension, making for crisper steering inputs with minimal need for mid-turn corrections and the such.

Given all these repairs, I figured I'd take the car to Group 2 and have them do all this work for me. They are my favorite car shop, and they happen to specialize in all things Alfa Romeo. The problem is that in mid-summer, every other AR owner in the region decides to also drive their cars (just like me.) Which requires lots of maintenance work, making for very busy times at Group 2.

So, I figured I'd just have the front end aligned, and do part of the rear end work: bushings, springs, dampers. I would wait until later in the year to do the differential work, as well as any bearing replacements and the such. That way, I could get the car back on the road and start chasing any of the numerous gremlins lurking in the car. And besides, it helps spread the cost over a longer timeframe, which is always a good thing.

Unlike the front end, the replacement rear end bushings were the polyurethane types from Centerline Alfa. The shop replaced the trunion arm-to-differential bushings, as well as the bushings in the trailing/radius arms, and the bushings holding the rear sway bar in place. Polyurethane ("poly", for short) tends to be stiffer than rubber, and hence, helps keeps uncontrolled supsension motions to a minimum, but at the possible cost of a bit harsher ride. However, this upgrade is a very popular one in the Alfa community, and worth trying out.

The shop also installed the AR Ricambi springs I had purchased earlier in the year (which matched the already installled front springs), as well as Koni Sport dampers (again, matching the front end Konis I had installed already.)

The service was very much worth it. The car now rides in a much truer fashion. During turns, the car feels predictable and easy to point within the turn. There was still a bit of tail-end lateral slide, which can be tuned down by using these poly washers where the trunion arm connects to the chassis.

Regarding the rear springs, the car rides a bit higher in the rear than I would like (about half an inch.) The ride is not harsh, however, so from a spring rate perspective, I am very happy with them. Correcting the ride height involved cutting down springs, so I asked the shop not to do this for now, and see how the car rode before going drastic on the springs (one thing at a time...)
At first, the adjustable shock absorbers were set a bit too stiff (which was my bad - I had set them myself), which made the ride jarring and shakey - even over smaller bumps. After softening the fronts to almost full-soft, the car's ride became much less jarring while still keeping the car motions under control.

(I will be posting my spring rates, shock absorber settings, and other technical numbers in a future post.)

One thing that started worrying me were the brakes; they felt soft and spongy, with what seemed to me as too much pedal travel. This kept me from driving as I normally do - in traffic, I had to leave extra space between cars, brake earlier, and the such. This is something that needed to be addressed quickly.

But even with the soft brakes, I had a bit more confidence in opening up the car and seeing how it ran. And run it did - the car accelerated quickly and pulled strongly (for a 2.0 liter four-banger, of course!) Throttle response was quick and sharp. Steering inputs were much sharper and predictable. The transmission was loud, and shifting required very long throws and patient double clutching during downshifts (especially 3rd-to-2nd downshifts.) The clutch felt nice, though, engaging smoothly, linearly, and strongly. The sounds emanating from the engine compartment were nice - lots of fan noise at slow speeds, changing to loud engine intake noise, manifold growls, and a shade of valve clatter. Very good sounds indeed.

Highway driving was loud - lots of wind noise, road noise, engine noise, car noise. All this noise could be confused with music for Alfa addicts such as myself, however, and I was happy to put up with it during my short test drives.

Overall, the car drove very nicely, for a 34 year-old car.

I still needed to service the rear differential, and address the soft brakes. And I still need to chase all of them gremlins lurking out there...

Finally, Driving Impressions

So, after filling up the car with gas, I finally got to drive the car.

It must be noted that so far, only the front suspension had been rebuilt, and further, it is completely out of alignment. So although I was hoping for an improved ride quality, I wasn't expecting much in the way of handling and ride. And sure enough, the car seemed very twitchy. Bad alignments do these kinds of things to cars.

Further, the rear end felt sloppy and very soft. Again, I had not worked on that side of the car, so the rear springs were still the very soft stock units, dampened by very tired and old shock absorbers.

But overall, the car felt better. The front end seemed much tighter, with less slop and play. And the spring rates were tighter but not jarring. The dampers seemed a bit stiff, due to the fact that I started with a bit stiffer adjustment than needed.

In addition, the coolant leaks were gone - woo hoo! At least that part of the repair was done with, and I would not have to worry about that.

The newly-installed fan shroud made the car ride a bit quieter. The fan noise does not drown the various other sounds emanating from the engine compartment, and so one can hear the intake noise, as well as the valve train softly clanging away. Very good sounds, if you ask me.

But even with the slightly quieter engine bay, the car was still very noisy. Wind noise at 60mph drowned out any attempts at a conversation at a normal volume. Rolling the window up or down did not make much of a difference, so it was better to roll it down a few inches to get some ventilation inside the car. The car transmited every imperfection to the driver, which considering the current state of suspension tune, proved a bit tricky and surprising at times.

Mashing on the gas pedal made the car swerve off line, probably due to lack of alignment on one or both ends. But it did accelerate willingly and strongly - the car loved riding at or above 70mph, although at this speed, the car needed constant steering inputs to maintain a fairly straight path.

Even with all this stuff going on, the car hinted some better potential in ride and handling quality. Bumps were easily absorbed, without upsetting the car too much. And the car communicated the road to the driver - the feedback on the steering wheel, the controlled dive during medium brake application, and the engine noise when opening up the throttle.

I really could not wait to get the car sorted out.

So, after all this, what comes next:
- align the front end.
- rebuild the rear suspension: this involves replacing the rear springs, shocks, bushings, and maybe painting the various suspension links and the such.
- rebuild the rear diff: the thing is loud, and wide open. It also leaks a bit of oil. Rebuilding it will make the car work better off the corners, quiet it down a bit, and keep fluids from leaking all over my garage.
- Then, the details: wiring, interiors, etc.

It did feel nice to put some closure onto this part of the job. From now on, things should march along nicely.


Next up: Alignment and rear end work. More driving impressions.