I would like to get people's opinions/experiences with cars lowered via rear coilovers. I'm wondering specifically about spring preload. Everything I've read outside of this forum talks about the importance of properly set spring preload on coilovers. As far as I can tell all Fiero owners, myself included, with rear coilovers have no way to set spring preload when lowering their cars. I have my ride height set exactly where I would like it, but on full suspension down travel there is a gap between the top of the coilover spring and the top perch. Is this common? I have Koni adjustable dampeners and 12" springs.
My helper spring is fully compressed, is this normal? Yes, that is how it is designed. When your suspension is fully extended, the main spring is free to move around and can come dislodged from its perches without a helper spring. The helper spring is only there to retain pressure on the main spring when the shock is fully extended so that the main spring stays in place.
When shorter rear springs are installed, flipping the top plate over helps to put a pre-load on the springs which keeps them in place. When I cut the rear springs in my '84, it was a nuisance every time I jacked the back of the car up as the rear springs would come off their perches in the struts. When I flipped over the top plates, this reconfigured the strut assemblies and the rear springs were now held in place.
The two concerns to be addressed are can the spring come out of its seat when fully extended, and does the orientation (rotation) of the spring matter.
On stock springs, if there was not any preload, the springs could move around and slide off the side of the rear spring perches (large diameter spring, small diameter strut, lots of room for misalignment). Also, the ends are not perfectly flat as the cut end of the coil is about 1/2" thick. The lower a-arms up front and the upper/lower spring perches in the rear are contoured for the spring end to be in a specific position. So if there isn't any preload, then the spring could rotate and have the ends of the springs out of these seats and make them more prone to fall off the seat.
With rear coil overs, the ends are ground flat and the perches at both ends should be flat as well, so rotation of the spring to the seat doesn't matter. Since coil overs are small diameter and have a very close fit to the bottom adjustable sleeve (less than 1/8" gap), it falling off the lower perch isn't possible. The key is how you design the upper perch to ensure the spring stays in the proper place while at full extension. This is done by adding a side support/locator sleeve to the top. I use 2" or so length of exhaust tubing and weld it to position the outside of the spring at the top. Inside works too, but clearance to the strut body at full compression is very, very tight, so I use the outside. As long as this upper sleeve is longer than the gap in the spring at full droop, the spring can't be in the wrong position.
If you have lowered your car, you can help reduce the spring gap by flipping the top strut hat as it will shift the top of the strut up about 1" and reduce suspension droop by 1" as well.
Flip the top plates over so the bushing is on the top side. You will need to swap them side to side to do this, cut off the studs, weld on new ones, and weld a 2" length of exhaust pipe to keep the top of the spring centered.
Flipping the top hat will lower your current gap by 1" and the 2" exhaust pipe positioner should ensure they never come out of position, even if there is still a 1" gap.