dregg100

iTrader: (34)

i have been getting this vibration like something is out of balence or lose in my rearend. the strange thing is it only does it at certain speed. the vibration gives the feeling of the wheels being warped, but they are brand new. between 10-20 mph its kind of slow(back of the car moves slightly up and down), but here is the strange part, between 20-45 mph there is nothing then from 45-55 it comes back(faster) and after that its gone. so if it was a warped wheel, or tire out of balence wouldnt it be constant? any ideas or help is much apreciated! i have been pondering this for a few days and cant really come up with anything. thanks!

4CN A1R

iTrader: (6)

pretty sure its your pillowballs. the bushings in the rear end go bad after so many miles. after the bushings are replaced you should prob also get toe links and trailing arms to help get rid of rear end suspension shimmy

dregg100

iTrader: (34)

i actually have trailing arms and toe links with heim joints on the ends waiting to go on, i just havent found time. the pillowballs are the ones that there are 6 of right?

DaveW

The problem you are describing seems to me to be the rear sprung or un-spung mass (car itself or tire, wheel, and suspension) moving up and down at the natural frequencies (the 2nd is twice the frequency of the 1st, and if you went 80-110 mph, the 3rd would likely appear) of the mass on the combined spring rate of the tires and suspension springs. This says to me that there is not enough damping in the rear (if there were, it would damp this out) to control it, plus the new tires may have some out-of-roundness, or something rotationally non-uniform in the carcass causing force variations. If this is truly the cause of the problem, then a 5-psi or so change in rear tire pressure will change the speed at which this occurs (more pressure will raise the speed, lower pressure will lower it, due the change in tire spring rate). How old are your rear shocks? I wouldn't be surprised if a different set of rear tires (even, maybe, the same type) made this disappear.

I doubt that it is pillow *****, because that usually causes looseness, vague handling, and noise, not tuning-fork-like vibration of the unsprung mass.

dregg100

iTrader: (34)

i have KEI office type xt alpha coilovers with about 8k miles on them. i just wonder if i hsould pull the rears off and double check to make sure they are correctly balenced.

4CN A1R

iTrader: (6)

someone got a new thesaurus

DaveW

Since you have new coil-overs, I'd bet that they are quite a bit stiffer for low-amplitude inputs, and that you are feeling the sprung (car) mass oscillating on the relatively-undamped spring rate of the tire(s). This almost certainly means a tire/wheel is out of round or a tire is non-uniform. If it were a balance problem, it would get worse with increasing speed, and you would probably NOT be able to feel it at 10-20 mph, where it would take a HUGE amount of imbalance to be noticeable.

Dave

DaveW

Sorry about the engineering-ese jargon, but I work on air suspension for a living.

Dave

4CN A1R

iTrader: (6)

no, its cool. if you can help him, thats all that matters

DaveW

I had another thought - if it only happens when you first start out after parking for several hours, you could be feeling the tires flatspotting from being parked.

Dave

coneklr

iTrader: (2)

I think I have the same problem with different symptoms. When I go down a certain stretch of highway, the rear end of my car oscillates and sounds like it wants to tear itself apart until I slow waay down. I have short very stiff springs. Do I just need to get softer springs? My shocks are also very stiff, but they may be bad.

DaveW

Do you mean the rear suspension is jumping up and down (like wheel hop), or am I misunderstanding your description?

Dave

coneklr

iTrader: (2)

Well, I don't really know what it is doing because I am in the driver seat.

I think the road has bumps at a constant interval. It makes a pretty loud bang noise from the rear for each bump.

When I originally got the car, I had to replace one pillow ball that was causing a knocking sound in normal driving when taking a corner or something. But the other pillow ***** felt ok. -This despite the previous owner replacing the pillow *****. So whatever the issue is, it might be hurting my pillow *****.

adam c

iTrader: (2)

Get the tire balance checked before doing anything else. Are the wheels and/or tires new? If so, that is almost certainly your problem.

coneklr

iTrader: (2)

It was doing it with my bald tires, plus with my brand new balanced tires. The car also has nylatron bushings if that helps.

DaveW

A bang with each bump sounds like the suspension is bottoming out (springs going solid, shocks too long for the ride height, etc.), or the opposite - "topping out." Does it do it as the car hits the high part of the bump, or at the end of the rebound travel?

Dave

dregg100

iTrader: (34)

thanks for the help, i just wanted to go ahead and ask until i have time to do it(re ballence). i should get around to it tomorrow and report back to you guys. and no its not a flat spot(im not that dumb) as it does it all the time at the above speeds.

DaveW

Sorry - didn't mean to offend...

It's just that sometimes the simple things tend to get overlooked.

Dave

dregg100

iTrader: (34)

its all good man. i appreciate all your help, someone with real knowledge and not just BS is hard to come by these days.

ccanepa50

iTrader: (1)

might try a low tech troubleshoot option first........relocate the potential offending wheels....ie: can you rotate tires to check for imbalance (if all same size or will at least fit?) .....also .....rx7 non specific: any u joint going south can induce speed specific vibration
chuck

coneklr

iTrader: (2)

Wow, the causes you list sound as bad as the noise I am hearing! Here are pics of my struts and springs and one of the car to show the ride height. They are M2 Advance shocks with Eibach springs.

Would longer springs be the cheapest way to fix it? That should improve ride comfort also, right?

Edit - I am not sure which part of the bump it is on. The bumps that I am hitting are so shallow that I can't really see them. I can't feel the bumps when I am in my Saturn.

Edit Edit - My apologies for hijacking this thread.

DaveW

Am I looking at the 2nd pic correctly? It looks like the spring is not touching at the top. If this is correct, what you are probably hearing is the springs unseating, and then banging as they snap back into the seat, due to the rebound length of the shock being too long, allowing the springs to be loose at rebound. Also, is there supposed to be a rubber cushion at the top of the spring? I don't see one.

A possible cure would be more bump (jounce) damping, keeping the spring from compressing so much, and then rebounding fast enough to unseat. Also, more rebound damping would slow the rebound, possibly helping keep the spring seated. What these changes would do the the handling, I don't know, since I don't know how the shocks are set now. However, since, if my guess is correct, the shocks are now underdamped, this may actually help your handling.

Link to a handout from a handling seminar I gave: http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

Dave

coneklr

iTrader: (2)

The suspension came with the car. I don't know if it is missing anything. And yes, when the car is on jack stands, the spring just sits there without touching the top of the shock.

I have a feeling that these aren't the springs that originally came with the M2 Advance. It sounds like from your diagnosis, if I increase my ride height by getting longer springs, it will also solve my problem but without making it rougher?

For now, I guess I will muck around with the stiffness settings.

DaveW

Maybe you can get longer, but softer, springs that would take up the space w/o increasing the ride height too much, and at the same time, improve the ride. You should also get spring cushions for the top. These would also help take up some of the extra room.

Dave

coneklr

iTrader: (2)

Good idea. Thanks for your help Dave.