I recently lowered the car 2" all the way around and change to poly mounts up front. I had the wheels rotated and balanced and had the car aligned. I waited for about a week for my STB to come in and I put that on.

After all that it seemed to pull to the right a little bit, but I need to take it back and have it aligned again just to make sure.

The ride was a little bumpy, but expected, so I had my Sphon LCA relocation brackets installed at SpeedTek (local speed shop) and then...here is where my problem starts.

When going down the highway, I can feel a slight vibration all the way up to 70mph, but after that it is horrible vibration. If I slow down below 70mph then it will always give that bad vibration over 40mph. Sometimes it won't do it, like mabye it's the highway or something. I had the tires balanced, and they were fine.

The only thing I have yet to install is the
adjustable panhard rod. Is this the fix? I was thinking that the rear might be in a bind or something.

TIA

Daniel

 

u lowered it and u didnt put the adjustable panhard bar in , ummm, put the panhard bar in and see what happens, your rear end should be over to the left i think, when i lowered my car mine was to the left or right i dont remember , but anyways put the panhard bar .

 

i would check your pinion angle.

 

That is what my guess is. I didn't have time to put it on; waiting on shipping two jobs and sleeping.

I will continue post if this does not work.

 

really, panhard should not effect this.

and extreme pinion angle on the other hand, could.

 

I don't see how adding LCA relocation brackets could have changed the pinoin angle. As least not enough to need to get this fixed.

Wouldn't that mean that everyone that has used a 2" lowering spring and Spohn LCA relocation brackets would have the same issue?

 

if they were installed incorrecly, it could have effect the pinion angle, that along with the lowering, might be your culprit

 

While you are checking the pinion angle, check the ujoints for some play. I had a similar sounding vibration once. I checked the front end, balancing, tyres, just about everything. Once it started getting worse, I thought about the driveshaft. At first I thought maybe it was bent, turned out the rear joint was broken and allowed to move about a 1/2 inch!!! Glad it didn't break all the way...

 

Okay, checked it out last night:

U-joints still good.
Stock panhard rod, little bent.
Wheels are still circles, not oblong.
LCA brackets look like they are staight.

After checking, I changed the panhard rod and set the rear down, appearently the hotchkis bar doesn't work with two inch lowered cars, so I need to fab it up, but everything fits.

I took the wheels off to see what the problem was, still trying to find that vibration. I noticed that one of the lugs on the two rear wheels rear lighter than the other four for that wheel. This would mean that it would throw the balance off by a little bit normally, but it would be more if they were to be at the top of the tires at the same time.

So I changed them, and took it out for a ride. I don't know if it was the panhard rod, or if it was the lugs, but one of them fixed it.

 

I lowered my car and put the Spohn relocation bracket's on aswell, I have the exact same proplem!! I have a bunch of other susp. mods aswell, but have yet to change the panhard bar When I put one in I will be able to tell you if it was the panhard or not(my guess is that it was not the lug). I hate vibrations, they drive my nut's.

 

I would suspect your pinion angle it will change when you lower the car. The LCA relocation brackets change the angle of the LCAs which may correct fore/aft location of the axle in the car slightly and could change pinion angle all changes due to lowering the car. It sounds like your driveshaft is in a bind. I don't see where you installed an adjustable torque arm so I'm not sure how you would adjust your pinion angle.

After suspension mods on my Z I adjusted my pinion angle due to a vibration which moved the vibration from 60 mph up to 75-80 mph but ultimately cured it with an LPE aluminum driveshaft. Now it's smooth and vibration free. The fastest I've run it since is about 140 mph.

 

PINION ANGLE....

SPOHN LCA brackets do not KEEP the stock geometry... Im not trying to start a war on this issue BUT when installing mine I noticed that the angle of the holes was off..Im not trying to discredit the product...I have them...I Like them...Ive liked everything that Ive gotten from Steve..But when Guido and I installed mine we both looked at each other and said "ID be pissed if I did not have adjustable LCA...That could cause a problem..."
I had Adjustable LCA's so I was able to correct the geometry problem....But If you have non adjustable ones or do not make the adjustments the Rear end has to be rotated to get the bolts in.... On a STOCK torque arm there is Enough GIVE to allow the control arm to rotate the housing to a small degree thus changine the pinion angle... Mine the Bolts would not go because of the aftermarket torque arm... I had to adjust my control arms approx 1/4" to make the goemetry correct...

Initially I ASSUMED that it was the fact that I had a Moser rear and his moser brackets and that the tolerances between the 2 caused the problem...But in talking to many others about this I Found that all had similar stories...

Take it FWIW... Check the pinion angle...

 

Kenwood,

i meassured a set of spohn brackets too.

the holes were drilled on a 9" radius, the stock are is about 19.5"

i thought the same as you, and am in fact getting my own set made instead.

 

How do you check the pinion angle?? Could you not change the angle with an adjustable torque arm??

 

an adjustable torque are IS how you change it.

you do it by measuring the angle of the tranny output shaft, compared ot the angle of the pinion.

 

Thanks man.

 

I "COULD" have changed my torque arm....But I had already dialed it in where I wanted it... Instead I changed the control arm legnth.... My thought was if I left the Control arms SHORT and adjusted the torq arm shorter to line the angle back up that I would risk shortening the wheelbase by bringing BOTH forward...So I opted to legnthen the control arm to keep the bottom of the rear where it was and keep things how I had them.... I wanted the rear to Sit exactly where it was before the install...

 

Could someone give some measurements or what the required/desired pinion angle is and the legths of the control arms. I have this problem and after reading this tread I have some things to work out.
I have a set of installed LCA brackets from BMR with an adjustable panhard bar but stock LCAs and Torgue arm.

 

This is definatly a thread that will help people with that pesky vibration problem!! Really good info guy's. Everybody always thinks they need a new driveshaft.

 

I'll bring some more tech to this....well, tech i am going to link... agood2.8 posted this before he was banned for the third time

here is the best explanation i have seen on this site for adjusting the pinion angle.



Example of Diffrent Ride Heights


The full thread can be found here:
https://www.thirdgen.org/techbb2/sho...hreadid=193352

 

Okay, I am starting to believe this pinion angle thing. What gets me is how guys like Graboid and Mr. IROC-Z don't have this problem.

Anyway, it came back last night, I noticed that I could feel a little bit of the engine during accel at higher speeds, the vibration was just like my friends car when his angle was out too.

 

because, diffrent cars react diffrently, everything has to work together. if hte LCA brackets are off by say 3/4" so it has rotate the axle just a bit to comensate so the LCA's fit. then you have changed your pinion angle. when you are dealing with things that are as touching as the pinion angle, a fraction of an inch can change the angle by a couple of degree's which is enough to cause problems.

 

Well, I need to do some fabrication on the panhard rod to get the rear centered. I used a hotchkis and it still won't pull the rear end all the way to the center, 1/2 inch off.

What if the rear was not centered? Would this cause a vibration? I mean it would be pulling sideways against the driveline.

 

You need an adjustable panhard rod. That hotchkis one isn't doing anything for you but causing more problems if its not adjustable.

 

It is adjustable, it just doesn't adjust enough. I need to fab it a little bit to be able to work with my car. I need to headstock to turn one more time, but can only make 3/4 of a turn, even though there are threads left.

I am going to cut about a 1/4 of an inch off to allow me to get the rest of the turn that I need to properly set it up. My question was if the bar is still stock length, then will it make it viabrate?

 

I dont' have experience with the problem however it would make sense to me that it very well could be the cause of your problem. An offset rear will give vertical and horizontal misalignments. My car is also lowered like yours via the mods in my sig but I experience no bad vibrations. Maybe due to the adjustable panhard bar... I dont know...

 

When I lowered my car with lowering springs I got an adjustable panhard rod, too. It didn't make the vibrations stop but did align everything right when adjusted properly.
I even put in my old springs back in the rear to see if the new springs were to blame but it wasn't.

What I don't understand is how can the pinion angle cause the vibration at only highway speeds but not when the engine is reved pretty high.

 

Agood2.8's drawings show the angles well- what it doesn't show is bad angles. Here's what happens-

Lets say the tailshaft (TS) to driveshaft( DS) angle is 3* and the DS to Pinion (P) is set correctly at 4* (One degree down for thrust compensation- Under throttle this will rise to 3*) the harmonic vibrations cancel out because the angles match under throttle.

Now if the DS to P angle is 0*, then under throttle it inverts to -1* bringing the cross cancel to 4* off and a harmonic vibration will be the result.

We miss agood.

 

think about how this all works.

from the tranny back, everything inceases with speed, 3k rpms in first gear (say 20mph) you engine may be spinning farily quickly but you driveshaft is only spinning at (in a t-5 1st gear is 2.68:1 ratio) 1119rpms now in 5th gear at the same engine speed (5th gear is say a 0.75:1 ratio gear) the driveshaft is spinning at 4000rpms. big diffrence in driveshaft speeds, same engine speed.

if it is a driveline vibration, he can put the car in neutral and the driveshaft speed doesn't change, becuase it turns anytime you are moving. now you can have a 4000rpm driveshaft speed, and a 650rpm engine speed

hope that clears things up.

 

Alright, I took the headstock off last friday night and ground it down so I have the ability to turn it in 1.5 turns. I only turned it 1 time to see if it was better, and it was. The vibration doesn't start until 80mph now, and it doesn't vibrate as hard.

I still need to measure to see if the rear is sitting properly under the car or not, and adjust it in or out depending on how it is set. I think I will need to use the other .5 turn.