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Since I am going through the rear while the wheels are out being refinished. (Group Christmas present to me, I am very thankful for it as well.) I am curious as to figure out if my rear spring is shot... For starters while I am under there I am pulling the batwing to do the bushings and thus pulled apart the spring bolts. To my suprise it appears that they were almost maxed out... The cotter pin hole was a few threads below the bottom of the castle nut and looking at the witness marks on the bolts the nuts appear to have been pretty well tightened. The cars ride height seemed about right according to the FSM and the spring arched down some while loosening the nuts. It also maintained contact with the bushings until the castle was a thread or 2 above the bottom of the bolt. I am just wondering if maybe the spring is failing and the PO had tightened them all up to fix a ride height issue... Or are these perhaps lowering bolts that are longer and to get stock ride height need to be tightened... I will update with photos in a bit.
I wouldn't take this as a sign that your spring is bad. I would remove the spring and look it over carefully for any cracks, though, just to make sure. It does look like the previous owner may have wanted to raise the ride height for some reason. Check the rubber bushings for being broken or squished. I don't know what they are supposed to look like, as I always had poly on mine while I owned it. Also check the center shim stack at the diff: if someone previously removed the spring and didn't get the shims in the right order it could explain why it's riding lower than stock when the nuts are at the factory position. Typical lowering bolt setups don't have a hole drilled for a cotter pin and castle nut - they have "nylock" nuts instead so you can set them at any thread height you want.
I wouldn't take this as a sign that your spring is bad. I would remove the spring and look it over carefully for any cracks, though, just to make sure. It does look like the previous owner may have wanted to raise the ride height for some reason. Check the rubber bushings for being broken or squished. I don't know what they are supposed to look like, as I always had poly on mine while I owned it. Also check the center shim stack at the diff: if someone previously removed the spring and didn't get the shims in the right order it could explain why it's riding lower than stock when the nuts are at the factory position. Typical lowering bolt setups don't have a hole drilled for a cotter pin and castle nut - they have "nylock" nuts instead so you can set them at any thread height you want.
So the shim stack is a thin laminated plywood fiberglass piece, rubber, spring, rubber, two aluminum shims then diff housing. Not sure if that's correct.
It's a BMF spring and has about 3 inches of arch. I am replacing the bolt bushings with polybas the rubber was pretty squished.
Edit: there is another laminated piece between the top rubber and two aluminum shims.
So the shim stack is a thin laminated plywood fiberglass piece, rubber, spring, rubber, two aluminum shims then diff housing. Not sure if that's correct.
It's a BMF spring and has about 3 inches of arch. I am replacing the bolt bushings with polybas the rubber was pretty squished.
Edit: there is another laminated piece between the top rubber and two aluminum shims.
Unfortunately I don't know the correct order for your car. It changed with different years and springs. I'm including a picture of the various specs from the 96 Factory Service Manual just so you can see how things changed even within a single year, but that's not a good reference for your car. You need the 1984 FSM (or someone else who has one) to verify the right order for the insulators and spacers.
Last edited by MatthewMiller; Jan 16, 2021 at 11:45 PM.
Unfortunately I don't know the correct order for your car. It changed with different years and springs. I'm including a picture of the various specs from the 96 Factory Service Manual just so you can see how things changed even within a single year, but that's not a good reference for your car. You need the 1984 FSM (or someone else who has one) to verify the right order for the insulators and spacers.
At least now you know. Ruling out possible causes is nearly as valuable as identifying them! I'd say you should carefully check the spring while you have things apart, then try the new poly bushings and see how it sits with the nuts at the correct factory position. Maybe it will be fine that way.
ETA: Oh, I just thought of one other thing. Are your rear shocks in good shape with good gas pressure? All modern shocks have a gas charge which reduces cavitation and foaming of the fluid, but which also provides a bit of spring rate on its own. I don't know how much that really changes ride height, but if your shocks have lost all their charge then that extra spring rate is no longer there and the car might sit a bit lower because of it. Your dampers should generally have enough pressure to extend on their own after you compress them (with the shock out of the car, of course). Just one more thing to verify before you put it all back together.
Last edited by MatthewMiller; Jan 17, 2021 at 10:56 AM.
At least now you know. Ruling out possible causes is nearly as valuable as identifying them! I'd say you should carefully check the spring while you have things apart, then try the new poly bushings and see how it sits with the nuts at the correct factory position. Maybe it will be fine that way.
ETA: Oh, I just thought of one other thing. Are your rear shocks in good shape with good gas pressure? All modern shocks have a gas charge which reduces cavitation and foaming of the fluid, but which also provides a bit of spring rate on its own. I don't know how much that really changes ride height, but if your shocks have lost all their charge then that extra spring rate is no longer there and the car might sit a bit lower because of it. Your dampers should generally have enough pressure to extend on their own after you compress them (with the shock out of the car, of course). Just one more thing to verify before you put it all back together.
I'll snap some pictures of the spring later. Looks like the resin has shrunk but no damage. And the shocks are 2016 bilsteins. The seem good and stiff still.
Originally Posted by JoBy
The fiberglass spring will not sag over time like a steel spring. As long as there are no cracks or other damage it should be good to reuse.
I kind if figured. I'd assume any settling happened at the factory. The deck on my boat is fiberglass only and in the last vouple years settled about a quarter inch. Door is no longer square but it's 10 feet of fiberglass with no supports I'm the middle so it's expected.
Looks to me like someone didn't know what they were doing and just ran the nut up as far as it would go. You can see it was bottomed out on the threads. They probably didn't know the pin is supposed to go between the cutouts in the castle nut.
Like I had said. Seems like the resin is receeding more than anything.
Nothing wrong in those pictures, looks like the typical "grain/texture" on every Corvette monoleaf I've seen! Spray it down with some simple green and use a bucket of hot water and a gentle (softer bristles) scrub brush and clean that baby up. Then visually inspect all surfaces closely, if you don't see any delamination, cracks, chunks missing or individual strands of glass letting go....you should be good! My bet is the spring is perfectly fine and as suggested above, someone just ran the nuts up too far, good luck 👍
Nothing wrong in those pictures, looks like the typical "grain/texture" on every Corvette monoleaf I've seen! Spray it down with some simple green and use a bucket of hot water and a gentle (softer bristles) scrub brush and clean that baby up. Then visually inspect all surfaces closely, if you don't see any delamination, cracks, chunks missing or individual strands of glass letting go....you should be good! My bet is the spring is perfectly fine and as suggested above, someone just ran the nuts up too far, good luck 👍
I agree with that and everyone else. The rubber was pretty crushed. I bet they tightened it to compensate for that. The new poly mounts are almost twice the thickness so on top of the bolt being pushed higher the preload from the bottom from the thicker bushing will raise it too. I'll bolt it up per the FSM and check ride height after a drive at the set measuring points and if need be, adjust from there.
The spring is back in. I cleaned it up and it appears to be good. However I was able to install the nuts on the bolts by simply holding the top with a wrench and using a plain old socket with no ratchet to tighten the nut underneath and compress the spring until I was able to install the cotter pin. I feel it should take more then the force of turning a screwdriver to compress the spring no? I mean it was greased though... just curious. It definitely fought me off about half the bolt but it could've been rusted that much too.
It depends on how stiff the spring is. With the car lifted my rear spring ( 1984 Z51 ) is completely unloaded. The spring is very stiff so it is quite straight unloaded to get the correct ride height. A softer spring will start with more arc to get the same shape ( ride height ) under normal load.
It depends on how stiff the spring is. With the car lifted my rear spring ( 1984 Z51 ) is completely unloaded. The spring is very stiff so it is quite straight unloaded to get the correct ride height. A softer spring will start with more arc to get the same shape ( ride height ) under normal load.
It is the base 84 spring... BMF. But reading it seems to be similar to the 85 z51 so maybe I'm okay.
Right, your base 84 spring is still one of the stiffest ever installed on a C4. I see nothing to worry about here.
We'll see in a few days... the Spicer joints I had ordered had one come damaged. So with the rear apart it'll sit a bit... whoever designed that setup I low key hate... like just awkward enough working around everything under there... no good way to attack those damn strap bolts. And I continue to be paranoid.
Rear spring question...
