Gear Leg Shimmy

[tonyr]

We had an instance of gear leg shimmy at a flyin this weekend past.
The Sonex in question normally operates off grass strips with 5.00 x 5 tires but during takeoff from a bitumen runway, shimmy from the RH main was noted by the airport operator sitting in his flight hut office...a couple hundred yards away! The pilot of course felt it as well.
We jacked up the aircraft on its return and indeed the leg was able to be rotated slightly in the gear socket, and the axle was slightly loose on the leg.
The bolts were able to be turned with the nut held, indicating the bolts were not broken. The LH hand side also had some play in both areas.
After retorquing the nuts, the play in the legs was gone. We suspect the wear and tear of landing on grass had allowed some extra wear in the bolt holes. SB-006 says they should not be overtightened but if they're too loose you get this problem I think. The top bolts couldn't be removed for inspection a they were inserted from the rear during construction.
Because of that they are still the stainless bolts by the way, (the SB says replace if broken or as soon as possible).
Anyway it was enough to get the aircraft safely back to home base.
The aircraft is due an annual so we will be pulling legs, doing the SB and inspecting everything!

Highlights that these bolts need to be regularly checked, especially operating off grass with 5.00 x 5 tires..

Cheers
Tony

[MichaelFarley56]

Thanks for sharing Tony. I've noted similar shimmy on my gear this past spring and found the exact same thing; slight movement could be detected on my gear legs where they plugged into the mount. I didn't have any issues with the axle bolts and nuts but those get checked often.

I performed the SB before I even flew my airplane and tend to agree with you; I don't think you want to really torque those nuts too tight which could set up for a bolt failure, but you do need them "snug" enough to prevent movement. I operate about 50/50 on grass and pavement and have never had an issue while landing on grass; the shimmy only happened on pavement. Since I adjusted the bolts and nuts however, I have yet to have another issue. Hopefully my luck holds...

Good tip!

[hickej]

Leg shimmy is a recurring issue and there doesn't seem to be one cause. ( by the way, this is also an issue on some RVs and I've had my share of Cessna 172 shimmies). I had a shimmy that was noticed by my buddies on the ground and felt by me. It cracked the wheel pant. I balanced the wheel and that seems to have cured it for now. I think the elongated bolt holes is the result of the shimmy and not necessarily the cause. I switched to the 5" tires with the thought that it would slow down the tire rotation and change the dynamics of the landing system. So far so good. I think this issue deserves the same attention that the 'burps' issue is getting. If we put our collective experience to work we can help everyone else avoid or fix the issue. The factory says to check the alignment and the bolts which we should all do. I suggest adding wheel balance to the annual inspection as well. Any other good experience out there ?
Jim Hicke