Rear Shackle Geometry - how are you adjusting it?

Folks!

Alright...so my Coupe is no drag racing machine, however proper setup should never be lost on anyone, so I've started to look at the leaf-spring geometry seeing that last time out at the track I was showing dismal 60' times and the video of car launching showed barely any chassis movement with a good amount of wheel spin (275-65R15 Nitto NT555Rs, so there is plenty of tire there).

NOTE: I suppose I shouldn't be surprised by this seeing as the car is much more focused (setup wise) on street handling: big sway bars, tight shocks, etc.

...but the angle the spring rear shackle makes has always bothered me. I don't have a picture handy at the moment, but the lower part of the shackle is ever so slightly leaning forward (at the spring eye), which is telling me that when launching the spring doesn't really have the "room" to flex and cause the body to rise - which normally is what you want to do b/c that body rise is actually what plants the tires against the pavement.

So off I went measuring my sping front and rear segments: 24.5" on the front and 34" on the rear, which is per spec (allow for +/- 0.5" here seeing as this isn't precisely off of the center bolt to the center of the front/ rear spring eye).

I am using a 5 leaf pack, and the rear of the car has always been somewhat high. I feel like it would be better for it to be about 1" lower.

All of this begs the question: so when our F/M/J-body cars needs such geomtery adjustments, what are you guys doing here?

Our rear suspension is different from the earlier Mopars where you could shim the shackle mounting bracket and that would in turn move the shackle pivot forward thus allowing the lower part of the shackle to lean/point back.

It would seem that the only way on our rides to accomplish the same thing is to re-locate the shackle mounting bracket forward on the frame rail and well, given that this is all boxed in I honestly can't quite fathom doing this w/o cutting things up, etc.

I did consider making a plate that would itself mount into the current as-is shackle bracket mounting points, but which would effectively relocate the NEW mounting point forward...that seems doable, at worst it requires some clearancing holes to be drilled in the framerail and of course the plate itself needs the nuts welded on so that the shackle mounting bracket can attach to it.

....but even that approach seems like it's very R&D (LOL)...so is there an easier way that I'm simply overlooking here?

Maybe a spring slider like these Calvert pieces (SPRING SLIDERS for Calvert Springs (SL-300CR ) – Calvert Racing, Inc. ...but even these would still require some creative way to mount them onto the rear frame rails!

As always, thanks for ideas and suggestions!!!

Body lift is more than likely controlled by the leaf springs, I use SS springs with the appropriate hangers but the rear hangers have always leaned slightly forward. I get four or five inches of body lift from the rear springs with the car capable of running 1.3x 60 foot times. Do you have you have the clamps removed on the rear half of the springs? Also when I used stock rear springs I also used traction bars, the only down fall was the wheels up launching, the long slapper bars tended to lift the front end also.

I hate to sound dismissive , but it sounds like you may be "over thinking " this. Sounds like the suspension is just way too stiff for what your hoping to do and you've already mentioned that the rear body sits higher than you want. Sounds like a simple spring replacement , (maybe 4 leaf) would help the situation with much less work and expense. A taller/adjustable pinion snubber wouldn't hurt either.

Oldiron440 said:

...I use SS springs with the appropriate hangers but the rear hangers have always leaned slightly forward. I get four or five inches of body lift from the rear springs with the car capable of running 1.3x 60 foot times....

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As it happens I have the required front hangers as well to install the shorter (front segment) SS springs, but honestly was hoping that would not be necessary. My best 60' time was 1.870 and most of them are over 2.0.

Oldiron440 said:

...Do you have you have the clamps removed on the rear half of the springs?...

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Nope, I have the clamps placed at all the factory locations.

Infiniti Dave said:

I hate to sound dismissive , but it sounds like you may be "over thinking " this. Sounds like the suspension is just way too stiff for what your hoping to do and you've already mentioned that the rear body sits higher than you want. ..

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No, you do not sound dismissive at all. I am well aware of the limitations of my setup right now, or to be more correct the limitations of the adjustments I can make given my primary criteria (that being road handling), but I figured I'd run this by the group here to see what others may have done in the past.

Infiniti Dave said:

...Sounds like a simple spring replacement , (maybe 4 leaf) would help the situation with much less work and expense. A taller/adjustable pinion snubber wouldn't hurt either...

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You are probably right in that a 4 leaf setup would drop the ride height and be the right move for drag racing performance focus...it would also go the opposite way I want it to go re: road handling though.

Yes, I have an adjustable pinion snubber already installed, 1" from the floor and honestly I doubt it actually manages to travel up that far. But but...that brings up a good point: I'm going to set it up dead against the floor, aka manual trans style to see what that might do...worth a try!

If you want your cake and eat it too then you need to start with double adjustable shocks on all four corners.

Something else I forgot to mention is that I use shocks to that are the length of the shocks on the rear of a half ton pickup, 2” longer than the stock shocks.