Angier,
The shimmy damper is a vertical piston that must displace fork oil when the strut is turned. The oil escapes through the clearance gap between the sleeve and the outer strut wall. The oil is captured in this general area by O-rings to the north and south of the damper. Some struts do not have these O-rings and their damping is much reduced as the oil in these struts can escape vertically to the rest of the strut interior. (I have attached some photos of the these various parts)
The "null zone" (after all air has been removed) is quite common, but it is not present on all struts. There are a few possible sources.
First is the clearance between the guide key and the
sleeve. Yours parts are new so wear is not likely, but they may have a little clearance simply due to manufacturing tolerances. Also, clearance between the main shaft and the sleeve will also contribute a little bit of movement prior to engaging the damper. Another source is movement of the balls in the shimmy damper check valve. When the strut direction is reversed and fluid was drawn into the damper these balls must travel a small, but finite distance to engage and block the flow of oil out of the damper. During this short time there is little resistance. This is the source that I find most intriguing. The jump in rotational resistance is consistent with the check valve engaging. I have one strut here that has such a null. As a test, I have been tempted to place some light springs behind the balls so that they are immediately
engaged upon direction reversal.
While a little unsettling, the good news is that this null zone by itself does not seem to be a trigger for shimmy as it is present in many struts. Logic would however say that it can't be desirable.
The advice from Bucky, Scott and others should keep you out of trouble.
Just to add another data point: I use 35 psi in my nose wheel.
Chris Zavatson
N91CZ
360std 1,400 hrs
From: "Greenbacks,UnLtd." <N4ZQ@verizon.net>
To: List Lancair Mailing <lml@lancaironline.net>
Sent: Saturday, February 23, 2013 2:31 PM
Subject: [LML] Re: Nose Gear Strut
I removed the fork this morning and inspected the strut which was dry inside.
Bled off the pressure and removed the little valve stem piece thingy, collapsed the strut and no oil came out.
I added a bit more than 1oz of fork oil which was just enough to have the oil flow from the valve with the strut collapsed.
Worked the nose wheel side to side and could hear some air inside so kept doing this as well as collapsing/extending the strut. After a while, no air bubbles could be heard. But I have one observation and don't know if it's normal or not since I don't know how the dampening is accomplished inside. With the wheel centered or in any other position right or left, I can move the wheel perhaps a degree or two either side and don't feel as much dampening as I do when turning the wheel further left or right. Seems like a dead or null zone either side of where the wheel is positioned. I'll check again in the morning and then pressurize the strut
and then see how it feels.
My tire pressures are 60 on the mains and 55 on the nose as recommended by a well known Lancair GURU...
Thorough inspection of the engine mount shows all normal and wheel bearings are correctly loaded.
Angier Ames
N4ZQ
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