I got my car running with a new engine and started driving it again 2 weeks ago. Every time I hit a bump with the rear tires I can feel the axle jumping. Hard. It's so bad that people riding in the car with me comment on it. It's an 87 Trans Am with a Pro Kit, KYB select-a-ride struts up front, stock shocks from 87 out back.

Normally I'd say the shocks are the culprit, but they aren't leaking, and if you bounce the rear end it goes down, comes up and stops. Shocks appear to be fine going by the normal checks. Is the stock valving just too weak to deal with the higher spring rates of the Pro Kit? Is this just something I'm going to have to deal with because of the springs?

I don't want to buy Koni's, Bilsteins, etc. I'm not looking for better handling characteristics. With 255/65R15's out back it ain't gonna happen. I just don't want to hit a bump going around a turn and have the car swap ends. So if it is the higher spring rates of the Eibachs will the same thing happen again if I go with new stock style rear shocks (either parts house shocks or OEM)? Or should I just stop overthinking this, and get some KYB's?

 

You answered your own question correctly

Yyou are dribbling across the road like a basketball. You are also most likely bouncing off the rear bumpstops since it is lowered- combination of the two will make the rearend jump really badly.

 

So would it be safe to assume that if I replaced the rear shocks with another stock style from a parts store, I would have the same issue?

 

You may soften the issue with new stock shocks, but its not only the shocks that are the problem.

Here's what happens. When you go to a stiffer spring rate, you increase the compression and recoil frequency of the spring and it will bounce shorter and faster cycles than the original springs. Stiffer springs REQUIRE stiff rebound (recoil) damper on the shock vavling. Hence why you need a better quality of shick valving- not just stiffer vavling, better valving Cheap stiff valved shocks genearlly have slop between the comression and rebound strokes leaving a bbole feel that can cause not to control the jumping dribble.

You other problem is being lowered. you may be also bouncing off your factory bumpstops and abruptly causing the suspension to stop and the impact causes the tire to compress and release- heance making the car jump also.

 

I kinda had the same problem during bumpy cornering..
I used a 4thgen V6 rear swaybar and most of that went away.. stock WS6 bar was bway bigger but i use the rear WS6 bar at the drag strip.. and non in front.
Just another idea to try.. also if you have poly bushings they bind real bad.. use some antiseize (instead of their sticky lube) to lube them between the metal and the bushing..or go to rod ended control arms and panhard bar..