What's better? Divorced rear suspension or non?

[yamahammer]

Anyhow, this is what the GC rear units look like:



Things to note for installation:

- First issue was the upper mount didn't fit into the strut tower. The inner shell of my tower is still there, and the boxing is overlayed. To fix this I had to take the mounts off and cut them up a bit, repaint them and through them back on, no big deal.

- Next issue, was the spring collar interfering with the wheel well. The upper strut mount is offset so that the center line of the coil is moved outward. It isn't quite enough, so in the below picture I have outlined where you have to cut out and rebuild further inboard. I cut out a fair amount of wheel well and patched it further in to ensure there is never contact.



- The third problem is the lower coil perch rubs on the CV boot when in full extension (green arrow points to where the contact is). In the picture below, you'll notice the lower link is offset (red arrow), this allows the coil to be moved slightly further away from the boot. When the car is on the ground loaded up it doesn't rub, but as soon as you jack it up it contacts. I am looking into some slim boots.



I have 850 lb springs on there now to try out. May end up with a stiffer, long spring eventually. Before when I still had separate spring, I ran 1100lb units and it was not enough. The car squatted so hard coming out of corners, it was kind of shocking the compliance you get on the spring out of the stock suspension in an e30.

Anyhow, we will see next season how these work, just feeling the car compress vs the old set-up it seems way better. The coil angle is not bad in respect to the suspension axis. So in the below picture you can see what I am talking about, the smaller the angle between the two lines, the less actuation per compression you get, obviously the angle becomes a bit smaller once it's loaded up, but it's minimal.

[Brezzi]

Anyhow, this is what the GC rear units look like:

*edited for length*

Awesome thanks for all the detailed info man. Do you have a build thread? I wanna see the rest!

[TobyB]

You know, you're carrying a ton of extra unsprung weight back there....
;)

By the way I figured it, a long time back now, the stock spring is at
about 75% of wheel distance, so the spring rate is something
like just a bit over half wheel rate (it's a squared relationship- it kinda mentally hurts)
The shock is something like 108%- but I forget the actual number.

So for the equivalent wheel rate, you'd end up with (approximately) half the spring rate
at the shock that you had at the stock position.

From what I remember.

The spring vs wheel rates in the stock position are truly phenominal- there's a lot of
force multiplication going on there....

t

[yamahammer]

Awesome thanks for all the detailed info man. Do you have a build thread? I wanna see the rest!

I do not have a build thread unfortunately. :(

You know, you're carrying a ton of extra unsprung weight back there....
;)

By the way I figured it, a long time back now, the stock spring is at
about 75% of wheel distance, so the spring rate is something
like just a bit over half wheel rate (it's a squared relationship- it kinda mentally hurts)
The shock is something like 108%- but I forget the actual number.

So for the equivalent wheel rate, you'd end up with (approximately) half the spring rate
at the shock that you had at the stock position.

From what I remember.

The spring vs wheel rates in the stock position are truly phenominal- there's a lot of
force multiplication going on there....

t

Yeah, two calipers isn't helping anything, but that entire setting is going away anyhow, need to make a big set-up for it.

**Corrected Below**

I have always just made spring rate changes based on how the car feels on track lol.

[TobyB]

Yeah, don't quote me on the dimensions.
I just remembered that the shock mounts
slightly behind the rear axle.

And yes, the motion ratio is linear. But the spring rate is a squared relationship.
I typed up a paragraph, it made no sense whatsoever.

So I hereby cop out and offer the same links I found when someone pointed out my
error of equating spring rate and motion ratio:

http://ismasupers.com/downloads/tech-talk/Tech-01%20Motion%20ratios-4.pdf

http://www.truechoicekoniracingservices.com/worksheet.pdf

http://performancetrends.com/Definitions/Wheel-Rate.htm

http://eibach.com/america/en/motorsport/products/suspension-worksheet

http://www.autospeed.com/cms/article.html?&title=Spring-and-Wheel-Rates&A=2904

actually copies the E30M3 performance data, and that site seems to be gone.
Which is too bad, because I think it had a really good explanation...
But I recognize the .67 motion ratio number, and I think THAT may actually be a real
number for the rear of a E30.

And yeah, :p once I had a 'first guess' on how stiff to make the rear spring when
going from stock to coilover, like you, I then changed based on how things handled!

t

[yamahammer]

Yeah I wasn't thinking this morning when I rolled lout of bed. I realized I did it wrong an hour later . I forgot factor compression distance into that calc. It should be: [1100*(75^2)]/(108^2). Gives around 530 lb. Which gives a percent decrease of %51.8. Forgot to divide the required force by the desired vertical distance (this is what gives the squared values your talking about.

Basic formula is simply: current_rate X (displacement_ratio)^2

Will strikeout my delirious early morning post