Mcola - I can get confused sometimes. Geoman started this thread, then it seems to have shifted to your similar problem. I guess it's easy to get lost when addressing two different problems. I make some assumptions based on one car's descriptions, then get mixed up when a different car gets involved.
Let me try a simple overview.
I did a quick check, and didn't see any mention of replacing the lower front ball joint. This is a key part in the steering as it carries all the load of the front suspension, and wears heavily; the upper ball joint can last the life of the car, the lower barely 50,000 miles. No easy test without removing the front springs, then it's usually obvious. And once you take enough apart to test it, it's cheap enough to just replace it.
The inner front bushings should be replaced (when needed) only with the OEM spherical joints, both in the lower A-arms AND in the single upper inner joint. Poly bushings in any of these have cause many Alfisti problems, especially on the inner upper arm. The only marginally good place for polys in the front end is the caster bushing (at the outer front of the upper arm), and in the sway bar body bushings. Sway bar end link bushings seem to be fine either OEM or poly.
In the rear, poly's have mixed reviews in the trailing arms. Generally no obvious benefits over the stock rubber bushings (which require a shop press to remove and replace); some folks have had serious problems with polys here (and some love them!). The T-bar center bushing seems to be a good place for poly, as well as the rear sway bar bracket bushings. Poly spacers are good in the outer ends of the T-bar.
Centerline's yellow springs are pretty stiff for a street car. IAP's yellows are a little softer, but still mostly for a track car. The car will tend to dance and jump across most road bumps. Even on a race car, the yellows are often NOT used on the rear. Stock springs (with adjustments for ride height) seem to be the best choice. I'm lost on which of you has which springs.
Stock front sway bar with stiff front springs is a bad combination.
Yellow springs in the rear can cause some of the problems you describe. You want everything in the rear as soft as possible, as the car is very light in the tail.
For shocks, the most popular set up is Koni Reds, set to medium stiff front and full soft rear. FOr a stiffer car with yellow springs and a BIG front sway bar, Koni Yellows do well in front. Keep the soft reds in the rear. Gas struts all make the springs stiffer, and do not adjust soft enough in the rear nor hard enough in the front for non-stock springs.
RJ's point is really important if you have the super low yellow springs. The lowered front end changes the steering geometry badly. You get what is called "bump-steer" where the steering angle changes as a front wheel goes over a bump. Racers that hit the raised curb at the apex of a hard turn get major adrenaline bursts, and occasional off-track excursions at speed from this. The solution to this is to modify the steering geometry to match the lowered car. This can be done with a nice kit made by Max at Alfaholics in the UK, or with dropped (bent) steering arms that RJ and others can do. RJ makes one of the best solutions by modifying and re-welding the entire upright spindle to relocate the joints ("dropped spindles").
Then there are variations in the limited slip differential ( zero on early 1600's, 20% on stock 2L cars, 40%+ on modified diffs.
Finally, there are all the alignment issues, wheel and tire sizes, and tire pressures. Low profile, wide tires on too large rims almost never improve anything but parking lot looks.
You should be aware that these cars vary a lot in weight, so any general advice is iffy. My early Duetto with lots of stuff left out for lightness is under 1900 lbs, while a late S4 can be well over 2700 lbs. That's FOUR+ passengers extra! I've raced GTA's that were under 850 KG.
Unlike simpler cars such as MG's and most small Japanese cars, the ALfa stock suspension is an extremely well balanced set up. More often changes from stock upset rather than improve the car's feel and handling. Even for a dedicated track car, changes that make it faster often make it harder to drive. Every issue I mentioned above matters, and interacts with all the others. Get a ride in a 1960's 1600 Gulietta and you will see how absolutely exquisite the stock suspension design is, even on stock skinny tall tires.
Now to address your (two) driving problems: I will summarize: uncertain and unpredictable, skittish steering in normal driving at highway speeds on highway curves that seems to be tail happy.
None of this is oversteer or understeer. Those terms refer to the front vs rear balance of tire slip angles as the tires approach their traction limits, and the car starts to slide at the front (understeer) or rear (oversteer).
You are having skittish steering at normal traction. That's either looseness and sloppy steering (including bump steering) at the front, or poor tire traction at the rear, including tire and axle hop and similar issues.
If the issue is in the front end, some of the causes are mechanical problems, and can be dangerous as failures can be catastrophic. For safety, you should (and may already have) check all the moving parts of the front suspension. After all, these cars are two to four decades old. Everything is worn. Check and replace every moving part. carefully check the steering box for play with all the links disconnected. There should be essentially NO play that you can feel. With the wheels off the ground, the tires should turn the instant the wheel is moved, with absolutely NO center slack or sloppiness. Inspect the aluminum box for cracks and the body for sheet metal cracks and tears - both of those are life-threatening issues. Check every ball joint for ANY play and replace anything you can feel.
[BTW - No mention has been made of wheel bearings - check those immediately!]
There is so little weight in the rear of the car that it's easy to make the tires hop and momentarily loose traction. All the stiffness things you do add to that problem.
In the rear, again check all the joints for tightness. The trailing arm bushings can dissolve or tear; ploys often stretch, tear, and distort if not correctly lubed on installation, and can do so in a day or week or year anyway. The upper T-bar has a big cone bushing at the center - this is the best place for poly on the entire car. The OUTER ends of the T-bar have thin (ca 5mm) spacers on each side that MUST be in good shape - otherwise the T-bar itself will slide sideways. This is another excellent place for poly. You must use a good lithium grease for any poly installed; otherwise it will grab, stretch or tear, and fail very quickly.
Finally, the front end alignment has a lot of effect on the car's handling. Other than tracking the rear behind the front, there is not much to do with the rear alignment. In the front, you have caster and toe readily adjustable, and camber if you have the adjustable upper arms. [These must be used if you've lowered the car noticeably - the camber cannot be set correctly on a lowered car without them]. Stock settings for alignment - WHEN USED WITH ALL THE OTHER STOCK BITS - makes the car a nimble dancer. Changes in parts often require changes in settings to come close to that. Be sure the right and left steering rods are exactly the same length with the steering wheel centered, then adjust the center rod for toe.
The causes for your skittishness are most likely in the front. But if you have any stiff pieces in the rear, your tires can merrily hop around and seem like you're going to spin. Low, wide, stiff tires are bad (hence significantly lower rear tire pressures than front should be used), stiff rear springs or shocks. Stiff rear sway bar. All these get in the way of keeping the tires in good contact. Tracks are MUCH smoother than streets, so we can use a lot more stiffness all around the car on a track than will work well on the streets, and thus get significantly higher cornering forces (which means faster corners). The same choices makes street driving a chore, or worse. Remember - higher cornering forces on the track means faster lap times. It does NOT always mean nice easy handling.
On cars that have a lot of mods to the suspension - ones that are well chosen and interact with each other well - the rear sway bar is often the last adjustment to achieve that elusive perfect balance that the car started with from the factory. I have two bars, one of which has multiple link points, so I can choose a F/R balance to suit the nature of the track or street I'm going to be on.
SO:
1. Check your front lower ball joints, and wheel bearings, steering box and idler, and adjacent body sheet metal - those are life threatening issues.
2. Check your T-bar outer spacers. and other bushings. While your at it, check your guibo for cracks and tighten and lube your u-joints and the slider on the drive shaft.
3. Lower your rear tire pressure and raise the fronts.
4. Report further.
Robert
