Eccentric Rear suspension Bushings Explained........maybe

This is a hard one to put into words.  

Each rear suspension trailing arm has 2 bushings. Their mounting bolts pass through a metal tube centered in the rubber bushing material. The rear trailing arms pivot at these bolt tubes. Eccentric polyurethane bushings are different. Their mounting bolts pass through offset holes in large metal inserts set inside the polyurethane material instead of tubes. Rotating the inserts move the bushings in orbit around their now off-set mounting bolts. When the alignment spec is met the bolts are tightened locking the inserts, and the alignment settings, in position. This shifts the trailing arm's pivot points off-center. Now the trailing arms don't pivot at the bolts. They pivot around their new offset inserts.

The toe-in spec is adjustable via a threaded rod between the rear subframe and trailing arm. When adjusted it moves that side of the trailing arm fore or aft along a horizontal axis, flexing the inner bushing, and angling the rear wheel in or out with respect to the car's centerline. There are eccentric bushings sold that will adjust the toe-in but they are not needed or recommended by our friends at the Omega Owners Forum (OOF).

The outer bushings set the rear camber and aren't normally adjustable. The camber specs were locked in when the suspension was designed. In this case there is an available eccentric bushing that adjusts the camber which helps reduce rear tire wear, especially on lowered Cateras.  When adjusted they move the trailing arm's outer pivot point higher or lower along a (vertical axis) with respect to their mounting locations on the suspension subframe. To make the adjustment you rotate the eccentric inserts between their 12 o'clock and 6 o'clock positions along a vertical axis. Rotating the eccentric will cause the trailing arm to move its pivot point (the inner eccentric) in or out from the stock position. This in turn causes the top of the tire to tip in or out to meet its spec. Once the bolt is tightened the eccentric, and camber, is locked in.

Remember, changing one adjustment may affect the other. Since the outer eccentric really moves the bushing in an arc instead of a straight line it's necessary to play with the eccentric and recheck the camber after setting the toe-in (and visa-versa).  You may have to check both settings several times before buttoning it all up. Any experienced alignment specialist will have seen these types of adjusters before so there shouldn't be a problem. Don't forget the camber alignment bolts for the front control arms. They extend that adjustment's range by +/- 1 degree over stock and go a long way toward reducing front tire inner edge wear.

I hope this makes sense because even after several re-writes it's still pretty confusing. I can visualize it but have trouble verbalizing it. The 1st picture is a Catera showing the front & rear suspensions. The other diagram below is that of a 2004-2006 Pontiac GTO which is basically a widened Catera. Its semi-trailing arm suspension almost identical to the Catera's. Hopefully they help clarify what I'm talking about.


Catera X-Ray


2004-2006 Pontiac GTO Rear Suspension