[Amc-list] truck arms

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

Date: Tuesday, April 17, 2007 11:48 AM
From: Tom Jennings <tomj@xxxxxxx>

Mark, one thing I don't see in a lot of these truck-arm schemes
is explicitly allowing for roll. They all go up and down fine. I
realize also that the angled arms helps, but a flat pivot that
flexes in only one plane simply can't work.

------------------------------

"Truck Arms" do allow for roll, but as you noted roll is limited. The
angle of the arms and pivots allow for some roll motion, and the length
of the arms. The original Chevy truck arms are I beams, not boxed. The
beam itself twists as the vehicle rolls. It's designed specifically to
twist. The beams are relatively heavy in cross section, but are made
from 10-11 gauge material. They have a high resistance to bending in the
vertical plane due to depth, but a low resistance to twisting in the
horizontal plane. There is at least 6" of flex from one wheel to the
other, though I believe it is more like 8" IIRC. One wheel can be jacked
up for changing easily without affecting the other, or rather affecting
it no more than it would a leaf spring or torque tube suspension. 

Boxed beams are used on NASCAR chassis. Those are much stiffer than the
I beams used by Chevy and Hot Rods to Hell. HRTH sells the arms and
other components for custom installations
(http://www.hotrodstohell.net/catalog/catalog.htm). Stock Car Products
(http://www.stockcarproducts.com/trkarm.htm) also sells them for about
the same price (SCP $225 each, HRTH $220 each). SCP has a universal
crossmember for $96, so it would be about $550 for the parts to put one
in a 63-66 Classic (the most likely recipient) with a little welding.
I'd cut the crossmember to the correct width and weld on end plates that
would bolt under the car. 

I've though to build a set using 1"x3" 16 gauge rectangular tubing. That
should handle moderate power (300-400 hp) and be thin enough to flex
some. Not as much flex as an I beam, but should be enough to mimic a car
with a rear roll bar. I just wonder if a length of tubing could be
placed diagonally from one bar to the other to eliminate the need for a
panhard rod. There would have to be rubber bushings or spherical rod
ends to allow the arms to twist, but there's little enough movement and
pressure on rod ends in that application that they should last. They
won't last on the street when used to replace main suspension bushings,
such as on strut rods and A-arms.  

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