When setting an M3 for high performance driving, properly setting the rear camber and toe will yield significant results. The factory eccentric suspension bolts have a reasonable amount of adjustment range for a variety of ride heights and possible alignment settings, but are not a perfect solution.
Eccentric bolts are adjusted by rotating the bolt within the suspension arm, which causes two major issues:
The first is that tightening the bolt into place will inevitably change the alignment. As you turn the nut to clamp the bolt into position, alignment will change slightly from the intended setting, losing precision.
The second issue is that as high loads are applied, the bolt will tend to slip from its pre-set position, creating alignment changes while the car is being driven around the race track which are frustrating at best, and unpredictable and dangerous at worst. Such movement of the factory eccentric bolt has been documented by Fall Line Motorsports in their many years of professional racing.
The rigid camber shims will allow a technician to precisely set camber by choosing one of 5 shim offsets to install in the rear suspension. The shims can be used in either the normal orientation, or flipped, resulting in 9 different positions, divided into 1mm increments. This will give an exact camber setting, and ensures absolutely no change in alignment even in the harshest driving conditions.