[BaadAssGremlins] Front Suspension Concerns

[Jay Nogg <nogg@xxxxxxxxxxx>]

Saw this on amc-list.com Archive.

Sounds like something we all should check
our cars for.


Jay









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
momentas 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
absorbimpact pressures from road hazards. 


There are a whole lot of weird monkey motions and strange pressure
profilesgoing 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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