I just installed some BC racing coilovers, and the fronts are on good and make no noises.
The rears are set all the way down, and when I had the Z in the air, the spring did not move up nor down, but you could turn it fairly easily.
I went for a testdrive, and every little bump I go over in the road, I can hear the rears making a thumping sound, so I know it's the preload.

Any idea on how to adjust/fix the preload? And perhaps where it should be at?

 

Probably normal, pillow ball style coilpvers do make noise and its easily heard in the rear of the Z. If you want to make certain , just thread the rear shocks down a little, its a 10 minute adjustment.

 

The rears are set all the way down with the dampening at 15/30.

 

First turn the rear up to around 20, your to low to run softer settings. Next take the rear shock and thread it in and make it slightly shorter. This should take care of your problem, if it not just the normal sounds that rear coilovers make on our cars.

 

Assuming your talking about your springs. But is your damper set all the way down too?

 

Springs are set all the way down, (all threads).
Shocks are on 15/30 dampening.

 

I can't understand what any of these responses have to do with spring preload....

The spring seat should be threaded up to the base of the spring until there is no more play. Simple as that.

 

Yes, I understand that about your springs. But is your shock/damper threaded all the way down also? If your springs are all the way down, the shock body itself has to be adjusted down too, to get the correct amount of piston travel. I also suggest stiffening up the dampers some too. Almost sounds like you have too little travel and are hitting the bump stops maybe?

 

Are you talking about the OEM rear spring locastion or coilover rear?

 

I'd be interested in the best way to do this when the springs are kept in the OEM location.

 

simple - on springs that use OEM spring locations, the spring itself will sit on a threaded collar, and so will the shock, or damper, itself\

preload is adjusted by threading the collar that holds the spring. Whatever change you make to the spring, you should also then translate and make to the damper.

To the OP, I doubt preload is your issue - sounds like you left something loose such as the center nut on the damper itself, etc

 

So I have springs adjusted to the lowest setting and cut the rubber, what should I do to set the preload?

 

Colin - not sure what you are trying to accomplish? Preload should be set to a minimal amount, where the spring is not able to wiggle around - basically so that it is secure in its perch and not able to be moved up and down when the car is in the air (suspension unloaded). From there, you really only should be altering preload via corner balanacing, with the car on scales.

 

Just trying to eliminate any preload, it just didn't seem as easy to tell if there was any preload or not on the rear.

 

When I spoke to Pete from BC, this is how he explained adjusting the rear springs and dampers.

1. You have to remove the lower mounting bolt from the dampers so it's disconnected from the rear suspension arm.

2. Assuming the springs are set to desired ride height use a jack and raise the lower suspension arm up 1/4" from this point, this sets the location for where to line up the lower shock mount.

3. Now thread the shock body up or down as needed to line up with the hole. Tighten up the lower lock ring.

Hope this helps.

 

Ah, something I clearly overlooked when putting the rears on.
When I put them on, I left them from the previous owners settings.
Then, when I put the rear springs all the way down, I forgot to adjust the shock all the way down as well. AH!

Well, I just finished adjusting it, and the spring is crazy snug now, under load and under no load.

Well, I went for a test run, and wouldn't you know, the sound went away slightly, but it still makes a thumping sound when I go over bumps/etc.

I took a small vid just to describe the sound to see if that helps,
You can hear a "thump" sound at 8 second and then again at 2 and 1 seconds left.


I tripled checked every loose thing, and found nothing out of place...gr, this is bothering me...

 

check the torque on the center nut on the damper itself

 

Is there a specific ft/lbs setting it is supposed to be at?

 

I can ask the guys at BC tomorrow morning, but if I had to give my best estimate, 50-65 ft lbs is typical for coilovers for that center nut

also while you're there just check to make sure there is no slop at the rear upper bearing in the mount - I've never heard/seen of one going bad, but mind as well check while you're there

 

Ok, thanks Adam!
I'll check it out in the morning.