Message-ID: <01BEC4E7.0C4C77A0.ed@testelectronics.com>
From: Ed Armstrong <ed@testelectronics.com>
Reply-To: "ed@testelectronics.com" <ed@testelectronics.com>
To: "'lancair.list@olsusa.com'" <lancair.list@olsusa.com>
Subject: Nose wheel shimmy
Date: Sat, 3 Jul 1999 00:00:22 -0700
Organization: Test Electronics
Mime-Version: 1.0

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Nose wheel shimmy is caused by the oscillating combination of structure 
flex and wheel mass. It is easy to understand if you just picture a nose 
wheel with its attached structures rolling down the runway. The first thing 
that starts the oscillation is a sideways force. There is always going to 
be a slight sideways force from something no matter how straight the runway 
is. This sideways force bends the nose wheel and the structures connected 
to it in the direction of the force. We are talking only of the forces 
between the area of contact between the runway and the rubber. Even if you 
have a very stiff structure, you could still get the tire side walls to 
bend. When the nose wheel bends, it leans, and turns into the direction 
that it leans. It turns into the force exerted by the runway underneath. It 
turns because the tires are round. If you don't understand this, think of 
how a bicycle turns when you lean when you are riding with no hands. As I 
said, it turns into the force, as this turning progresses, this primary 
force diminishes, but the turn angle stays the same because the tires and 
structure have a mass which tends to hold it there. This is where the 
trouble starts. The turn angle stays the same, but the airplane tends to go 
straight, so a secondary force then rebuilds in the opposite direction. The 
nosewheel then turns towards the secondary force and overshoots because of 
the mass of the system, then we are back to the same force direction as the 
primary force.
Here is an example. Lets start with the force from the runway to the tip of 
the nosewheel pushing the nosewheel left. The nosewheel side walls and 
attached structures bend left, so the nose wheel steers right and its 
physical mass holds it there, but the airplane is going straight, so the 
force then shifts to the right. When the force shifts right, the nosewheel 
side walls and attached structures follow, bending to the right, the 
nosewheel then steers left. After the nosewheel steers left, the force from 
the runway to the tip of the nosewheel is pushing left, the nosewheel 
steers right and this whole thing repeats (oscillates).

The theoretical way to prevent nose wheel shimmy is simple. Increase the 
rigidity of the nose wheel and attached structures, and reduce the mass 
(weight) of the nose wheel and attached structures, and reduce side forces. 
The reality is you can't really reduce side forces, they are usually just 
there, and reducing mass usually reduces rigidity.

Suggestions
Increasing rigidity:
The obvious one is check all nuts and bolts.
Get higher quality tires with less side wall flex (stretching).
Higher air pressure will work on high quality tires that don't stretch. If 
the rubber in the tires stretches, lower pressure will reduce the distance 
from the side walls to the area of contact on the runway. And could reduce 
the bending part of the flex which causes the steering, but this can cause 
other problems. I wouldn't recommend this, get better quality tires.
The side walls of old tires are not as stiff as they were when the tires 
were new. The tire may have little wear, but the fact is the rubber gets 
old. The rubber cracks, and the reinforcing fibers loosen from the rubber. 
If you are experiencing shimmy and it was not there when your tire was new, 
you may just need a new nose wheel.
Jeremy F Fisher wrote about grooved nose wheels. Too bad they don't make 
Good Year Aqua Treads in our size. The grooved wheel reduces the length of 
the side wall. It reduces the side wall length from the rim to the tire tip 
(area of contact on the runway), down to the length from the rim to the 
tire edge. Since total side wall length is reduced side wall flex is 
reduced. I wish I could get a tire like this.
Another idea, larger diameter rims and shorter side walls. Too bad we don't 
have a very big selection of tires to choose from.

Reducing Mass:
The self centering strut effectively reduces the mass by holding it from 
over steering.

Reducing side forces:
Some side forces may be caused by warped tires. I know the tires are 
spinning much faster than the shimmy, but the shimmy can be induced at a 
sub harmonic of the forces induced by tire warpage. Warped tires could be 
caused by a simple twisted inner tube. Inner tubes can twist and bunch up 
inside if you have ever ran the tire at too low of a pressure. You can fix 
this by letting all the air out of the tire, then massage the tire, then 
add just enough pressure to fill the tube, less than 1 pound massage the 
tire again, then fill it to pressure.
Old tires can also warp as the reinforcing fibers loosen up over time.
Warped rims are a more expensive problem, you can detect it by lifting the 
nose and spinning the tire next to something that you can reference, see if 
it wobbles. I think this would be unlikely though, these rims are very 
rigid, but it is worth a check if all else fails.
Balancing the tire can also reduce vibration forces that can start off 
shimmy, but I don't know where you would put the weights, These tires are 
more rubber than rim.

Hope this helps
Regards,
Ed Armstrong
Watsonville CA.


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