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Reply-To: lancair.list@olsusa.com
From: "Gary Casey" <glcasey@gte.net>
To: <lancair.list@olsusa.com>
Subject: Super ES taxi shake
Date: Thu, 8 Nov 2001 06:28:44 -0800
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>>I've read in the Piper Owner's Forum that shimmy can be caused by an
out-of-balance nose wheel.  Improper mounting, uneven wear or a defective
tire can cause this.  A few days ago, there was a posting and link here on
some excellent material from Dunlop that described, among other things, the
proper mounting procedures.<<

I'm new to this list (today!) so pardon me if I repeat someone.  I have
owned a Cherokee and a couple of Cessnas all with some degree of shimmy, so
my research into the problem goes back about 20 years.  Here are at least
some the potential causes:

1.  Shimmy damper low on oil.  This obviously doesn't apply, but as I
understand it our nose struts depend on internal friction for damping.
Anything that would reduce this friction could be a problem.
2.  Any play in any part of the nose gear - mountings, fork, wheel bearing,
etc.
3.  Any appendage (like the wheel fairing) that is loose, not even
necessarily on the wheel itself, just in the neighborhood (like the strut
fairing).
4.  Tire pressure either too high or too low (higher or lower than what?
Than what it takes to make the shimmy go away.  It might be different than
the recommended pressure for any individual plane).  Tire pressure has an
effect on the damping and on the center of effort of the tire patch.
5.  A difference in the "trail" compared to the design.  This would most
likely be the result of a bent fork that might not be noticeable.
6.  Out-of-balance wheel assembly, especially side-to-side.  You need to do
a dynamic balance to detect this as a bubble balance isn't enough.
7.  A change in the moment of inertia of the rotating assembly.  This could
be a worn tire (less tread weight) or most likely the substitution of a tire
of a different design, like one with more plies.
8.  A change in the inertia (weight) of the fixed portion of the wheel - the
fairing - was it repaired with more layers of glass, for instance.
9.  A change in rake - this is the angle of the steering pivot (strut) with
the ground.  Most likely the result of an error in build or a different nose
or main gear height causing the rake angle of the plane to change.

The list goes on...As I understand it, any "castoring" wheel, whether it be
an aircraft nose wheel or a shopping cart, has an inherent tendency to
shimmy.  Whether the gear is steered or free-castoring doesn't make much
difference except that the steering linkage is another source of play in the
system.  Motorcycle riders know all about this and worry about dampers,
geometry and all of the above to prevent shimmy, which is triggered at some
high speed.  I have never been able to completely cure shimmy tendencies in
my planes, but they are admittedly old and have some looseness in all areas
that I am not quite ready to fix.  I lift my nosewheel off a little early on
takeoff and lower it at the lowest possible speed on landing - and then try
to distract the passengers during the rollout.  Yeah, I know I should
replace everything, but I would rather work on my ES.

Gary Casey
C177Rg, ES project

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