Tension Shackles

[tsm1@domain.elided (Tom Mandera)]

We've had discussions in the past about 3/4 eliptic suspensions and four
links and other fun stuff...

A friend of mine just finished shortening his '78 F250 and swapping a
Reverse '60 front and '70 rear (5.38s w/ Lockrights... woah!)... he
mentioned that he went from a tension shackle (spring above the shackle
mount) to a compression shackle (spring below the shackle mount), removed a
leaf, ditched some blocks, etc to come up with a softer rear suspension...
but with a little more axle/spring wrap up now.  I joked that he should've
put a 4-link in while he had it apart....

Combine that with a post to the BinderBulletin TechTalk a while ago
regarding Jim Maulis converting Scout IIs over to a rear tension-style
shackle arrangement... because it allows more droop... a thread on the NW4x4
list about articulation and droop being key... and it got me back to
thinking again (you knew I couldn't stay quiet for long, right?)

Most of this applies to my '72 Scout II project, but I'm planning on
springing-over the rear on my '77 in the Spring... using some flat '73
non-HD non-XLC rear springs.. since the 4" Skyjackers are just NOT tall
enough in the rear.. so it could apply to it...

Front suspension design for the '72 is currently looking like 69-75 3/4T
Travelall rear progressive rate leaf springs, reverse shackle, 3/4 eliptic
"buggy leaf" in a spring-over-axle setup.  Soft and pliant, yet strong
enough for the weight.. the "buggy leaf" will allow for extra droop over a
"conventional" setup.  Probably add "rails" just for peace of mind to keep
the buggy lyin' where it's supposed to lie when the suspension is "at-rest".
That design is pretty  much worked out.

But recall our previous discussions on the rear suspension... a 3/4 eliptic
setup isn't as easy to implement, since the rear springs are mounted
outboard of the frame, not directly under it like up front.  Sure, you could
move the rear springs *under* the frame (and net a few inches of lift) and
then do the buggy... but is that what we want to do?

Which brings me to thinking of the Jim Maulis' conversions of the Scout II
to using the tension style shackle in the rear.  This allows for more droop.
Just looking at the photos I took when testing my 33s out... my longer rear
shackles are pointing directly in-line with the spring when it's at
max-droop.   But the shackles are raked to the rear "at-rest."  If the
shackles were flipped (that is, inverted to a tension setup), at-rest would
have the shackle roughly vertical, maybe slightly rearward still.  As the
spring compresses, the shackle would move rearward (spring elongates) just
like a regular setup.  But as the spring droops, it would contract, pulling
the shackle forwards past vertical.. and, should the spring lengths, shackle
mount and length all be correct, it should also pull the shackle past the
horizontal position and even DOWN to point in-line with the spring *just
like it does now*... only the difference would be between the "max-droop"
level of the spring eye compared ot the "at-rest" level... with my current
setup, the difference is almost nill.  With a tension setup, that difference
could be 4" to 6".  Which translates directly into a lower tire aka more
droop aka more tire to the ground = more go.

The downside to the tension shackle setup?  You *lose* lift.  Either that,
or you have to build the shackle mount MUCH lower to compensate.. which just
means one more thing for the rocks to attack.

Maulis' solution (as I'm told) is to use a 4" lift spring... but you end up
with NO LIFT in the process!  Lots more droop though.

Hmmm... doesn't seem like a bad idea to KEEP my 4" Softride rear springs,
and just go to a tension shackle when I do the rear spring over on the '77.
Sounds like a plan.  They are sufficiently soft.. provide lots of droop
as-is, and I could see to it that the Scout sat level when I was done.

I wonder if I would need to "flip" the stock rear shackle mount over as-is,
or if I would / should need to move the shackle mount farther rearward a
little bit... if it's just a flip, hey... sounds like a plan!

That's fine.

Back to the '72 ground-up project... I had been planning on doing a rear
spring over and adding the appropriate mounts for a rear 4-link
suspension... but NOT implement it (yet)... thinking that I'd try it first
w/o, and if the wheel hop was bad, it's a lot easier to have had the mounts
in place when the body was off.. so just fab the links and go from there.
Because I want to play on the rocks, a 4-link would involve double
shackles.. one shackle at the front of the spring and one at the rear, so
the axle can follow the arc of the links w/o binding.  (one could almost
argue for a coil spring conversion after all this!  But... .naaahhhh)

So now I'm just trying to work it out in my head how to go about designing a
4-link setup with double shackles with a *tension shackle* in the mix.. 

Or maybe better yet.. do it with TWO... I suppose I could sit down with some
paper and some of my calc and trig n' engineering texts and start doing the
math and describing the arcs n' angles involved on how to *best* formulate a
mad creation like this.

I think two tension shackles would be ideal...

Ideally, set it up so at-rest both shackles are near vertical... compress
the rear springs, the shackles would both flip "outward" to near
horizontal... when drooping the spring would contract.. engineer it so that
the springs would contract sufficiently to bring the shackles inward to
perfectly horizontal and then *down*.

Who needs a buggy leaf in the rear?

Obviously lots more research and math will need to get involved before I
even buy the rod for the welder... but that's the beauty of a long-term
build-up... before I even get started I re-design it 5 times... ;-)

So... there is Tom's Thoughtful Engineering Concept for the day.. sorry for
such a long dry spell. ;-)

-Tom Mandera, Helena MT
http://www.tmcom.com/~tsm1/scout
'72 Scout II ground up project... gets more and more engineering thought put
into it every day...