As promised, here are the two remaining views of the rear frame rail and cradle with the control arms from the front suspension superimposed on them. Again, this is just a crude way to get an idea how well or ill-suited the stock lower frame rail is oriented to locate some mounts for an upper control arm and a pushrod shock. The rear view was posted earlier, so here is the top view with the lower control arm being blue and the top one red:



Once again, I'm not planning to replace the '88 lateral links with a lower control arm. The only reason I left the lower control arm on the diagram was to show the relative positions of the lateral link mounts on the cradle vs where they would need to be to retain an identical geometry with the front. The lower control arm's function is replaced by the two lateral links which are roughly 35 mm shorter than the aft leg of the lower control arm and roughly 65 mm shorter than the forward leg. This view shows clearly that the front geometry cannot be incorporated lock, stock, and barrel onto the rear, but that doesn't mean it's not possible to achieve a similar performance.

Since the lateral links are shorter than the lower control arm, the result would be decreased camber gain in jounce vs the front suspension, if the upper control arm were identical. But this drawing (and the rear view one posted earlier) show that the upper control arm would need to be shortened to bring the ball joint in line with the top of the knuckle. The impact of that change would be to increase the camber gain in jounce... perhaps enough to compensate for the effect of the shorter lateral links.

Here is the side view:



Of course the upper control arm is angled for anti dive when anti-squat is what's needed, so ignore the front to back slope of the upper control arm. The other thing that jumps to mind in this drawing is the amount of caster, indicated by the misalignment of upper and lower ball joints vertically. Caster is needed in the front suspension to make steering more stable, but not needed in the rear so the upper ball joint location could be moved towards the front of the car if it were useful to do so. The only other notable point I can think of about this drawing is that it shows how much room there is rearward for a fore-and-aft oriented coilover similar to Datsun1973's design.

Overall then, the layout and location of the stock rear frame rail appears to be amenable to an SLA-like suspension, only in htis case it would be more like a four(?) link design (is the shock pushrod considered a link?). Certainly worth further development in my opinion, so I'll continue refining these drawings by looking into a suitable upper control arm pivot design or two, and a pushrod shock system. Questions or constructive comments are always welcome.

(Edit: corrected side view drawing to illustrate correct ground clearance)

[This message has been edited by Bloozberry (edited 09-16-2012).]

I've spent the last couple weeks working on and off trying to refine the location of a new upper control arm. I must admit it's taken much more time than I expected mostly because I needed to leave it alone on many occasions to let things sink in, and because I don't have simulation software. I had to try many iterations using a digital stick model approach to narrow down the best coordinates for the new arm. I wanted to make things as easy for myself as possible in the fabrication stage too so I began by setting some design limitations. Though it didn't necessarily make things easy at this stage, I decided to:

a. leave the locations of the lateral and the trailing links intact (following the 6" track width increase);

b. use the upper or lower strut mounting holes on the knuckle to locate the outer pivot point of the new upper control arm;

c. use the 25 mm raised cradle; and

d. leave a 125 mm ground clearance to the bottom of the center of the cradle.

Starting with the rear view, my goal was to locate the coordinates of the inboard pivot point for the new upper control arm, such that the roll center movement and camber gain were improved over the stock configuration. In my head it seemed simple enough to locate a single pivot point, but practically speaking it was anything but.

Initially I just picked a random point near the lower frame rail for the inboard upper control arm pivot, then I rolled the body over 6 degrees and noted what happened to the roll center and the camber of both tires. To keep the impact of jounce out of the equation, I made sure that when I rolled the body I kept the ground clearance as measured at the centerline of the cradle at 125 mm. This may not prove to be what happens in real life but I needed to hold this constant to prevent too many variables from wreaking havoc on my simplistic methodology.

I then changed the location of the inboard pivot point, and repeated the experiment until I got a feel for how the camber and roll center were impacted by various vertical and lateral displacements of the pivot point. I was amazed at how sensitive the camber gain was with only very minor pivot point changes, but I was eventually able to find a location near the bottom outside corner of the lower frame rail that seems to be a good compromise between both roll center movement and camber gain. Whether this pans out when these coordinates get plugged into the Lotus Suspension Analyzer software or not is yet to be seen, but as I said, it's a good start.

Using the best coordinates I found for the uppper control arm, this first drawing shows the static location of the rear roll center being about 92 mm above the ground. If you recall, the static roll center of the modified front suspension is lower at 72.4 mm. The result is a roll axis that is raised slightly at the rear. This is a good characteristic and is what FieroWannaBe was referring to earlier as the roll axis inclination. I won't get into details at this point but suffice to say that this new design is on the right track.



(I apologize in advance for the poor resolution of these drawings, but unfortunately it's a draw back to using PIP.)

This next drawing shows how the same suspension behaves when the body is rolled 6 degrees to the left, as it would be in a high G right hand turn. The most important things to note are that the camber change is almost perfect (excluding the effects of bushing deflection). The outside (LH) wheel counteracts the body roll by tilting the same amount in the opposite direction, which keeps the tire contact patch flat with respect to the ground, giving maximum traction. The inside (RH) wheel comes within 1 degree of countering the body roll angle, which is a significant improvement over stock.



The other important thing to note is that the roll center moved considerably less in this design than my earlier stage 1 or 2 modifications. Here, in a 6 degree roll, the roll center migrates only about 7 mm down and 320 mm outboard as compared to nearly 470 mm down and 2670 mm outboard in my stage 2 design using Chapman struts. Again, I still have to verify that my drawings are correct using the Lotus Software. To do that though, I'll need work on the top and side view drawings to get the additional coordinates in the x-plane.

(Edited to rename both drawings)

[This message has been edited by Bloozberry (edited 10-20-2012).]