[Amc-list] truck arms

[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.

They might "get away with it" due to combined tolerances, flex
and the give of rubber bushings, but it is at least stressful
on critical parts.

(By roll I mean, hold the car level, raise the left rear wheel
and lower the right rear wheel.)

Torque tube AMCs behave perfectly here.

Open Hitchkiss style with rear leafs throws the pinion angle
off very slightly in two planes but obviously not enough to
worry about (it's worked for 80 years).



When I was contemplating such a thing for a torque tube car,
the simplest fittable scheme I came up with was two very long
(6 foot) "arms".

At the trans crossmember end, they have threaded ends like an AMC
front suspension strut. They connect at the trans crossmember
with ordinary AMC front strut bushings into a suitable (1/4"
thick) bracket on either side of, and as close as possible to,
the driveshaft.

At the axle end, the arm end fits into a tubular socket (say
6" long for ease of construction) affixed to the axle tube.
This socket would be angled on the axle tube, looking down from
the top.

With 6 foot arms there would be little motion at the strut
bushings.  Threaded ends would allow rear alignment. Axle
wrap-up would be easily held by the long arms. The motion at
the bushing end would be no more than in the front suspension.

It STILL doesn't properly handle the distortion of the
axle-arm-arm triangle. If you look at old Model-T type rear
wishbones, they have an actual ball and socket at the x-member
end; this lets the axle-arm-arm triangle rotate without
distortion.


(I think copying the Model-T geometry, with a "V" of rigid arms
attached to the rear axle tube, the apex of the "V" connecting
to the trans x-member with a single ball joint, would actually
be best.  To keep the ball joint off the ground it would need
some U-shaped up-and-over the driveshaft or something.)



For body roll: the force on the arms is somewhat complex.
The triangle of the axle+arm+arm wants to rotate against it's
virtual apex, which would be a foot or more in front of where
the bushings mount into the x-member.  So it distorts in two
planes (this is the critical part) causing stress on the arms
and joints. This translates to the arms *rotating in place*,
twisting lengthwise, but not simply. A pivot in the rear
and rubber in front will relieve nearly all of this, but not
all. There is non-fore-aft force on the strut bushings during
this time, but with the arm length no more than on the front
suspension strut.

Even doing the whole thing with heims at both ends of the arms
does NOT eliminate the torquing of the axle-arm-arm triangle.
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