Tension Shackles...

[jmbrodsky@domain.elided]

Devin,

        Unfortunately to burst bubbles, almost EVERY Big 3 truck, and most IH 
pickups have Tension Shackles.  Virtually every truck of 3/4T or greater has 
tension shackles as well.  Take a gander under a truck and look at the rear 
spring, rear shackle.  Most are in tension.  It has a more predictable action, 
and tends not to distort the spring pack to the extent that a compressino 
shackle does.  Also the brackets that hold the shackle to the frame are inverted
from what you consider to be 'normal'.  I have seen people with Chevy's cut the 
bottom of a tension shackle bracket out, and make a compression shackle and net 
about 6 inches of lift while retaining the factory spring rate.  It does greatly
reduce the strength of the bracket, though.  This setup is also preferred on 
larger trucks with overload springs since the design allows more clear room 
above the spring pack for the overload pack.  Thus allowing a longer overload 
pack to be installed.  There is a 5 Ton Mack at my Uncles shop now, with this 
setup.  Loks like you only get about 3" of travel, then the overloads, then rock
hard like stone!  The other benefit of a tension shackle setup is the 
elimination of the potential flip-lock action of leaf springs and shackles.  In 
a compression setup the shackle is free to rotate almost 180 deg. back and 
forth.  The tension setup rarely moves that far, more like 45 deg, in either 
direction.  Take a look.  Let me know what you see.


     -Joel Brodsky
        www.physics.arizona.edu/~jbrodsky

         '76 IHC Scout II 345/tf727 RE8000, Cibie40s, custom 
                rear tire rack, 6 different color body panels.
                Not much sport, all utility.
         '75 IHC Travelall 150 4wd 392/tf727 Hummer brush guard, 
                3/4T rear springs, Hella Rallye 2000s, Con-Ferr 5x7,
                Class IV hitches front and rear, 3" Exhaust.
                1st Place, Rocky Mountain IH Rendezvous,
                3rd Place, Tulare IH Western Regionals.
         '74 IHC Travelall 150? 2wd 392/tf727 "The 'new' one"
                Factory AM/FM, Cruise, Tilt, AC, Captains Chairs,
                Rblt, Alt, carb, fuel pump, ps pump, water pump, dash,
                seats, rear door, starter, master cyl, what could be left?


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Date: Sun, 13 Dec 1998 17:22:45 EST
From: TERNOV@domain.elided
Subject: Re: tension shackles

It would certainly be interesting to see such a setup. However, it seems to me
that there are a few basic flaws with the design. Again, I have never seen any
such a setup up close, let alone riden in a truck equiped with them, but it
seems that while the setup would theoretically offer massive droop, the
transition from horizontal shackles, and the past horizontal, nearing vertical
once again phase would be very jarring. In fact, I don't even think its
feasible for the shackles to pass horizantal, and droop down further, unless
there were enough upward force on the opposite wheel to force the spring down.
The spring would have a tendency to keep the shackles at a horizantal
position, because this is the position where the spring is the shortest
lengthwise, which is by definition, maximum droop. The very function of
shackles actually  defines the fact that even if the springs were forced to
invert the shackles, the spring would flatten proportionally such that the
wheel would be in the same vertical position and no net travel would be
achieved. (Draw a picture, my words are probably hard to piece together) Sorry
to rain on your parade. Its a good idea. Buggy spring is probably a good idea
if you're looking for that kind of travel.
Devin Smith
'65 Scout 80 turbo

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