Re: [Amc-list] Re; 'Flexible Flyers'

["Jim Blair" <carnuck@xxxxxxxxxxx>]

A: The upper arm could be done in tubular as it's the same as the 2wd cars. 
I know I'd be interested in uppers if they are available and not too much 
money (in the near future as fixing up my Jeeps and cabin is the current 
priority and my broken rib has slowed me down somewhat). My driveway is 4 
miles of rough road that punishes and sometimes claims cars and trucks alike 
(case in point: My '73 J4000 is only 1 of about 50 cars it claimed last 
year, not counting the cars stolen from town and dumped there by thieves)


From: "John Elle" <johnelle@xxxxxxxxx>
Subject: [Amc-list] Re; 'Flexible Flyers'
To: <amc-list@xxxxxxxxxxxx>
Message-ID: <001c01c7bd9f$085b2830$7cde0d82@john1>
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Brien,

SNIP
So boxing in / strengthening or recreating my Eagle control arms
in tubular steel - looking to get out all the flex - will lead
to
binding and excessive damage ?
SNIP

Read completely and understand. ARM not ARMS and specifically;

Snip
The
lower control arm is a scary poor design as is the geometry it
works
with. But it does work the way it was designed to work. It must
flex to
be functional and flex it does.
Snip

And according to my TSM the Eagle front suspension in terms of
the
lower control arm is the same as the two wheel drive AMC cars so
yes, I believe so otherwise I would not have stated it as such.

The design for the most part is a compromise of function vs cost
and
if a piece can be designed that will function according to the
objectives
and meet what ever safety standards apply and be the cheapest
possible
way to make the part, compromise enters the picture.
The first compromise for the lower control arm is they are
interchangeable
side to side thus needing to make only one part.
Another compromise is that they are used on all model AMC cars.
There is

no discernible difference that I have found in any 1970 plus
lower
control arm application from 1970 to end of production. The same
part
works for a Gremlin as it does for a fully loaded AMBO. This is
also
true
of the upper control arm.
How does it function?
It pivots on a bolt located on the engine support cross member.
This
same
cross member locates the inner end of the lower control arm. The
bolt
also operates as a cam to control front wheel camber.
The rubber bushing not only insulates the chassis from road
shock
transmitted to it from the front wheel but also allows some
resistance
to movement due to the fact that the center sleeve and the outer
shell
of the bushing is vulcanized to the metal, so nothing rotates
there, but

the rubber flexes allowing the lower control arm to pivot at
that point
as it moves up and down. The outer edge of the lower control arm
describes
and arc as it moves. The dimensions of the arc are determined by
the
radius
length of the lower control arm to the pivot point.
Solidly attached to the lower control arm is the strut rod. It
triangulates
the outer end of the lower control arm to the chassis and it's
pivot
point
is the strut rod mounting bracket located on the chassis about 2
feet
back
of the lower control arm, thus as the lower control arm moves up
and
down
the strut rod does too, along with a little bit of pivot action.
However the end of the strut rod also goes through an arc as
determined
of the strut rod describing a radius of a circle.
The pivot action of the strut rod is taken care of by the strut
rod
rubber
bushing design however in order for the end of the strut rod to
ride up
and down with the lower control arm there has to be some give
some where
and that give is in the rubber of the strut rod bushing.
Also if the end must prescribe and arc, and in part the
properties of
the
arc are determined by the adjustment of the Caster of the front
wheel
which is determined by the length of the strut rod and also by
the arc
moment of the lower control arm moving up and down. The bottom
line is
that
the lower control arm and the strut rod go through a compound
arcing
moment
as the tire moves up and down and the flex in the system is a
combination of
the following.
The strut rod is mounted in rubber allowing a pivot point for
one arc.
The lower control arm bushing is only pressed into one side with
the
other side a clearance hole around the bushing to allow some
flex of
the lower control arm at the bushing.
The bushing is rubber which gives some flex to the bushing lower
control

arm combination.
In addition the strut rod bushing and mounting is used to
position the
strut rod to maintain caster adjustment of the front wheels and
to
absorb
impact pressures from road hazards.
There are a whole lot of weird monkey motions and strange
pressure
profiles
going on in this front suspension design which in an of itself
is a
compromise to cost and function and application in the first
place.
Why was it done this way I dunno other than it probably worked
over the
design objective criteria as analyzed, if it stems from a
previous
design
tooling was probably in place and could be re-used with out a
capital
expense
and it probably met usage criteria assuming factory recommended
maintenance
took place.
Plus past warranty for the most part hands could be washed of
it.
What problems could have entered to alter assumptions over the
years?
The
change in Caster from - numbers to + numbers for one. - number
Caster
made
it easier to steer the car with out power steering as front end
weight
increased. Plus numbers favors self centering which could
eliminate
wander
at speed. If the original design was optimized for - caster,
simply
adjusting
+ caster in would put it towards one end of it's design
parameter
causing
possible reliability problems.
What I see when I go after cores, the lower control arm is bent,
twisted
and
broken at the bushing point along with the hole wore out so a
new
bushing
will fall out of the control arm.
This is probably caused by improper maintenance for the most
part as the
older
and more neglected these cars got the worse the wear out issues
became.
As far as polyurethane goes! Installing polyurethane bushings in
the
lower
control arm and strut rod locations eliminate the flex that is
required
for
proper operation of these parts in that location.
There will be incredible resistance to any of these parts moving
the
arcing
motion they should do.
What will happen to them. The lower control arm will bend and
flex and
fatigue and break at the lower control arm bushing location.
Why?
Everything
there is designed to flex, anything you do to eliminate flex of
the
lower
control arm over the normal operating range will force parts to
bend and

flex and possibly break some where else. Not good. If you want
to limit
the
movement of these parts and then re-enforce them you might be
able to
get
away with it but the next question for me is how long.
You asked, this is my answer from looking at damaged parts,
damaged
parts
don't lie.
I will not install polyurethane bushings in either the lower
control arm

or the strut rod for the above reasons.
What you do with it is your decisions.
You asked, this is my answer. And as the refrain goes I'm
sticking to
it.
John.

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