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I recently purchased a set of front and rear UMI weight jacks for my 87 IROC along with the KYB AGX adjustable shocks and struts. After installing them the fronts turned out great. I got the car as low as I wanted and the ride was not harsh at all on bumpy Chicago land streets. The rears on the other hand were disappointing. The ride was harsh over bumps and when I would drive and park over a section of my slanted driveway I could hear a clunk in the rear coming from the shocks/springs-weight jacks??? I did go low with the car, is it possible it’s too low and I need to come up? I had the shocks set at 1 which is supposed to be the softest setting but it didn’t help? I will do some more inspecting but for the time being I suspect that it’s the weight jacks cup/plate that’s making contact with the rear end??? Is it possible that I went too low and I need to come up some? I did come up about a half inch and adjusted the shocks to 3 with no change. Has anyone else encountered this issue? The springs are the recommended 175lbs. Any advice? Do I have to settle for coilovers? Thanks in advance.
Yeah, I spent a lot of time trying to make the rear ride better.
1. The front has much more travel than the rear to the bump stops. Once you hit the bump stops the ride becomes very harsh. This is most likely your culprit.
Look at how close the bump stops are to the top of the axle tube and you will see you have little to no clearance. Either shorten the bump stops or raise the car.
One important consideration.... Most aftermarket wheels position the tire further outward such that it could hit the top of the fender lip. You will find that the factory bump stops are perfectly positioned to prevent contact. Personally, I would not shorten the bump stop. It was put there for a good reason.
2. And despite that, the rear will always be a harsher ride than the front because of the unsprung weight of the axle. The better the shock the better the ride. Fortunately rear shocks, even really nice rear shocks, are not terribly expensive like the front struts. Usually people have budget to experiment a bit with different rear shocks.
3. Shocks have height ranges where they are rated for use. If the car is too low, making the shock shorter than it's work range, then the ride quality will suffer. Just make sure the shock you are using is rated for the extension length where you run.
4. Spring rates matter. Your weight jacks are a constant rate spring at 175 lb/in. Most conventional rear springs are variable rate, meaning that sporty 145 lb/in aftermarket spring is actually running at about 110 lb/in cruising down the road. 110 is a softer ride than 145, and waaaaaay softer than 175.
Thank you so much for the detailed information. Yes I was definitely frustrated! I really think I have to raise it unfortunately, which sucks because I love the stance. I wonder if I will have the same issue with coil springs???
You are probable hitting the bump stops, you need to get in there and measure. Pictures might help.
Coilovers wont work any differently at the same height if you are already hitting tye bumpstops. If you can measure for clearances to important parts, you can probably shorten the bump stops and see if things improve.
Thanks for the info and advice. I will get in there and see what’s going on. I just think there’s only so low you can go with these cars whether it’s with weight jacks or coilovers and I’m learning that the hard way! I guess if I want the stance I’m looking for I have to use airbags on the rear????
Update….lifted the rear 3/4” and it drove great! I guess you just can’t have it too low or it will bottom out…I’m happy with the stance and the ride. Thanks to everyone for the advice.
I have umi weight jacks front and rear. I had the rears on the lowest setting and the fronts pretty low. It looked awesome but drove like crap. The front would bottom out on bumps, the rear only had 1 in of travel before hitting the bumps. It was scary if I hit highway expansion joints at speed. I was also miserable to get on my trailer. I went up 3/4 of an in and it made a massive difference.
your car has a better stance, thats how mine looked for a while. I just got to the point where i was stressing over the bottoming out. Also, speed bumps didn't happen and driveway ramps were miserable.
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Last edited by Firechicken82; May 9, 2026 at 05:12 PM.
Thank you. I love the blacked out look on yours. Yeah it definitely had me annoyed with the rough ride. It’s crazy what a world of difference 3/4” makes. The front end is pretty low so you become limited to where you can ride which does suck sometimes.
Yeah, totally can be faster. But in my infinite wisdom I've still kept my car low enough that its a total pain to jack up. I can't just jack up the front. I need to get a side up a little and then get the front x member. That one of the reasons Im currently building a garage w a lift! These cars are such a pain to jack up.
On my GTA I had weld in subframe connectors which made it so much easier. I plan on adding them to my IROC. Mine is also pretty low but I’m able to adjust the bolts on the weight jacks with it on the ground. It’s a slight challenge but as long as you can reach under and get your arm in there with a 3/4 ratchet you can get it
What's the measurement from the ground to the lip of the fender and quarter panel? I'm getting Ground Control weight jacks and want to have an idea of where I should start with the ride height. I'm thinking 26" front and 26.5" rear.
