Ground Control: Strange popping from rear

[rwh11385]

So my car was making some creaking noises during Auto-X, but now there is a very distinct popping noise coming from my back end.

Does anyone know the cause? Is this an issue? Can I resolve it easily?

Typical GC rear E30 perches near the top + 5" spring + plastic cones on the top stub.

Video:

http://s20.photobucket.com/albums/b217/rwh11385/?action=view&current=RearSuspensionPopping.flv

thanks

[z31maniac]

What so you put racecar parts on your car and then are pissed when it makes noises? ;)

[jlevie]

Get the car up in the air and inspect all of the rear suspension for cracks.

[Jonny Cash]

I certainly hope you got under there and poked around before you asked the internetz?

[rwh11385]

What so you put racecar parts on your car and then are pissed when it makes noises? ;)

Yes, I know. It doesn't bother me a ton, but I want to make sure it's not a serious issue. The cone shaggers were making mention of it.

Get the car up in the air and inspect all of the rear suspension for cracks.

Suspension is fine. I'm 95% sure of what is going on, but I want unbiased confirmation of that is going on from people who have had this happen. (It's is hard to believe it is happening if it is what I think it is) I also might know the potential fix, but don't want to take that step without knowing for sure.

[Hellabad]

crossposted from beanerforums

I also might know the potential fix, but don't want to take that step without knowing for sure.

This gets very complicated of course.

So let me explain. Follow the logic here, and you will see how GC parts end up the way they do. There is always a reason, and when the reason is understood, then your solution will be found.

First make sure all the parts are correct. ie , not 2.25 mixed with 2.5

1) The height of any GC rear adjuster is a very specific height, usually very short. The reason being that the rear springs we use have LOTS of travel in them. Lots.

2) This is to retain (not get) better handling through consistency.
Rear spring coilbinds=spring rate change=inconsistent handling.

3)If we believe this, then also rear coilover hardware touching each other is bad.
Rear spring hardware touches=spring rate change=inconsistent handling.

Conclusion) Therefore, the GC rear hardware cannot have a very tall taper on top if used with a GC or Eibach spring because we want to avoid "Rear spring hardware touches=spring rate change=inconsistent handling."

So that is why your GC rear hardware cannot be tall and tapered like others.

Example: An Eibach 5 inch long, 2.25 ID, 650 pound/inch spring can compress down to 2.132 inches tall*. This means ANY coilover hardware will crash into itself if protrudes into that spring more than 2.132 inches

So then where does the noise come from?

In contrast to popular belief, it is not the spring coming unseated, but is actually the spring refusing to bend. The e30 rear arm moves through extreme angles, which a short stiff spring cannot follow. You simply need to accommodate the angle between the two. Of course the e36/e46 weight jack is a great solution, but the same part for the e30 is illegal for almost every AX class.

So the solution is to accommodate what is happening. The bending is caused by the necesssity to use a stiff spring if you want better handling.

You can have one or the other other: A soft spring that bends, or a stiff spring that doesn't bend as easily. The factory has obviously chosen a soft spring for ride comfort, if they chose a stiff spring for handling, they would have to do things differently. (On the rear of an AMG Mercedes with stiff springs, the spring is actually bent as it is manufactured.)

In the world of race cars, there are compromises. GC chooses not to make the compromises in a manner that slows the car down, so the rear coilover hardware is short and rigid. (incidentally, the GC STREET rear hardware for the e30 is tapered and taller)

What to do about the compromise of using a spring that doesnt like to bend? Simple. Situate the spring so that it doesn't make noise as it bends. This is done by rotating the lower coil so that it points at the rear shock.

So thats it. Analysis. Explanation. Reason. Solution.