What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

anti-shimmy leg stiffeners

Bluebeard

Active Member
For years I have been annoyed with occasionally severe shimmy on landing my RV-6 on tarmac runways. I keep my tire pressure at 24 psi (unchanging with Airstop tubes), it is very sensitive to the pressure. If I remove the spats there is no shimmy. It seems to be a resonance in the leg spring, reducing the mass eliminates this.

Once after a severe shimmy on landing at an unfamiliar airfield, which I thought might indicate some undercarriage problem, I spent hours fruitlessly examining the aircraft. Later I discovered that a few lines of transverse bars had been painted on the runway for a previous spot landing competition, these lines seem to have excited the leg spring into resonance.

Now as an experiment I'm fitting some pine stiffeners from Vince Frazier.

I have attached the wood with multiple strong (stated to be 175 lbs) cable ties set in grooves, so that (i) they can easily be removed if unsuccessful and (ii) there is nowhere for moisture to get trapped against the metal leg. I've checked this out with multiple fast taxi runs and a couple of landings on a rather bumpy grass runway. Next test will be with spats, on a concrete runway.

I wondered whether there was some reason why - as far as I can see - no-one else here mentions using cable ties for this purpose :confused: .....?
 
Perhaps folks are thinking along the same lines as cable ties on the engine mount tubes? There's some evidence to suggest that over time, vibration causes the cable ties to move around slightly, grinding dirt between itself and the tube. The dirt acts like sandpaper and eventually causes measurable wear to the metal.
 
I'm certainly not an expert on cable ties, but my concern would be that they have similar strength and durability to the typical mounting procedure, i.e. strips of fiberglass cloth. That glass has far, far more tensile strength than 175#... but that isn't the only factor to consider.

Long term, I'd keep an eye on them for heat or oil degradation too.

Having said that, it's experimental. If it works for you, you must report back with your experience and where to get the super duty ties!

Same thing if it doesn't work! :eek:

Of course, if it works for you, you can always add the glass later.

Keep us posted!
 
Kurt makes a valid point. You will need to protect the gear legs from getting a stress notch worn into them. Maybe a scrap of aluminum formed to fit and placed under the tie?

Or just replace the legs at each condition inspection. :O
 
Is there room for a stainless worm-drive hose clamp? If so, 'gut' an appropriately sized Adel clamp & use the rubber core around the gear leg, & a long hose clamp around everything.

Or maybe the entire Adel clamp, bolted to an aluminum 'U' around the wood. The clamping force of the adel clamp should keep grit away from the leg.

I don't think the grit/nylon wire tie issue should be ignored. Proseal, silicone caulk, or some other sealant that won't move on the leg would be a good idea, if you keep the wire ties.

Charlie
 
Wood Stiffners

Installed on a buddy's 6A a couple years ago and it immediately cured the long term shimmy/wobble issues. Put them on with 6 very heavy duty ty-raps per side. Monitoring the install and so far all is great. I suspect the wear issues happen when a ty-rap is loose enough to move around.

Don Broussard. A&P, IA

RV 9 Rebuild in Progress
 
I plan to use fiberglass filament packing tape to try my stiffeners out. Might just leave it if it works.
 
For years I have been annoyed with occasionally severe shimmy on landing my RV-6 on tarmac runways. I keep my tire pressure at 24 psi (unchanging with Airstop tubes), it is very sensitive to the pressure. If I remove the spats there is no shimmy. It seems to be a resonance in the leg spring, reducing the mass eliminates this.

Once after a severe shimmy on landing at an unfamiliar airfield, which I thought might indicate some undercarriage problem, I spent hours fruitlessly examining the aircraft. Later I discovered that a few lines of transverse bars had been painted on the runway for a previous spot landing competition, these lines seem to have excited the leg spring into resonance.

Now as an experiment I'm fitting some pine stiffeners from Vince Frazier.

I have attached the wood with multiple strong (stated to be 175 lbs) cable ties set in grooves, so that (i) they can easily be removed if unsuccessful and (ii) there is nowhere for moisture to get trapped against the metal leg. I've checked this out with multiple fast taxi runs and a couple of landings on a rather bumpy grass runway. Next test will be with spats, on a concrete runway.

I wondered whether there was some reason why - as far as I can see - no-one else here mentions using cable ties for this purpose :confused: .....?

I used cable ties. I have 100 hours on my 6A and just had the leg fairings off for painting. All cable ties intact. I used rubber tape where the tie wrap touches the steel leg to avoid wear to the steel, just as I did on my engine mount.

Larry
 
Thanks folks for the helpful replies.

The grinding point is certainly a good one to think about, as dirt will surely get inside. Maybe I will put a thin rubber layer under some of the cable ties and a thin piece of aluminum under some others, and check after some months whether there is any sign of surface damage on the leg under any of them. One of the advantages of cable ties is they can easily be cut off/replaced if desired or if they deteriorate.
 
Thanks folks for the helpful replies.

The grinding point is certainly a good one to think about, as dirt will surely get inside.

How about a fillet of RTV after the cable tie is installed. Keep things nice and clean where the tie is touching the gear leg.
 
..... 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.
 
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.
 
Shimmy

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.
 
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!
 
After a while, they look like this...

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.

0419161632.jpg



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... :rolleyes:

0419161633a.jpg
 
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
 
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
 
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
 
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.
 
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.
 
Last edited:
Now I can report that the stiffeners I have recently fitted do eliminate the shimmy I previously encountered on hard-surface runways.

Maybe now I can safely increase my 24 psi tire pressure a bit.

I'm planning to inspect the stiffeners closely at the end of the summer season and see whether the cable ties (apart from ones where I used rubber backing) have marked the legs, loosened or deteriorated in any way.
 
Good to hear that the stiffeners worked for you. I haven't balanced any of my wheels/tires and run pressures of 40 psi all around. I've heard stories of people blowing tubes up front as a result of too little pressure. Make sure that the break-out force is within spec!
 
A friend ripped the valves off his RV7's tubes due to running low pressures and braking too enthusiastically .......
 
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

What exactly did you use for the "rubber buffer"? Silicon self fusing tape or something else?
 
Now I can report that the stiffeners I have recently fitted do eliminate the shimmy I previously encountered on hard-surface runways.

Maybe now I can safely increase my 24 psi tire pressure a bit.

I'm planning to inspect the stiffeners closely at the end of the summer season and see whether the cable ties (apart from ones where I used rubber backing) have marked the legs, loosened or deteriorated in any way.

So - what exactly did you use for a rubber backing? (Seems like I am conducting a survey about rubber backing now :rolleyes:)
 
proseal

The steel gear legs are rust prone by nature of alloy. I chose a slightly different approach, and epoxy primed, urethane pained the legs, then set/bonded the wood stiffeners with Proseal and taped in place. After Proseal cured, I wrapped with 3 layers of glass and sealed top and bottom with Proseal. 5 years in service, no cracks, corrosion and no shimmy. I don't recommend any RTV around steel, as it's acid will cause corrosion.
 
What exactly did you use for the "rubber buffer"? Silicon self fusing tape or something else?

It is a rubber tape that comes on a roll for electrical. It is not really self- fusing, but it does kind of stick to itself. It is maybe 1/16" thick. Get it at home depot
 
The steel gear legs are rust prone by nature of alloy. I chose a slightly different approach, and epoxy primed, urethane pained the legs, then set/bonded the wood stiffeners with Proseal and taped in place. After Proseal cured, I wrapped with 3 layers of glass and sealed top and bottom with Proseal. 5 years in service, no cracks, corrosion and no shimmy. I don't recommend any RTV around steel, as it's acid will cause corrosion.

Can you please expand on the statement about steel and RTV? I have been using this stuff on steel, cast iron, aluminum engine, TC, and diff parts for decades......
 
Certain RTV's

Bret,
The term RTV (Room Temperature Vulcanizing) hosts a variety of manufacturers and compounds. There are some that contain Acetic Acid which releases during the cure (smells like strong vinegar) that are harmfull to most metals and electronics. I work in Aviation Maintenance at my day job, and there are many RTVs that are formulated without the Acids..but if you dont know the type to get at the Big Box store aviation department, you may induce the acid corrosion problem. I am just a fan of the Proseal type products which bond well and seldom lead to any moisture incursion. Proseal is also paintable, RTV is not. Im not trying to say that everyone using RTV should worry, Im just saying there are better options if you are at square one.
 
Much bigger deal with electronics than with metals. A friend who raced rotary powered cars for decades used GE silicone bath caulk (acetic cure) in place of the block-to-exhaust manifold gasket. It's in contact with cast iron, aluminum, & probably some stainless or more exotic metal in the exhaust port. Rotary exhaust temps are legendary (of course, the actual metal is never that hot).

If you need something quick & want to avoid the acetic acid cure, just look for 'sensor safe' on the RTV packaging at the auto parts store.

Charlie
 
On my Six, after epoxy priming gear legs, I got 2 big pieces of electrical heat shrink tubing and shrunk it to each gear leg (before installing brake flanges). That stuff is pretty darn tough! I wound up not having a shimmey problem. I wonder if the heat shrink, or maybe multiple layers had any effect?
Jake
 
What size heat shrink?

I like this idea so am curious as to what diameter heat shrink you used? Thanks.
 
My RV6 gear leg fairings are the sealed variety that have been filled with foam along with the wood stiffeners. My close friend who is a master craftsman and built many aircraft (including Rv's) has been after me for year to remove it all to inspect the condition of the legs for corrosion. Also he says that the stiffeners concentrates the bending moment of the leg's entire length down to a short area. This can/will eventually weaken the steel in that area. He says this is contrary to the original design of the gear. He has removed this same set up from another 6 recently and found alot of corrosion. I haven't decided yet weather to remove mine as they are beautiful (to look at at least) and haven't caused me any heat ache yet. Has any heard of gear failure?
 
No argument on the corrosion issue; with steel it can never be ignored completely. But on the effect of the stiffeners: The Van's-recommended technique (and all the ones I've seen) show adding fore-aft dimension, with minimal change in up-down dimension. My impression, FWIW, is that the stiffeners effectively convert the round rod to a 'slab', similar to the RV-8's leg. The primary flex is up-down, and the fore-aft flex is just an unwelcome byproduct of the round leg. So the stiffeners shouldn't have the effect your friend is concerned about. Again, just my opinion...
 
So I have revisited this topic again with my nameless friend.. (Most of)His comments are as follows. The gear is tapered spring steel designed to distribute the load induced deflection evenly over the entire length of the leg. Hollowed wooden stiffeners placed securely on the legs prevent this (in all directions) and concentrate the bending moment to a small area of the leg. Contrary to original design. (not Vans). The "unwelcomed" fore and aft movement that you speak of is indeed a beneficial, designed improvement over cessna type gear (RV8). In that type of gear,those loads are transmitted directly to the airframe as the gear has no give in that direction. If one has the gear properly aligned while using good, balanced tires there is no need for strapping wood on the gear legs and filling the fairings with foam. There should be no shimmy.
 
By the way. Mine have the wood and foam. I have decided to remove the fairings which will get destroyed when I do remove them but I really want to know about the corrosion.(or not) When I replace them there will be no wood or foam so I will let you know what happens and whether this theory holds water! LOL
 
Back
Top