Your C4 Corvette has a great suspension but it could very well be worn out. Ok-it is worn out. The C4 Corvettes are old cars and the parts have lost their edge. It's not that any one item is really bad. The normal problem is that all of the wear has added up. A little bit of wear in a lot of places means you have a lot of total wear. It's a cumulative thing. Plus, you may not even notice how worn out things are since you've probably never driven a perfect C4. Your Corvette seems perfectly normal to you even if it isn't. You just assume that all C4 Corvettes drive the way your car does.
One of the reasons aftermarket parts seem to make such a huge difference with older Corvettes is that aftermarket parts are generally used to replace worn out factory parts. People put some new aftermarket part on their car and extol the virtues of that part. They would have felt the same difference if they had used a stock GM part. All new parts make a huge difference whether they are OEM or aftermarket. Sometimes though, GM doesn't give us much of a choice. They send us to the aftermarket. Let's take the rear suspension of the early C4 Corvette. A lot of the parts you need to rebuild your rear suspension simply aren't available from the local Chevrolet dealer. We're simply forced to the aftermarket to restore the ride and handling of our early C4s.
My track car is a poster child for aftermarket parts. This '85 Corvette has been beaten to death on the track. I'm closing out my tenth year of running track events. Each lap puts a little more wear on perfectly good parts. Every few years another part gets replaced. OK, every year any number of parts gets replaced. It all depends how I feel about my Visa card balance.
One rather important item to consider is that a lot of the aftermarket suspension items make alignment adjustments easier. The rear of my '85 is a great example of that. All of the Vette Brakes parts I've installed over the years make camber and toe adjustments really, really simple. I may not get the settings right all the time but I can really adjust the hell out of the toe and camber settings. Actually, I can adjust my suspension to the point where the car will barely corner but that's a personal problem, not a parts problem.
Before I get into the details, let's take a look at the rear suspension on the C4. The C4 rear suspension is really a variation on the '63 Corvette suspension. You just knew that transverse leaf spring in the back looked familiar. Also, keep in mind the C4 Corvette was designed around the tires. The engineers fell in love with the tires on the Porsche 928 and had Goodyear develop a P255/50R16 tire for the new '83 Corvette. Yes, boys and girls, that's how it all got started.
This new '83 Corvette was to have very low pitch, at least much less than the '63-'82 Corvette had. The engineers didn't want the nose of your Corvette diving to the pavement when you applied the brakes. They didn't want this new Corvette squatting down in back when the car accelerated, either. Both of these can be accomplished through anti-dive and anti-squat suspension geometry.
If you look at a side view of the C4 Corvette you need to think of a long arm that angles up and towards the front of the car. The ideal locating point would be above the car's center of gravity. The problem is that this single arm, like in the '63-'82 cars, would have to be so long that it would get in the way of the seats. That was a problem with the older '63-'82 Corvette. The attachment point for the trailing arm actually limited the driver and passenger area. Or you could say the driver and passenger area limited the rear suspension.
The solution for the C4 was rather simple-just use two arms whose virtual intersection point is above the car's center of gravity. In that respect the anti-dive and anti-squat characteristics of the C4 are vastly superior to the older C3 Corvette. At least when your car was new and all of the bushings were in good shape.