Paul, what you've learned applies to suspensions with double ball
joints, as I described, not to trunion suspensions.
A trunion can pivot about two axis only. Without the third axis of
rotation, it breaks if forced that way. And tweaking the upper and
lower suspension arms out of parallel travel to adjust caster requires
that third axis of rotation. Can you get away with it? Maybe...for a
little while. Metal bends before it breaks, and the nylon bushings will
deflect a bit. But in any case, that is how you break trunions, and why
the industry abandoned them for ball joints on the upper and lower
arms. With double ball joints, you can tweak to your hearts content
without worry of breaking things.
If you want a more detailed explanation, it's below. Otherwise, skip
it.
The first axis of rotation is for turning the wheels side to side, alo
known as steering the car. We'll call that "X". The second axis of
rotation is for the up and down travel. This is how the wheels stay
upright as the suspension moves up and down. Call it "Y". That's the
two axis of rotation the trunion has. There is no more available with
this trunion design.
If the upper and lower suspension arms are traveling perfectly parallel
in their arcs, that is fine. Those two axes are enough to do the job,
because it's only a two dimensional motion. But what happens if the two
arms aren't traveling perfectly parallel?
If the two arms aren't parallel in their travel, then the upright (call
it a kingpin for simplicity) goes through some rocking along the third
axis, called "Z". If you can't visualize it, try this. The upper arm
is perfectly flat, going straight up and down. The lower arm is tweaked
forward to impart positive caster. The lower arm would go back as it
goes up, and go forward as it goes down. So the kingpin would be rocked
into positive caster at the lower limit of suspension travel, and
negative caster at the upper limit of suspension travel. A trunion
can't do that though. It can only pivot along the x and y axis. If you
try and make it do this, something has to give.
What happens if you do impart some of this Z axis twist? If it's only a
slight bit, you'll flex things, causing a bind in the travel and wearing
parts out quickly (this is why some cars eat trunion bushings btw). If
it's more then slight, you cause stress fractures and quickly fatigue
things like the trunion, breaking it (which should sound familar to some
folks). This would also cause greater suspension bind.
This doesn't mean you mustn't use unequal amounts of shims. Quite the
opposite. You must use whatever number of shims are required to get the
upper and lower arms in perfectly parallel travel arcs, compensating for
bent frames, poorly machined brackets, and whatever other ailments
existed in manufacturing and the life the car has lived to this day.
As one more monkey in the barrel, if the upper and lower arms are not
equal length, it's not enough to have them traveling parallel arcs, they
must also be perfectly aligned over one another. Unequal arms cause
twisting action along the Z axis if they are merely parallel and not
aligned in their arcs. The greater the difference in arm length, the
greater the twisting along the Z axis.
Because of this inherent limitation with trunions, the automotive
industry abandoned them for ball joints on both a-arms. A ball joint
can pivot along all three axis in suspension travel. This makes it more
flexible in use, more tunable, and much less fragile.
You can find this binding problem pretty quickly in a Spitfire. Unbolt
the shock assembly to eliminate that restriction. Disconnect the
swaybar and remove the wheel (it's heavy). Now you can lift the
suspension through it's range of motion. If it's binding because of
this misalignment of the upper and lower a-arms, you'll feel it as
binding. There will be a change in the force required to lift the
suspension.
If you do find this binding, fix it, and then consider replacing the
trunion. Normally I'm not a big fan of simply replacing pieces, but if
a trunion breaks, it's a most unpleasant experience, and not worth
gambling on in my opinion.
|