Rear coilover spring length

[FredK]

I am doing a rear coilover conversion on an E30, where the motion ratio is .64 at the factory spring perch and 1.04 at the shock mounting location.

I understand what spring to use on the coilover to get the same wheel rate.

How do I determine what the best spring length is?

I'm currently using a 5" spring in the stock location that has a maximum of 2.7" of travel, which translates to around 4.4" of wheel displacement.

What makes sense to me is I need to find a spring that's around the same as my existing wheel rate, but also offers additional travel to account for the increased amount of displacement the spring could hypothetically experience. This corresponds to the travel of a 7" spring.

Are there any other factors that I'm missing?

Thanks in advance!

[ScotcH]

Makes sense ... FYI, we use 8" springs in the rear, but that's e46 (Motons). I think your spring perch adjustment would be the determining factor.

[aeronaut]

For a C/O, won't the length be determined by the distance between the upper and lower perches, then fine tune length for needed ride height adj range?
You'll probably want to do a little math first.....corner weight of a rear corner of the car / spring rate will get the preload distance on the spring (nothing fancy needed with a ~1 motion ratio). For higher spring rates, this isn't going to be very much, 3/4" or less. Then buy a spring length with a preload compressed length that puts you in the middle of adjustment range?

[aeronaut]

Add: Before buying, I'd also do a sanity calculation on of length any chosen spring at full suspension compression to make sure you don't get spring bind.

[FredK]

Maybe I need to install the rear shock on the car to see what the maximum extent of travel I would want is first. I did do a little math in terms of unsprung corner weight to see what the spring preload / static sag would be, and that seems to be fine, and I just need to ensure, as you said, I don't run out of spring travel.

[aeronaut]

I've looked at doing a true C/O in the rear, but it's pretty tight back there.
Are you moving the upper mount location?
Anything else for clearance?
What width rear rim/tire you going to run?
Any concern with the strength of the lower attach point?
Let's see a build thread!!!

[FredK]

Yeah, it's very tight. I think I did a quick sanity check calculation and I might have the width of a piece of paper between the spring and my tire.

I am, of course, driving an E30 which has even less clearance in the rear than an E36 for wheels and tires. I'm running 17x9 with 245 RE71Rs and I had to hammer stuff. Hopefully it works, otherwise I'm going to have to run the shock in a divorced configuration, which isn't the end of the world.

I may weld on a lower attachment point reinforcement, but I feel like people have done this before on E30s and haven't died.