I'm sure many, such as myself, have experienced a vibration while accelerating after lowering on whatever springs are out there.
I don't believe a resolution was posted anywhere. I found a thread over on G6P where the OP mentioned reading about the same issue on the cobalt forums, and the fix was a small tab on the bearing plate that needs to sit a certain way.
Well I can say with for sure that is the problem.
When I first lowered I got the vibration every time I'd accelerate, at right around 15 mph, always.
Today I replaced my shocks and struts and decided to pay attention to this bearing plate and it's tab. Now that it's back together right I get absolutely NO VIBRATIONS!
So here's some info that could help make this clearer to understand.
This is the bearing plate, It sits on top of the spring (with a rubber insulator in between). On top is a plastic bearing housing (white) and the top hat bolts on top of this:
As you can see towards the right is a little tab that sticks out with a green mark on it.
THIS TAB MUST FACE INWARDS TOWARDS THE ENGINE.
The plate is thicker on the left side of the photo than the right side where the tab is. That thick side is meant to take up the gap of the uneven spring coil, Problem is with lowering springs if you rotate it so it fits right, the tab will not line up to the center. So I had to compress my spring a bit more so I can turn everything the right way and then get that center nut on.
Here's a photo of the plate on top of the spring (tab facing the camera)
After tinkering and such I found the top hat only seems to fit the car in one direction. And that direction is with the flat edge also facing inside towards the engine.
So when assembled you should see the flat spot, then the tab, and it should line up on the back side of the strut, like this:
The flat spot and tab are facing the camera, the bottom of the strut where it bolts to the steering knuckle is facing away from the camera.
So what I think is happening is when all is said and done and the strut is back in the car, if that bearing plate isn't at the right angle, it throws the center line slightly out of center (the line that goes from the center of the top plate down to the center of the axle) So while the car can still take an alignment I'm thinking the plate pulls it out just enough to where the axle has to move a bit out of it's "comfort zone" and hence we get a little vibration.
So it makes sense to me that when the parts are aligned correctly so is the center line and the weight of the vehicle on the knuckle, so the axle can sit in perfect alignment.
(I'm thinking that slight difference might pull or push the axle slightly at an angle, causing a vibration from within the CV joint)
Now that's all speculation, I may be wrong and I'm interested to hear other's thoughts.
But it does seem that the one inch difference in the bearing plate is enough to throw off some kind of alignment.
So make sure, that tab faces inwards towards the engine.
Also here's the post on G6P that prompted me to check the little tab: