Rear Springs and general Spring questions

[E3012933]

I have a 91(92) 318i convertible. Previous owner installed the BC racing coilovers. The rear spring rate appears to be 160lb based on the part number and behavior. I have raised the adjuster all the way up(or down) and the car will settle right back down on the tire. Rate seems preposterously low.

Question 1 - what is the recommended spring rate for a convertible? Would like it lowered but not the tire sitting on the lip (this is a street car)
Question 2 - why is it so hard to find manufacturers spring rates for these cars instead of just marketing style naming?
Question 3 - does anyone sell rear springs only for the BC setup? Ideally, I would call a vendor and get a BC replacement spring with a different spring rate and swap it out, but BC doesn't list part numbers or spring rates. Again, seems absurd.

Not uncommon in even the muscle car world to just go to the catalog and get springs by length, diameter, and rate. Am I missing something?

Thanks for any and all help. Pretty new to the e30 world.

Edit - I'm checking the spring part numbers against swift spring website.... 62-160 is the part number for the front spring. but that appears to be installed on the rear of the car. so, likely the spring is too short. But it should be a 450lb spring.

According to the same site, both springs for the kit are 8k / 450lb springs. does that sound right? or should I get a higher rate for the back?

[reelizmpro]

From a quick search...BC states they come with 8K front and 8K rear unless custom rates were requested and they don't discern the difference between a coupe/sedan or convertible. 8kg/mm is about 450 lbs/in. Is the rear a true coil over (spring over shock) design or is the spring perched on the trailing arm?

Convertibles are heavier with a lower rear fender arch. So lowering one will typically cause the rear to appear too low and rub. If you adjusted the rear all the way up and it still settles down too low then you need a longer spring or spacer/pads.

[E3012933]

Just confirmed the rear spring part number 62-120-008, which is the right spring part number for the kit. Which is a 450lb srping.

The spring sits on the trailing arm, not a true coilover in the rear. The kit has adjustable spring perches which adjust about 2inches. There seems to be no difference in ride height even with the spring perch at its tallest setting.

Still very confused why I cannot raise the rear of the car, even when adjusting the spring perch.

Any suggestions for replacement springs? The shocks themselves seem to be fine.

[reelizmpro]

That makes no sense, there should be some difference if the adjuster is all the way up. Inspect the perch on the body and trailing arm to make sure they aren't cracked/broken. This can happen when the adjuster digs into the perch. You can install large spacers underneath to help spread the load and raise the car a bit.

[E3012933]

Agree, does not make sense. This is happening on both sides. No cracks anywhere. I take the spring and perch out to adjust height and then reinstall.

My theory is the spring rate is way too low and is collapsing.

[reelizmpro]

A 450lb "race" spring shouldn't collapse from just the weight of the car though. Besides, even if the spring were completely collapsed, you would still see some change in height from the adjuster. Did they remove the stock spring pads ? The top pads in particular have a protruding nipple hanging down through the middle of the spring. If these are used with short coil springs they could be bottoming out, keeping the spring from moving. Look at the suspension when it's fully compressed to see what's happening.

[E3012933]

Here is a question - does the BC spring adjuster go on the top of the spring or the bottom? I don't have a set of instructions. The adjustable spring perch is on the top and I left it there. But, I see installation videos with the perch on the bottom. In theory, should not matter, right?

The nipple is still there.

[E3012933]

The rear spring on this installation video looks much longer than what I have.

edit - maybe not. second picture looks very close.

[digger]

no issues with subframe bushes?

[E3012933]

Could the subrame bushings cause the body to drop regardless of spring perch height? Are you thinking the trailing arm bushings? What is your thinking?

I've only ever worked on live axles. When it stops raining here I'm going to turn the springs around and I will see if anything is deflecting abnormally when I put a jack under the arm.

[mtech325]

If you know the rear springs are confirmed to be the 450 lb. rate, you can try experimenting with a stiffer pair of springs like Eibach. You can buy them new off places like Summit, or check eBay for used ones. Assuming the front is 450 lbs, you'd probably want something around 600 lb. or more in the back in 5 or 6 inch length with the corresponding diameter (usually 2.25 or 2.5 inch depending on the diameter that your rear coilover adjuster accepts).

Something to note is rear springs below 600 lb. tend to be barrel shaped and taller than 5-inches on e30s because they will otherwise experience coil bind. I ran into this while softening up the spring rates on one of my cars, and to run a softer 500 lb spring in back, I had to switch to a longer 6-inch tall spring to use the linear non-barrel shaped ones.

[digger]

Could the subrame bushings cause the body to drop regardless of spring perch height? Are you thinking the trailing arm bushings? What is your thinking?

I've only ever worked on live axles. When it stops raining here I'm going to turn the springs around and I will see if anything is deflecting abnormally when I put a jack under the arm.

I was thinking shot subframe ushers or trailing bushes could cause issues or the sway bar is limiting movement