REFLECTOR:Nose Gear Shimmy

[Ronnie Brown]

John, I really think you ought to try balancing your nose wheel/tire.
Shimmy starts from an out-of-normal condition such as out of balance, warped
nose wheel, distorted tire, etc.  If you have ever had an old car, with worn
out front spindles, you can be driving along and all is fine until you hit a
bump, and then it shimmy's like crazy.  To stop it you have to slow down or
sometimes you could jerk the steering wheel to one side or the other (back
before there was any traffic on the roads).  The rear brakes causing a
shudder can certainly excite the nose wheel and start it to shimmying.

My Velocity has the shudders in the brakes at about 20-30 knots, but it
stops and starts with application of brakes.  So far, no nose wheel shimmy.
But I balanced my nose wheel as suggested using pieces of angle iron and a
rod.  And like Mack, I bought a cheap fishing scale that reads up to 25 lbs
and I use it to set the nose fork nut, and set it to 15 lbs as recommended
by the factory.

I have checked my main brakes for disk runout and have none.  As others have
suggested, there is a resonance in the RG main gears that allows the mains
to shudder at a certain speed (20-30) knots.  My brakes work just fine at
higher and lower speeds, no shudder and plenty of stopping power.

Ronnie




----- Original Message -----
From: "John Dibble" <aminetech@dixie-net.com>
To: <reflector@tvbf.org>
Sent: Saturday, August 09, 2003 11:47 PM
Subject: Re: REFLECTOR:Nose Gear Shimmy


| I also doubt that wheel balance is a shimmy issue even though when I spin
the nose
| wheel, it does seem to be out of balance.  But I never get shimmy on the
take off
| roll, only when I brake during landing.  I'm comfortable with the nose
wheel at 16
| psig, in fact I think that it might squeek by at 0 psig, but I don't plan
to try that.
|
| John
|
| Chuck Jensen wrote:
|
| > At this point, about 12 hours and as many landings with nary a hint of
| > shimmy.  Lightly loaded and gross.  No difference.
| >
| > And, the times when the nose drops on a little harder (rare, I assure
you),
| > I'm nearly positive that the "thump" is muted by the softer tire, yet I
| > don't really see any "squishiness" in the front tire when loaded....and
this
| > at only 16 psi.  And, as John observed, the nose is lightly loaded
compared
| > to the mains, so the low pressure may not result in much tire
deformation.
| >
| > Its fair to say there is some interesting "stuff" associated with this
nose
| > wheel thing.  I grew up on a farm (which probably is a surprise to no
one)
| > and the hay rake had a castering wheel.  Sometimes, for reasons unknown,
| > it'd shimmy like it had gotten into some bad home brew.  Then other
times,
| > it'd track like a dog.  Who knows.
| >
| > The wheel balancing is certainly a good idea as that little wheel spins
like
| > the dickens at 80 mph, but I have some doubts that the out of balance
issue
| > is a causative factor in the shimmy problem.  I think the shimmy is
likely
| > initiated when we land and, with a little cross wind, the nose wheel
touches
| > slightly out of alignment with the runway, though in perfect alignment
with
| > the plane.  Upon contacting the pavement, the wheel is kicked in the
| > opposite direction, which then kicks it back and so on and so forth til
it
| > self destructs and is sliced-an-diced in the prop.
| >
| > Clearly, the stiff resistance of the tightened washers on the forks
kills
| > this oscillation.  Perhaps, the additional friction of the soft(er) tire
| > also helps reduce the oscillation vector.  To date the stiff washers
| > definitely work (even though its a little bit of a cuss to steer) and
the
| > jury is out on the reduced air pressure in the front tire.
| >
| > I'm just ignorant enough to try the continued loosening of the washers,
| > and/or, increasing the air pressure to see if I can induce the shimmy,
yet
| > get to a grass field to land and rectify.  Wouldn't it be a kick if all
we
| > had to do was reduce the front tire pressure to 16 psi, then we could
loosen
| > the front forks so a light tap on the brakes would turn the nose ever so
| > subtly?  There, but for the grace of god, goes a stearable nose wheel.
| >
| > And, besides, if the test is a failure, MT can always use the extra
money!
| >
| > Chuck
|
|
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