Corvette C4: The Rear Suspension

Your C4 Corvette has, or maybe once had, a great suspension. It could very well be worn out by now though.  C4 Corvettes are old cars and the parts have started to wear out. It's not that any one item is really bad. The 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. 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. 
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. Again the rear of my '85 is a great example of that. All of the 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.

The Design
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 rear suspension. You 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 asked Goodyear to develop a P255/50R16 tire for the new '83 Corvette. 

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. Neither did they 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.

Checking the Rear Suspension For Wear
At least once a year you should check the rear suspension for wear. You have two places for wear to take place in the rear of the C4. The most common wear problem is the wheel bearings. Raise the entire rear of your car off the ground on jack stands. Now grasp the bottom and the top of the tire. Then try to wiggle the tire in and out. 

Place your hands on the top and the
bottom of the tire. There should
be no movement.. 

I've generally found that all C4s have a very slight amount of play in the bearings with the emphasis being on slight.  After you repeat this process on the other side, you will now know if you need rear wheel bearings. With the wheel bearing diagnosis completed you should now place your hands on the front and rear of the tire, with one hand reaching all the way to the inner edge of the tire. Again try to wiggle the tire back and forth. You shouldn't have any play in the wheel.

Place your hands on the front and rear of the tire. If you have movement
it will most likely be from wear in the toe rod. That will require having
someone look under the car while you move the wheel. 

If you find you have movement, you'll need to crawl under the car and locate where the play is taking place while your partner moves the tire back and forth. This wear can take place at the outer tie rod end or at the inner ball socket where the tie rod mounts to the differential cover. There are some very specific specifications for the amount of play you should have in the rear suspension of your C4 Corvette but most of the time this play is rather obvious. 

If you have serious wear you'll notice a huge difference from the right side to the left side. It's unusual for both sides to have serious problems at the same time. Then again these are old cars so use some common sense.

Once again I decided to use heim joints. These have grease fittings. I'm not convinced that's a good idea though
because grease attracts dirt and dirt creates wear. 

GM used two different types of toe control rods during the production of the C4. The early cars use a male tie rod end. In 1992 they switched to a different arrangement. The problem is that the parts for the early cars are no longer available. If your tie rod ends are worn on an '84-'87 Corvette, you're going to have to replace the entire assembly. You can also convert to a heim-jointed arrangement shown here if you have a track only car. 

Rear Camber
The second area of concern with the rear suspension is the camber setting for the rear tires. The goal is to have the tires at a 90-degree angle as you go around a corner. If your tire is vertical to the pavement, then you have all of the tire's tread on the pavement.

I replaced the rubber bushings with heim joint. They
are much better than polyurethane bushing but they
may not be ideal for the street because of dirt. 

The other goal is also to have everything stay in place. You don't want things moving around as you drive through a corner. Camber changes in the middle of a corner are not fun. If your suspension is worn out, the bushings compress rather than transmit movement. This is a case where new bushings can be a big help. You can purchase a new set of bushings, or you can do as I did and purchase a whole new camber rod. I wanted the rapid adjustability, which you may not need. It behooves you to look at all of the options available to you.

Polyurethane Bushings

Everyone sells polyurethane bushings. They're not very expensive, and if you believe the ads they're a miracle cure for your Corvette's handling. Let me just say that I've used them in every possible point on my suspension. Let me also say that if I had an alternative I would get rid of them. I would replace all of them with Delrin or even aluminum. Delrin is just much harder than polyurethane and compresses even less. The problem is that Delrin bushings are just not available, and I'm too lazy to machine my own Delrin bushings.

Polyurethane bushings are not all the same. The hardness varies widely from one company to another. Some are really soft and deform under use. Keep in mind that polyurethane is not like rubber. Once it deforms it will stay deformed. I used some soft polyurethane bushings in my rear control links and they actually ovalized in the course of a year. 

If you have over 100,000 miles on your C4 you're probably a candidate for polyurethane bushings. They're not as easy to install as some would have you believe but it's not all that bad. You can install them on one of those weekends when nothing else is going on. It's just a matter of taking your time. New bushings may very well be the least expensive way to revive your old tired suspension. I think it's worth the effort, just don't believe all the hype that you're suddenly going to pull 2.0 Gs in a corner.

The Original - Well Almost Original

Now the rear toe can be easily adjusted. If you participate in solo events or track days this is important.