rear springs part 3

Choosing spring rate will control how how much travel is available in droop before the spring is unweighted.  It will also be related to how easily the car bottoms out.  Especially since inclined springs mean that I have a falling rate suspension.  The shocks that I have give three numbers, fully compressed length, fully extended length, and ride height.
For my shocks ALN5855P we have:

Extended height: 16.375 inches
Compressed height: 11.125 inches
ride height: 13.5 - 14 inches

Since coil overs are adjustable, I can moved the collars up or down to put the shock length in the ride height range regardless of the spring rate I choose.  However, suppose I choose a spring that is way too stiff. So that the spring only compresses .5 inches when I put the car onto its wheels.  If I put the ride height where recommended (14 inches), then we run out of load on the wheel at 14.5 inches extension.

Ideally, I want the wheels to become unweighted at full extension of the shock.  I previously measured the unsprung weight of the car at a surprising 330lbf or (165lbf per side) so a spring that sags 2.375 inches from 165 lbf would be 70 lbf /inch.  Since I have a 14° installation angle that becomes about 80 lbf / in.

That seems really low.  I have concerns that changing fuel load or passengers will significantly change ride height, and that the heavy axle when moving upwards in bump will be too likely to reach the bump stops in street driving.  I bumped the rate up to 150 lbf / inch.  Interesting to note that if you buy the cheap summit springs, you can see that it has QA1's logo faintly through the finish...

I finished the mount and put the springs and shocks in.  Here is a shot from the back.  I think this flies under the radar nicely.

I got in the trunk and jumped up and down.  The suspension moved freely about the same magnitude as any passenger car would.  Encouraging.  A few things were rubbing / not lined up how I wanted, so I will take a couple days to adjust some things, remove and paint all the rear suspension bits and attach them permanently.

Hey, kids it's me! I bet you thought that I was dead!

I have actually been working, just not posting.  I expected to do the rear spring mounts really quick and then just post it all when it was done.  It turned into an epic and still isn't done, but close.


I had to drill out this slot to .5 inch.  It grabs a drill bit the instant it touches it. Sooo

Welded a cover over the hole...

Drilled the hole...

Then cut off the cover.  Bolt on this bracket:

 

 Make the other side:

weld on a standoff:

For where the shock mounts to the car, I had to move the mounts up quite a bit from the bar I made back in part 1.

Figure it out with cut up cards.  From the calculations (I will post these shortly) we see that as the springs depart from vertical orientation we get a falling rate suspension.  In other words the more the spring compresses, the lower the wheel rate becomes.  You want wheel rate constant or at least rising, so that there is nice compliant ride at small displacement, and then becomes stiffer as the spring deflects to help prevent hitting the suspension bump stops.

If I mount the springs vertical and near the differential like they do on drag cars, then I have a suspension that for a given bump stiffness is not stiff in roll.  So I am aiming to have the shocks as close to vertical and as wide as I can. I also have to setup in the range the manufacturer lists for ride height for the shock.  And pick the springs that will support this ride height.

 

Lay it out (handy to have the plasma cutter here to let me nest these close)

Had to awkwardly tig tack these since the mig won't reach. 
Right hand holds the torch
Left hand jams the filler
Other left hand holds the parts

 I am further than this, but don't have enough pictures to show the story.  Will post again later...