HOW DO YOU KNOW AN EVEN NUMBER OF SHIMS WOULD
PUT THE TWO A-ARMS IN PARALLEL MOTION TO BEGIN WITH!
My factory fresh 1980 Spit did not have an even number of shims to begin
with when I bought it new back in 1980.
So... anyone blindly believing that you should only have an even number of
shims better bust out there ruler and measure measure measure!
Paul Tegler wizardz@toad.net http://www.teglerizer.com
-----Original Message-----
From: Nolan Penney <npenney@erols.com>
To: spitfire-enthusiast@egroups.com <spitfire-enthusiast@egroups.com>;
triumphs@autox.team.net <triumphs@autox.team.net>; spitfires@autox.team.net
<spitfires@autox.team.net>; nass@egroups.com <nass@egroups.com>
Date: Friday, October 27, 2000 11:12 PM
Subject: Re: Door handle seals and # of spacers on A-arms
>
>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.
>
|