Control Arm Tests
These tests may satisfy us, but that doesn't mean they have to
satisfy you. If you're building yourself a car, you're going to
have to decide for yourself what is good enough and what is not.
We'll give you honest test results and hope they'll help you make
an informed decision, but the bottom line is, if a suspension
member falls off your car, you and/or others may be injured or
killed. So take what you're doing seriously, and in your search
for information, consider how it relates to your particular project.
our latest upper control arms are different from "standard
Locost parts" (as if there were such a thing) we felt they
needed structural testing before they hit the streets. Our concern
was the tab-and-clevis junction between the front and rear tubes all
the other components have a long history in LocostLand and
though the connection looked good on paper, we're belt-and-suspenders
folks and like to back up our calculations with physical tests.
Here's a right control
arm, as seen from the bottom.
a closer look.
tab is laser cut from 1/4" mild steel plate. Hole diameter
is .377" (lasers aren't perfect), and at the thinnest
point, the edge of the hole is .310" from the edge of
ball joint shown is a Moog ES323R, a component I don't currently
recommend for reasons you'll soon see.
pneumatic ram (above, left) has a 7 square inch piston,
which is hoisted by regulated air a maximum of 840
pounds ram force at an air pressure of 120 pounds per square
The upper arm
(see closeup below) has four lifting points, which use leverage
to increase the ram force by a factor of 3, 4, 5 or 6, and
convert the force from compression to tension.
above is experiencing 5000 pounds of tensile force; 120
pounds psi times 7 square inches times 6/1 leverage factor...okay,
that's 5040 pounds, but there are some friction losses so
we've rounded it down to 5000.
By the way,
at 4500 pounds tension (I back the air pressure off 10%
before I get near the thing) that 3/8" chain feels
like an andiron.
that's the tensionizer. The structure is made from 1-1/2"
.095 mild steel square tubing with 1-1/2" .125 mild
steel plates at the fulcrum.
| Load carriers
are made as needed, from 1/8" mild steel, 3/8" hardened
chain, and 1/2" grade 2 bolts (which did dent some during
the 5000 pound chain pull test).
our first pass at a control arm carrier. Essentially it
simulates the frame, and is made of the same materials as
the tension generator.
of holes on the lower arm of the fixture allow us to pull
the control arm from a variety of angles. The settings above
simulate a combination of braking and cornering forces;
roughly 75% braking and 25% cornering, with a 2400 pound
load. The 1" square tubes along the sides are guides
to keep the control arm from flopping over to one side as
the loads go up. Note that the 1-1/2" square tube that
is the base of the control arm fixture is bent. After the
control arm passed the 2400 pound test, I tried for 3200
pounds and the base tube yeilded--I couldn't get the load
over 2800 pounds.
That was the
most interesting and surprising result of the tests. I hadn't
bothered calculating the load and strain on the fixture
Well, because Locosts are made out of 1 x 1 x .065 and that
base tube is three times (a horseback estimate, but close
enough) stronger and stiffer than the chassis tubes, so
why even check, right? The fixture kept bending until the
piston topped out at 2800 pounds.
it was staring to yeild at 2400 but I can't say for sure
because I didn't check it that pass...let's see; to get
a 2400 pound load straight ahead, figure a 1600 pound car
(1350 plus driver and fuel) with a weight shift on braking
of 75% front wheels, 25% rear wheels is 600 pounds per front
wheel, so you'd get a 2400 pound load from roughly 4 Gs
braking force...that's a lot but you wouldn't expect permanent
frame deformation, would you?
preliminary results imply that the chassis may be the weak
point if one is, for example, screwing around on the street
and finds an unexpected speed bump in the way and hits the
brakes hard at the wrong time.
we bult a new control arm carrier, using 1-1/2" x 2-1/2"
.083 rectangular tubing nothing engineered, just the
strongest stuff we could find at the steel store and
ran it up to 3200 pounds. Also we traded the steel rear for
a Rod End Supply swaged aluminum radius rod 'cause I've been
curious what it would take to pull the threads out of those.
practice, 3200 pounds is an impact load for a Locost, and
unless you're building yours with a Chevy Rat or a Ford Windsor
under the hood, you'll have to hit something solid to put
this sort of load into your upper control arm. Still it's
nice to know there is ample safety margin in the tab-and-clevis
3200 pounds pretty well uses up the safety margin of the
5/8" rod end on the spindle side of the control arm.
It had shown the teensiest symptom of yeild from an earlier
pass (the jam nut, which had been put on finger-tight, felt
slightly draggy to the touch after 2400 pounds), but it's
unmistakeable here. This particular ball joint is an EM10
from RES's "economy" line; since this was expected
to be the first yeild point I figured I'd bend a cheap one
instead of an LXM10.
The tab yeilded
a bit too. The 3/8" hole elongated .007" and flared
the steel on the load side about .002" which
was enough to make it hard to disassemble but not something
I see as a failure risk.
If I ever
hit an unyeilding object (a pothole, a fireplug, a Chevy
Suburban) I'll probably look here first to see if the upper
balljoint/rod end/track rod bent. And I think I won't use
ES323 track rod ends any more, because they're a half inch
longer than a standard 5/8" ball joint, and thus have
a longer cantilever and thus more stress where they spigot
into the control arm.
The Moog ES312RL
is the same length as the ball joint, so that's my standard
for the time being. Its only drawback is it's only available
in left thread. Oh yes, and you need a tapered reamer to
get its stud to fit full length into the Miata spindle.
be interested to know if the history of minor Locost accidents
(and there's gotta be a few by now) shows the upper outboard
rod end yeilds before the chassis in real life. I'd wager
the chassis bends first.
that 1-1/2" x 2-1/2" .083 rectangular tubing after
the 3200 pound test. It's bent. After the load came off,
it stayed bent. The load/distance on this tube is about
double what it is on the book frame's FU1 and FU2, but still,
this is a pretty sturdy tube compared to 1 x 1 x .065".
Since we have
CMC Locost frames stacked up like cordwood around here,
I'm sorely tempted to reproduce these tests on an actual
frame, but I'm pretty darn sure the result will be permanent
deformation forward of the firewall. But that's another
test; this was a test of control arm strength, and I credit
it as a Pass With Honors. Well, maybe a B+ for the outboard
rod end, but an A for the arm itself.
that the clevis-and-tab arrangement is sufficient to the
needs of this control arm, so as of 10/28/06 the clevises
and tabs are available for purchase. The clevises are $4.75
and the tabs are a whopping ten bucks; at which price the
first 30 cars worth will cover our expenses drafting,
converting the drawing to code to drive the laser, materials
and cutting charge, fixtures and test samples and
the next 30 will reimburse me for my time on this particular
project, at roughly minimum wage. After that I'll be rolling
in dough, but for now I'm just lucky I enjoy this work.
For those who
are even lower on the wage scale than Yours Truly, we're
providing a tab drawing (see below) so you can make your
own tabs for your own car.
For those who
are higher on said scale, we'll deburr the clevis, sand
the tab down to a snug fit, and package them together for
$5.25 a job you can easily do yourself with some 100
grit sandpaper, a sanding block, and a popsicle stick.
For more fabrication
information regarding these control arms, click here.
That will put you in the middle of an earlier [Sage advice>Works
in Progress] page about Quick Adjust
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