Cferrel4

So I have a 99 Firebird Formula with 167k miles on it. Last week did a lot of work to the suspension set up. BTW this car is still factory, original drive train never tore apart (besides an aluminum drive shaft & flowmaster muffler welded in). This car is a daily driver for my wife/me, one of us drives this car everyday to work or where ever. It is not our designated race car.
I have searched, found other people had tire issues, other bad suspension parts and a couple that needed to adjust pinion angle; however, they dropped their vehicle more than I have.

Last week the bird received new front and rear monroe sensa-trac struts/shocks and new strut mounts since they were hammered. It also recieved new front and rear sway bar links, new BMR non adjustable rear control arms (original bushings had separated from the sleeves causing noise) and a BMR non-adjustable pan hard bar. I had a set of 1.3" Eibach lowering springs around, #3831001 & #3831002. I installed them, because the springs could be changed at the same time and I wanted it to handle better. Also has 4 brand new BFGoodrich tires on it and it received a new alignment!

Car feels amazing now except for a rear vibration that can be felt in seat from about 40 mph and up. I have had these same springs installed in other f-bodies and never had an issue. But it is the only thing I can think of to cause this vibration.

Will a 1.3" lowering drop cause the pinion angle to change enough to cause this vibration?

Daniel Richards

iTrader: (1)

they say on here, it's different for each car, one car can be dropped without needing it while another can need the pinion adjusted, that's kind of the basic rule, install the springs and if you get vibration get the adjustable torque arm to fix the pinion angle.

whats NOT helping is running SENSA-TRACS with lowering springs, those things are not valved for that kind of drop.

a NON-adjustable panhard bar and non adjustable torque arms are a WASTE of money, the being off center (left to right, not the rotation controlled by the torque arm)from that drop could cause some small vibrations because now the angle of the driveshaft is off in 2 directions, rotation angle and centering.

Cpt.Derrek

iTrader: (13)

any change in ride height will effect the pinion angle as it is set to -2 degrees from factory at factory spring height. You can try adding washers to lower your transmission cross member and try to get your transmission more parallel to your axle pinion angle. You might want to pickup a digital angle finder as its a lot easier.

Also your transmission mount could be shot as well, both my birds factory mounts lasted until just over 95k.

Sam Strano

iTrader: (41)

You changed a lot of things and almost any can contribute vibration. The much more solid control arms transmit a lot more NVH into the car than the stock arms with the fluted rubber bushings, that's why the stock arms were that way.

The tires could be slightly out of round or balance and that will be felt more through the stiffer arms.

If there is a vibration from springs changing pinion angle it's because something else is wrong, a worn trans mount something. The change in angle is so small because the torque arm is so long it's never a problem.

But the choice of shocks was not a good one either. You have shocks that on purpose don't damp well at small movements, but you put lowering springs on which demand more damping control form the shocks because you have less working travel. And shocks control unsprung weight on compression side of things, which those have little of. In short I don't know what caused the issue, and that's why I am always trying to get people to slow down and NOT make so many changes at once. But I suspect it might be a combination of things.

Cferrel4

I am sorry, I had the o.e. spectrums installed. After talking with their rep, made it seem like they would work.

The trans mount was replaced about 10k ago and still appears to be in good shape.

I am going to pick up a BMR adjustable torque arm from a friend this weekend and see if that makes difference.

Cferrel4

I sure hope the brand new tires are not out of round. Good to know that the springs won't affect it to much. I can pick up an adjustable torque arm from a friend this weekend to see if it will make a difference.

Unfortunately I was in a spot that required me to replace the separated bushings. I had the arms and panhard bar hanging around from another car that I had saved in the corner of the garage.

Daniel Richards

iTrader: (1)

Spectrums?

the OE shocks on these cars where the orange colored DeCarbons, and they weren't all that great to start with and again not valved for lowering applications.

this is why Nothing cheaper then the Bilsteins is suggested, Koni Sports are actually meant for lowered applications if you have the budget.

Corvett z07

iTrader: (2)

I'm one of those oddballs that didn't receive any vibrations after lowering. I couldn't tell you why I got lucky, kind of a hit or miss I guess

blackandgold

iTrader: (2)

I drive an almost 17 year old Camaro...if it didn't vibrate I'd be worried.

JD_AMG

I never got any either. I think its more of a oddball situation to get vibrations that it is to not.

Sam Strano

iTrader: (41)

No, that's not odd at all. It's normal unless something else is amiss like a bad u-joint or something.

Cferrel4

Got some work done on my firebird since. The left rear rim lost an ounce of weight, that affected the vibration I could feel in the seat. Still had a rotational/gear noise coming from the rear at 55mph to 65mph. I noticed the transmission mount was dry rotted, but not broken or split. I replaced it with a new unit. Still had the rotational/gear noise from the rear but much more subtle. Almost like a rhyme-tic woo sound. I was still thinking pinion angle. I got from a friend a polyurethane torque arm mount that was brand new. The noise is virtually gone.

leadfoot4

iTrader: (1)

Good to see that you're working through the issues. If I read correctly, you didn't install an adjustable PHB. With that much of a drop, with the springs you installed, your rear axle is now somewhat out of line with the centerline of the car. Since this affects the alignment of the driveshaft, laterally, that might be where some of your vibration is coming from, especially if your u-joints are worn.

Cferrel4

I installed 2 brand new u-joints with the M6 aluminum drive shaft I put in the car.

I am going to see if I can source out an adjustable panhard and torque arm from some friends for trials. The noise and vibration though is only really prevalent at higher speeds and it sounds rotational. Anything up to 55-60mph, the car is smooth and quiet. I would think that if the pinion angle is wrong, I should be getting noise & vibration all the time.

leadfoot4

iTrader: (1)

Correct......I installed lowering springs on my '96 Z/28 when it was a couple of months old. I had both the OE torque arm, as well as an adjustable torque arm in it, and didn't get any unusual vibration using either one.

Cferrel4

Unfortunately it has been snowing and there is now salt on the roads. Before I had to put it away for the winter I tried an adjustable panhard bar, made no difference.

I think I am going to take off the rear diff cover and inspect the differential. It has had no issues before and was quiet. Not sure why at that same time it would have gone bad though. But the noise sounds rotational, so considering it.

2JAWZ

iTrader: (1)

I'm in same boat. This is a plague. I was told by BMR the pivot angle should be -2 to -3 degrees. I got her up, leveled and load bearing on rear and she was right at -3 degrees. When spring rolls around I'm taking her to speed shop and let them try and figure it out. I have adjustable everything so I guess the problem could be anywhere. PITA**

BMR Sales2

iTrader: (40)

A good base range for the pinion angle should be in the -2 to -3 but every car is a little different. What kind of noise does the car make and when does it do it? It definitely can be hard tracking down vibrations so you have to be a little patient when trying to figure out what the culprit is.

2JAWZ

iTrader: (1)

The car runs fine up to 70 mph. No noises or anything. After 70 its not so bad as long as you are accelerating. Say if you let off the throttle at 80 mph it feels like the damn car is about to rattle apart. Apollo 13 was probably a more smoother ride, but once she falls back under 70 shes fine again. I immediately thought driveshaft when I bought the car. Installed new Spohn steel driveshaft, not a difference. So I have adjustable BMR torque arm and then everything else is UMI adjustable. I guess Ill have to leave it to the pro's. I had a 96 Z lowered on stock shocks and BMR springs and she drove/rode damn swell. My 97 is a much nicer car and its almost no fun at all to drive because of this***

JP

My does the same thing,so I will get an eye on this thread. If i figure anything out,ill report it here also.