anti-shimmy leg stiffeners

[Bluebeard]

..... of course the main question is whether my stiffeners will work and reduce or eliminate the dreaded shimmy - I will report back!

If they do work, as Vince suggests, maybe it would be the time to consider a more permanent fixture.

I have had three past episodes of engine mount cracks, involving a serious amount of work to repair, twice with an engine-off situation. Shimmy can't be good for my engine mount.

[rightrudder]

I don't have any before/after comparison, but I installed the wood dampers from the outset, on both the mains and nose gear. No hint of shimmy whatsoever in about 43 hours of operation.

I attached them with flox first, then wrapped them with 3" fiberglass tape. i made sure to coat any exposed wood with epoxy for fuelproofing, etc. With a permanent install, you have to be very careful with alignment, because you can't really rotate the gear leg fairings around the gear afterward.

[jrs14855]

I have about 75 hours on my Wittman Tailwind, designed by Steve Wittman, the inventor of the round and flat spring gears.
I have had ittermittent shimmy since day one, gradually getting more violent. Tire pressure had no significant effect.
I finally drilled the rivets out of my metal gear fairings, made my own OAK stiffners and taped them in place with filament tape. One touch and go, no shimmy, on the next landing full stop the worst shimmy yet.
I removed the left wheel, put it on an arbor and spun it slowly. Marked the heavy spot with a piece of tape and balanced the wheel with screws and washers on the light side hubcap holes. I also reduced the pressure to 30#. Four landings, no shimmy. During this process I discovered that the tires are significantly out of round.
The stiffners are not optimum, they could be a bit wider and thicker, but they do drastically improve the landings. Without the stiffners anything less than a perfect touchdown would result in a slight bounce.
My next project is to put the wheel pants back on and try some balance weights in the nose of the wheel pants. Also try some new tires.
The odd thing about this is that only the left side shimmied, and always at relatively low speed. Never on takeoff, only on landing and taxi and sometimes I had to come to a complete stop to get the shimmy to stop.
I still can't believe that a wheel imbalance caused a violent shimmy at only 20 m/h or so.

[Canadian_JOY]

The aircraft I fly, a Davis DA2A, is the airplane that got Harmon Lange into the landing gear business, where he today produces all the RV rod-style gear legs. The Davis is well known for its landing gear shimmy. Two owners in the local area opted to reduce tire pressures to reduce the amount of shimmy; this works reasonably well with pressures in the 24-28PSI range.

Last year I changed tires yet again. This airplane has had Flight Customs and Air Hawks on it previously - both tires performed about the same in terms of shimmy. The new tires I installed are Monster retreads. They appear to be a MUCH stiffer tire. I'm running them at something like 32PSI and the landing gear shimmy is virtually gone. It's still there, but only in a very narrow speed band, not much above a fast walk. A very slight throttle adjustment gets us past the speed where shimmy occurs. I'd say the speed range for shimmy is now perhaps 3mph wide. For all practical purposes, it's as good as gone.

Needless to say, I'm happy with my Monster retreads. They wear like nails, and seem to have fixed the shimmy!

[KatieB]

Has anyone had problems with corrosion of the gear legs when using glass and epoxy to hold wood stiffeners in place? Mine have a few rusty patches around the gear legs near the top of the wood and fiberglass.

Here's the upper part, showing the original wood block that held the original plastic brake line and a blind nut for the intersection fairing, with the pine stiffener below it. The stiffeners have about a year in service and 5 sittin' around.




Lower part, showing a patch of rust beneath an overhanging piece of glass and resin. Looks like surface rust up close, probably from the resin hunk wearing the paint away as the leg flexed. Wondering if this might cause trouble down the road, or if anyone has taken off a glass sheath to find pitting on the steel underneath. I've got this fuselage completely gutted anyway, but gear leg refurb is a project I'd rather not touch...

[woodmanrog]

Be sure to read the instructions for the installation of the stiffeners fully. They are to be mounted in position with either a flox mixture or Bondo along the entire V groove. DO NOT SEAL THE BRAKE LINE INSIDE THE GROOVE. You must run the brake line on the opposite side of the stiffener. When applying the fiberglass tape, over wrap the top and bottom or seal the ends with epoxy so there is no moisture intrusion into or around the wood. I really don't recommend tie wraps or hose clamps for this application as they Will chafe or work loose eventually causing damage to the gear leg or defeating the reason for installation in the first place. RTFM

[lr172]

Originally Posted by jrs14855

I have about 75 hours on my Wittman Tailwind, designed by Steve Wittman, the inventor of the round and flat spring gears.
I have had ittermittent shimmy since day one, gradually getting more violent. Tire pressure had no significant effect.
I finally drilled the rivets out of my metal gear fairings, made my own OAK stiffners and taped them in place with filament tape. One touch and go, no shimmy, on the next landing full stop the worst shimmy yet.
I removed the left wheel, put it on an arbor and spun it slowly. Marked the heavy spot with a piece of tape and balanced the wheel with screws and washers on the light side hubcap holes. I also reduced the pressure to 30#. Four landings, no shimmy. During this process I discovered that the tires are significantly out of round.
The stiffners are not optimum, they could be a bit wider and thicker, but they do drastically improve the landings. Without the stiffners anything less than a perfect touchdown would result in a slight bounce.
My next project is to put the wheel pants back on and try some balance weights in the nose of the wheel pants. Also try some new tires.
The odd thing about this is that only the left side shimmied, and always at relatively low speed. Never on takeoff, only on landing and taxi and sometimes I had to come to a complete stop to get the shimmy to stop.
I still can't believe that a wheel imbalance caused a violent shimmy at only 20 m/h or so.

I fortunately do not have shimmy issues on my 6A. However, from all of my research I believe that shimmys are most commonly caused by tire balance issues. As a teenager I had a jeep with really big tires that were very difficult to balance. I would often get violent shimmys around 30-40 mph. They would only originate in that speed range and they would not stop until I was well under 20 MPH. They were usually initiated by hitting a bump or rut. I believe there are harmonic forces at play that I do not understand. Interestingly, the shimmy NEVER occurred in the rear, even when I rotated tires from front to rear. I always believed it to be the rigid attachment to the real axle that prevented the shimmy from originating.

Just some data points to consider as you work through the troubleshooting process.

Larry

[lr172]

Originally Posted by woodmanrog

I really don't recommend tie wraps or hose clamps for this application as they Will chafe or work loose eventually causing damage to the gear leg or defeating the reason for installation in the first place. RTFM

I understand the logic and rationale for this recommendation. In my case, I chose tie wraps as I was concerned about corrosion under the F/G wrapping. My experience has been to avoid creating situations where moisture will be kept in contact with steel when possible to minimize corrosion. I found several posts from other members finding corrosion similar to what Katie found. The stiffener is not structural and therefore I was not concerned if the tie wraps failed. I used a rubber buffer to eliminate chafing.

Larry

[springer]

Simple strapping tape wrapped around sealed hard wood stiffeners worked well with my -4 legs. Original tape was still on them when I sold the plane 22 years later.

[Neal@F14]

My 6 has recently developed some gear leg shimmy which is most prevalent when I'm slowing down after a wheel landing. Last annual, I flipped my tires around (AirHawks) to even out the wear pattern and get some more service life out of them. A couple weeks ago, I pulled the wheel pants to inspect my brake pads and add some air to the tires and sure enough, somewhere along the line I have flat-spotted the tires and now they have ugly wear patterns on them. I have wooden stiffeners fiberglassed to the gear legs too, but their effectiveness only goes so far when your tires are out of round. It's time to begin the next annual in a couple more weeks, so a new set of tires is definitely in order.

PS: My plane is now 13 years old, and there is no sign of corrosion on the gear legs under the wooden stiffeners... probably since it has lived its whole life in a mostly dry Texas climate.