1999 Firebird -- Rear end kicking out over bumps

[ThirdeYe]

Hey everyone,

My new-to-me Firebird is giving me issues when going over large bumps (especially around corners). I hit a fairly large pothole and the entire rear end of the car swung out a few feet and I nearly clipped the curb. It also does this when going around corners and hitting a bump while in the process of turning.

I got new tires on it (BFGoodrich Traction T/As, 235/55/R16s) and also had an alignment done. However, the rear end is still acting up. It did this before and after the alignment/tires. It has 77,000 miles on it right now, but I believe the shocks may have been replaced at one point in time. These came with de Carbon shocks (which I've heard are pretty bad), but the shocks on it now are Bilsteins. I've heard great things about Bilstein shocks, but could it still be the shocks causing this swaying and bouncing? Granted, I don't know when they were replaced. I have all of the maintenance records from the previous owner who had the car for 10 years/25,000 miles and I didn't see any records of him having them replaced, so I'm guessing they're at least 10 years old with over 25,000 miles on them. I tried pushing down on the wheels and didn't really notice excessive bouncing, but it's hard to press down enough to really get it bouncing since the car is so heavy. Any thoughts/ideas would be greatly appreciated, as it seems very dangerous to me at this point in time. I took it to a Chevy dealership and they sloughed it off, simply saying that "shocks would be a good start", but they acted too busy to really look at the suspension in-depth. I also wonder if maybe they didn't do the alignment correctly? I've usually been told that cars usually just get a front-end alignment, and perhaps they never touched the rear wheels? I apologize for my newbie-ness, but I know next-to-nothing about suspensions.

Thanks!

 

[javacontour]

Some of it is the behavior of a solid rear axle. If one tire is disturbed, it impacts the tire at the other end of the axle.

 

[cheesepuffs]

Originally Posted By: javacontour
Some of it is the behavior of a solid rear axle. If one tire is disturbed, it impacts the tire at the other end of the axle.


That's what I was gonna say. That's just a solid rear axle for you, really.

 

[bdcardinal]

There is no adjustment on a solid axle, well there is, but for the sake of the argument on your car no there isn't.

Trying to go from memory on those suspensions is it a 3 link with a panhard bar? When did the F-bodies start getting a torque arm? While it is natural for a solid axle to have some deflection, if it is excessive one of the bushings in one of the control arms might be bad allowing excessive movement.

 

[BayouTodd]

Their is NO rear end alignment needed for solid axle cars, you might try www.ls1tech.com and read up some more on your car. I have a 99 bird too.

 

[NHGUY]

Springs too stiff,too much air in tires,stiff shocks (Koni/Bilstein/air shocks).The rear end is not compliant,its stutter stepping and that can happen with unloaded pickup trucks too (stiff setup).

 

[IndyIan]

Any other suspension mods? A larger rear sway bar? Maybe some of the bushings are shot back there? What's the rear tire pressure?

 

[NHGUY]

82-up F bodies got the Monza/Vega torque arm.

 

[fdcg27]

Stiffish springs plus limited suspension travel plus a live rear axle equal side-step in bumpy corners.
Nothing wrong with the car, just the way such things work.
This is a car way out of your accustomed frame of reference.

 

[Rand]

Can be normal for a rear solid axle vehicle. The other suspension components will determine how badly it will happen..

You would really notice it on a highway ramp turn that has a sticking up expansion joint.

 

[MNgopher]

Sounds like what to expect out of a solid rear axle vehicle. Part of the criticism of "muscle" type cars as we moved on in the years. The Mustang will finally say goodbye to it for the 2015 model year. That pretty much will leave trucks with solid axle rear ends, which behave, well, like you've described.

 

[440Magnum]

Originally Posted By: javacontour
Some of it is the behavior of a solid rear axle. If one tire is disturbed, it impacts the tire at the other end of the axle.


This.

But it can be severely aggravated if the shocks are not right (too stiff or too soft can do it) or if there are bad bushings, a bad trackbar end-link, etc. etc. A well-set-up solid axle car should be noticeably different from an IRS car in that circumstance, but it shouldn't feel out of control.

Interestingly, on truly smooth pavement solid axles have an advantage in total lateral grip (all other things being equal). No camber change through the corner, more favorable roll-center for the whole setup. But one peanut on the tarmac gives the advantage back to IRS.

 

[ThirdeYe]

Thanks for the tips everyone. I had a feeling it might have been due to the way the car is just supposed to be (i.e. the live rear end). I've only owned FWD 4-cylinder cars in the past, so this is an entirely new experience for me. My sister had a few Mustangs/Firebirds when we were still in high school, but I only drove them a couple of times. I'm probably so used to the way my Acura handled in the corners that I was expecting a bit too much. It feels pretty bouncy/floaty on uneven roads, too, so I'm not sure how much more stiff the suspension is compared to stock. I haven't had a chance to check the tire pressure yet since getting the new tires put on, but I'll check them tonight. Lately I've just been trying to take it easy around corners if I see bumps in the road.

 

[fdcg27]

Yeah, a live rear axle makes for a different feeling beast.
The side-step won't be all that much.
If it were really the few feet you mentioned in your first post, you would have swapped ends right now.
It may feel like a very large displacement, but it's really only a matter of inches.
You'll get used to this as you drive the car.
Been a while since we've had such a beast.

 

[dailydriver]

The dampers, especially the rear dampers are very hard to get 'perfect', as far as valving to match the spring rates goes for great handling on these cars.

Even though the Bilsteins are high quality units, they may NOT be valved precisely enough for the (even stock) spring rates on the car.
This is one of the reasons I went to double adjustable Koni dampers on mine, so I could use much higher rate springs (especially on the front), and adjust the compression AND rebound damping to match the (higher) spring rates.

But yes, even when everything is 'perfect' as far as setup goes on these cars, the solid rear axle will hop/skitter on REALLY bad, 'washboard'/speed bump-like/bombed-out turns (COMPLAIN TO your local Public Works dept./DOT/etc.!!).
But then, so will a very stiff, modded for ultimate handling rear indie setup, on bad enough roads (albeit to a lesser extent) despite what the owners of these rides claim.

 

[fdcg27]

This is true.
On a real washboard surface, my BMW will dance around at the rear, but it takes a really bad surface for this to happen and an MB 123 will handle the same type of surface with aplomb.
IRS isn't the cure-all some believe it to be. Spring rates, travel and shocks matter as well.
Also, the traditional cure for evil handling was lowering and stiffer springs.
As the old saying goes, any suspension will work if you don't let it.

 

[InvalidUserID]

Welcome to F-Bodies. You'll get used to it.

 

[eljefino]

Do you have a locking/ limited slip rear diff? That will bind up the fast tire to go as slow as the slow one or vice versa going around a corner. It "shouldn't" feel bad during normal cornering but could be out of whack.

 

[chainblu]

Originally Posted By: InvalidUserID
Welcome to F-Bodies. You'll get used to it.


+1
You'll get so used to it, you'll even know when to expect it.

 

[ThirdeYe]

Originally Posted By: eljefino
Do you have a locking/ limited slip rear diff? That will bind up the fast tire to go as slow as the slow one or vice versa going around a corner. It "shouldn't" feel bad during normal cornering but could be out of whack.


No, it doesn't have the limited slip, as far as I know. It is pretty much the base model with just a few options like the wider tires, t-tops, factory tint, cruise control, and an automatic transmission. I checked the tire pressure and all 4 tires are at approximately 37 psi. The rear-end hop is literally at least 6 inches to a foot at times. Maybe more.

Slightly off-topic, but the car came with a white/orange pinstripe on both sides. Is this stock, or something someone would've done aftermarket?