Motion ratio help?

I'm trying to calculate the motion ratio for a Saab 900. This is a double wishbone set-up, with the spring perch on the top of the upper wishbone, like this:

My question:

Do I use the distance from the center of the spring perch to the UPPER ball joint or to the LOWER ball joint, which is farther out from the spring than the upper? My sense is that I should use the lower ball joint's distance because it has a greater mechanical advantage than the upper that it is exerting on the spring via the spindle. Or does it not matter how the force at the upper joint gets there?

The web shows only SLAs with the spring attached to the lower arm and I'm not finding any clarification, hence my question here.

EDIT: Now with 100% less Alabama, 100% more drawing...

FWIW, I only see an Alabama State flag with a note in the middle where your picture is supposed to be.

I think it is clear what your question is without the picture and I bet there will be actual answers in no time. Me, I'm not really sure. I would guess the upper ball joint because the spring is attached to the upper control arm. (That's just a guess).

i think the answer is "none of the above". motion ratio is the change in length of the spring for a given amount of vertical travel of the contact patch of the tire.

google "define motion ratio" for all sorts of helpful links.

from the wiki entry, which references Milliken as well as Carroll Smith:

The most common example is in a vehicle's suspension, where it is used to describe the displacement and forces in the springs and shock absorbers. The force in the spring is (roughly) the vertical force at the contact patch divided by the motion ratio, and the wheel rate is the spring rate divided by the motion ratio squared. This is described as the Installation Ratio in the reference. Motion Ratio is the more common term in the industry, but sometimes is used to mean the inverse of the above definition.

I would use the lower ball joint if it's outboard of the upper BJ and from the picture you posted it looks like it.

Maybe I'm just dense, but the Wiki isn't clarifying. I understand what the motion ratio is but actually measuring wheel travel / spring travel is much less convenient than simply measuring some control arms I have. (Living in the city has disadvantages.) Puhn's book (somewhat obliquely), a PDF from Eibach, and countless other websites instruct to calc the ratio by simply taking the distance from the control arm pivot point to the spring seat (d1) and divide by the distance from the pivot point to the ball joint (d2):

If you're planning to change springs, don't forget that wheel rate = spring rate * motion ratio ^2. The squared term makes a big difference.

OK, i'll go along with using the UCA dimensions since the spring mounts to the UCA. But if the spring is not vertical, you'll have to use the cosine of the angle between the axis of the coil spring and the vertical.
REM: cos 0 = 1; cos 90 = 0.

Could someone tell me what I'm doing wrong here? I'm (still!) trying to calc the wheel rate and I'm coming up with an answer that's higher than the spring rate.

My numbers:

d1 = 9.13" and d2 = 11.89"

Spring angle A = 12 degrees from vertical (Cos 12* = 0.978)

Spring rate = 550 lb/in

550/((9.13/11.89)^2*(0.978)) = 953

Either I'm wrong or Eibach is. I know that the control arm has a mechanical advantage over the spring, so the wheel rate should be less than the spring rate.

I'm pretty much useless when it comes to math but my inclination is to multiply the motion ratio and angle correction factor by the spring rate, not divide, and I see now that this is what kb58 has written. So, then, can I hang my hat on this:

550 * 0.978 * ((9.13 / 11.89)^2) = 317

Thanks...