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Metal Gear Leg Stiffeners

Ironflight

VAF Moderator / Line Boy
Mentor
I want to preserve this information in the RV-3 forum, even though it is partially copied from a thread over on teh RV-4 forum - the RV-3 gear legs are wet noodles enough that some form of stiffener/damper is generally required, and the wooden ones have always been a bit marginal for us.

I just finished doing this modification on our -3, something I have contemplated for quite awhile, but never got around to doing. At ourt annual a couple of weeks ago, I noticed that the wooden stiffeners had de bonded, and weren’t doing much…so it was time. And with only one test flight in the box, I can still give a pretty definitive answer that the metal stiffeners are better! Our wooden stiffeners were on there from day one, and with almost 800 hours, and were doign very little. The truth is, we have always had a shimmy with these spindly RV-3 legs - we just controlled it with lower tire pressure, braking, and avoiding certain taxi speeds.

Installing metal dampers was probably a total of about six hour’s work - I spread it out over a couple of days, but it was straightforward. Based on notes from Dave Paule, I used 1/8” x 1-1/2” aluminum, and seven Adel clamps. I removed the rubber from the Adel clamps, mostly because I was worried about the outside diameter affecting the fit of our gear leg fairings. Largest Adel is -18, smallest is -12, and they are spaced to where they are naturally tight on the gear leg for their diameter - note that this is NOT evenly spaced. There’s a single wrap of Gorilla Tape under each clamp, just to seat them better.

Spacers were required between the clamp and the metal bar stock to put the center of the bar stock through the center of the gear leg. Screws are low-profile #10 Allen heads, 1” long, and fastened with low profile nylon lock nuts. Washers and s machined bushings were used for spacers, and there was approximately a single washer difference between adjacent clamps. The existing gear leg fairing fit right back in place!

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Very useful info Paul, thanks.

Just added to the todo list for the resurrection project of mine....
 
RV-3 gear leg

Hey Paul.
Thats a great idea! the wood on my -3's gear legs has de-bonded about 3 times over the last 20 years:mad:

Do you have any information that you can pass along as to exact measurements, length, and anything that you feel vital to know. I had thought about metal along time ago but was told by a non-RVer that it could set up a harmonic vibration, even at higher speeds. So if you would be so kind as to post further information, it really would be appreciated.

That you.
 
Hey Paul.
Thats a great idea! the wood on my -3's gear legs has de-bonded about 3 times over the last 20 years:mad:

Do you have any information that you can pass along as to exact measurements, length, and anything that you feel vital to know. I had thought about metal along time ago but was told by a non-RVer that it could set up a harmonic vibration, even at higher speeds. So if you would be so kind as to post further information, it really would be appreciated.

That you.

About the only additional information I can give you that wasn’t in the first post was that mine are 25” long. I am pretty sure that if you’re looking to use a two-foot long piece you have handy, 24” would work just as well.

The line of attachment holes is 3/8” in from the edge of the piece. Spacing is where the various clamps are tight on the gear leg. AN3 hardware is used for attachment.

I flew it again yesterday, doing various medium-speed taxied, landings (wheel and three point), and take-offs, and couldn’t induce any shimmy. I’ll want to fly it to different pavements before I declare complete success, but it sure is an improvement over the wood already!

I used to run about 26 psi in the tires to provide some damping - now at 35 for testing, and its fine. I’ll probably take them up to 40 before next flight.

Paul
 
....I had thought about metal along time ago but was told by a non-RVer that it could set up a harmonic vibration, even at higher speeds.....

A harmonic vibration in the stiffener is unlikely to have any effect, assuming it even happens, which I'd doubt. In any case stiffener vibration would be out of the plane of the stiffener and of low mass, compared to the gear leg, so it certainly would not excite any shimmy. It would also not have a suitable vibration frequency to bother anything.

Dave,
An old stress and dynamics engineer, now firmly retired
 
RV-3 gear legs

Guys.

Thanks for the reply. I plan on doing this in the next 2 weeks, as the weather down in Florida is extremely unpredictable this time of the year, not too much flying is being done. When I do, I will post a report on what I have found.

Thanks again.
 
Why not steel ?

Great to hear AL works. I expect a modulus vs. weight comparison with steel could be revealing. How would holes in the flat bar enhance the outcome ? Great work David, thanks. Any energy left to optimize or discourage steel ?
 
leery

I would be leery of steel. I think the thickness of the aluminum helps to keep the stiffener from buckling. Cant see how the steel would accomplish this without also hindering the flex in the up and down direction. But what do i know, my only engineering degree is TLAR (that looks about right)
 
....How would holes in the flat bar enhance the outcome ?....

They would save weight and reduce the effectiveness of the stiffener, but probably not by a huge bunch. They'd be best located adjacent to the gear leg since the stiffener's effectiveness is based upon its width from the gear leg.

Dave
 
I would be leery of steel. I think the thickness of the aluminum helps to keep the stiffener from buckling. Cant see how the steel would accomplish this without also hindering the flex in the up and down direction. But what do i know, my only engineering degree is TLAR (that looks about right)

A small factoid might be of interest. Aluminum, steel and magnesium all have about the same stiffness to density (i.e., weight) ratio. But the buckling resistance is based upon both the stiffener's thickness and the stiffener's modulus of elasticity (more of both is better here), so the most weight efficient material for buckling resistance is the lowest density of these.

That said, we don't know what the stress is in the stiffener and so we can't estimate how close it is to buckling.

I doubt that a steel stiffener would significantly adversely affect up and down bending, especially if it were made thinner so as not to be three times heavier than an aluminum one would be.

Dave
 
OK

so the most weight efficient material for buckling resistance is the lowest density of these.

.

Dave
I have seen too many old 914s go up in flames with wheel fires, so I wont be using magnesium. I guess aluminum is best. Now should it be 6061 or 7xxx?
 
I'm not sure.

I like it and wish I had the training of a suspension engineer. It seems you have succeeded in stiffening the spring in the same direction the system's dampeners (tires) perform poorly. The higher inflation pressure suggests/confirms a shift in the harmonic.

It will be interesting to see how much the system wears over time. A failure of one leg's stiffener could prove scary.

I have no training for proper analysis so nothing has or will be acted on: I have often wondered whether a urethane encapsulated gear leg would improve stiffness in the fore-aft direction while also improving dampening. I have also wondered whether welding a rod to the leg (in place of the mechanically attached bar) would achieve the same result.
 
As for the material, what DanH said.

========

DO NOT WELD TO THE GEAR LEG!

You'd destroy the heat-treatment of it and add a stress concentration. In fact, except for the RV-3 slider, there's no welding on any part of any of the RVs, as far as I know.

Dave
 
Planning on doing this next week on our 3.

Has anyone used a taper pin in place of the gear leg/engine mount bolt? If so, could you please post the part number?
 
AntiSplat?

Maybe AL at AntiSplat should consider this issue as thought for a future product offering...
I think his clamping method would be more superior for this than the Adell clamp approach.
 
Antisplat clamp ?

Ralph~,
`Can you describe the clamp concept in case Antisplat doesn’t chime in. It is my opinion you can optimize bending modulus and section modulus to perfection, but loose it all with minuscule slack that decouples the beam from the gear leg.
 
I was referring to the style of clamp Al used for mounting his bar to the gear leg.

I guess AntiSplat has discontinued his Nose Job product. I assume the market for that product dried up with the introduction of the new style nose gear from Vans.
You can still view it on the AntiSplat website or on his videos.
 
I wonder about introducing some actual damping into this system. For example, if the holes in the aluminum plate were drilled 5/16" and then filled with short rubber bushings made from 5/16" neoprene vacuum tubing, or even better some harder rubber, or Delrin, with a 3/16" ID, then put the AN3 hardware through that.

With the rubber cushions removed from the Adel clamps, the only remaining damping is from the Gorilla tape and any slippage. I would anticipate that the Gorilla tape will get cut thru by the Adel clamp edges fairly quickly, leaving the clamps to chafe on the gear leg. I also wouldn't be surprised to find that the Adel clamps fatigue and crack at the 90-degree bend in the clamp.
 
I wonder about introducing some actual damping into this system.

Clean all the parts so bonding is good. Carefully align the leg fairing, duct tape the bottom, pour it full of room temp cure urethane rubber.
 
The idea of the stiffeners is to eliminate the fore-aft motion if the gear leg as far as is reasonably possible.

With no motion, there can't be damping - damping requires motion to work.

And using an elastomer as a bond agent merely introduces flexibility when that's not desired.

The whole idea of this sort of stiffener is to add a structural element that's integral to the gear leg. Making a mechanism out of it, even a fancy one, is not consistent with that, and by its nature, lets motion occur - it's exactly that motion that's not desired.

Dave
 
The idea of the stiffeners is to eliminate the fore-aft motion if the gear leg as far as is reasonably possible.

With no motion, there can't be damping - damping requires motion to work.

And using an elastomer as a bond agent merely introduces flexibility when that's not desired.

The whole idea of this sort of stiffener is to add a structural element that's integral to the gear leg. Making a mechanism out of it, even a fancy one, is not consistent with that, and by its nature, lets motion occur - it's exactly that motion that's not desired.

Dave

I still have concerns about this method. Now you have a much stiffer arm and the fore/aft forces are transferred more directly to the engine mount (gear leg mount tubes). The early RV-3 engine mounts cracked because of those forces and the two additional mount to firewall bolts were added. Wouldn't this much stiffer system require additional strengthening of the engine mount? Or was there some other reason those two attachment points were added?

Finn
 
I still have concerns about this method. Now you have a much stiffer arm and the fore/aft forces are transferred more directly to the engine mount (gear leg mount tubes). The early RV-3 engine mounts cracked because of those forces and the two additional mount to firewall bolts were added. Wouldn't this much stiffer system require additional strengthening of the engine mount? Or was there some other reason those two attachment points were added?

Finn

Since I went to the TLAR engineering school and I stayed at a Holiday Inn Express last night, I would think a shimmy leg would be worse stress on the mount than a constant force. As long as the stiffener does not tie in to the fuselage, I dont see any harm. But what do i know, I am getting kicked out of my hotel room due to non payment...
 
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Adding the stiffener has no effect upon the forces at the top of the gear leg where it meets the engine mount. But eliminating the shimmy removes that undesirable dynamic load, and that's got to be good.

I watched mtnflyer's video showing his RV-6a main gear shimmying, and found it hard to believe that the gear stayed on.

Dave
 
Antisplat on the right track.

The video for the nose wheel gets to the heart of the matter. The steel clamp on the MIDDLE of the of the steel rod is the critical ingredient . This is the end game for the Adel Clamp uncertainties. Time may prove Ironflights approach is totally satisfactory. If found to need more coupling security over time, the Antisplat approach might be a potential improvement.

The challenge appears to me to be the fabrication of a clamp that fits the tapered gear leg and does not crawl down the taper. A lot of smart people here, lets solve this problem and end the RV shimmy. Antisplat will probably kit it because the market is much bigger than nose wheels.

PS, anything that absolutely controls the deflection of the gear leg over its distance, Will concentrate the bending moment at the mount as previously challenged. This will creat a new unknown.
 
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An afterthought

Using the engine cylinder head bolt preload principal, you could stiffen the rod leg with the backing bar to resist the maxlmum deformation expected ( reasonably practical ) and it should act like RV8 gear �� if it doesn’t break off at the motor mount ��
 
Appreciate all the comments guys - and I’ll report back when the whole thing falls apart. But I should point out that I am not the first one to do this - I stole the idea from guys who have been flying with the same thing for years. I’m just giving it some visibility for those that want to try something different, but which has some significant history. And lest you worry, there is a bit of engineering consulting behind it as well.

As for the P-shaped Adel clamps versus a symmetrical design, that’s also a question I have. I might end up fabricating some new clamps in the future, but we’ll fly this for now, and see how it lasts. I know of at least one plane with this design that has been in regular use for over ten years without a problem.

YMMV of course …. This is experimental after all!

Paul
 
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Alternate

As for the P-shaped Adel clamps versus a symmetrical design, that’s also a question I have.

Paul

I think the originator alternated orientation of the P clamps along the leg. Is there any reason to alternate orientation as opposed to all the same way?
 
Thanks for participating

Thanks for all those participating in this thread.

I have decided to do this to my 9A; the data here is really helpful.
 
Adels

<snip>...
Installing metal dampers was probably a total of about six hour’s work - I spread it out over a couple of days, but it was straightforward. Based on notes from Dave Paule, I used 1/8” x 1-1/2” aluminum, and seven Adel clamps. I removed the rubber from the Adel clamps, mostly because I was worried about the outside diameter affecting the fit of our gear leg fairings. Largest Adel is -18, smallest is -12, and they are spaced to where they are naturally tight on the gear leg for their diameter - note that this is NOT evenly spaced. There’s a single wrap of Gorilla Tape under each clamp, just to seat them better.

Spacers were required between the clamp and the metal bar stock to put the center of the bar stock through the center of the gear leg. Screws are low-profile #10 Allen heads, 1” long, and fastened with low profile nylon lock nuts. Washers and s machined bushings were used for spacers, and there was approximately a single washer difference between adjacent clamps. The existing gear leg fairing fit right back in place!

Paul
I'm getting ready to order parts for this. You mentioned seven Adel clamps each side, 18 to 12 size Adel clamps. Do you remember how many of each size Adel clamps were used? One each from 18 through 12? However, it may be pairs of certain sizes. I prefer keeping the rubber cushions. That probably changes the sizes as well. Guess I can order extra. Just trying to avoid more shipping charges from ACS.
Thanks.
 
Remember, Larry, Paul did this to his RV-3B, the small taidragger. Might differ for your -7A.

Rick B. has a -6A and did this, so he might be able to give you some info.

Dave
 
Paul
I'm getting ready to order parts for this. You mentioned seven Adel clamps each side, 18 to 12 size Adel clamps. Do you remember how many of each size Adel clamps were used? One each from 18 through 12? However, it may be pairs of certain sizes. I prefer keeping the rubber cushions. That probably changes the sizes as well. Guess I can order extra. Just trying to avoid more shipping charges from ACS.
Thanks.

I brute forced the project Larry - I ordered four of each size from -21 to -9 from B&B and had enough of everything, plus a bunch left over (I was low on stock anyway, and they are cheap, so it all worked out….). I used one of each size on each gear leg - no repeats.
 
My way

I followed Paul Dye’s lead and used adel clamps, one of each size and one layer of gorrilla tape instead of the rubber inserts. I wanted some adhesive that would grip the legs and not let the clamps wiggle down the tapered leg.

I bent the adel clamps so the gap was centered on the hoop, if that makes sense. I ended up putting the metal between the adel clamps to align the metal with the centroid of the legs.

For screws I used steel button head allen type 10-32s and thin (shear) self locking nuts.

The clamps worked out to be exactly the same distance apart with the total distance between top clamp and lowest about 20.25”

I am really happy with the way it turned out, but proof is in the first flight. Tell then….
 

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Results?

My RV-3B has the shimmy problem. I've collected the Adel clamps and aluminum bar, read all the posts and puzzled about what it means to decouple lateral force from fore and aft shimmy. Rather than balance the wheel pants, run my tire pressure so low I strain to push the airplane, etc., I would rather remove the wooden stiffeners and install the aluminum. All that's holding me back is the lack of exuberant endorsement, ringing praise or a declaration that the problem is cured once and for all. So far it seems the results are positive but subject to further testing.
Are any of you satisfied that the aluminum stiffeners have solved your shimmy problem?
 
My RV-3B has the shimmy problem. I've collected the Adel clamps and aluminum bar, read all the posts and puzzled about what it means to decouple lateral force from fore and aft shimmy. Rather than balance the wheel pants, run my tire pressure so low I strain to push the airplane, etc., I would rather remove the wooden stiffeners and install the aluminum. All that's holding me back is the lack of exuberant endorsement, ringing praise or a declaration that the problem is cured once and for all. So far it seems the results are positive but subject to further testing.
Are any of you satisfied that the aluminum stiffeners have solved your shimmy problem?

While there is still some noticeable “flexibility” to the main gear with the metal dampers, on our airplane, they are a huge upgrade from the wooden stiffeners. With the wood, which was actually beginning to delaminate (so not doing a lot of good) there were taxi speeds that I really didn’t want to spend any time at. Now, I find the airplane is fine in all operations.

The only real caveats I’ll give is that no two RV-3’s are identical, and so far, no two installations of metal dampers have been identical, so we’re still experimenting, and results coudl vary. But I have yet to hear anyone say that they want to tear out the metal and go back to wood.

Paul
 
Shimmy?

Getting closer to mounting the gear on my 3/ 3B and after reading through this thread was wondering if all RV3's suffer from the shimmy in varying degrees, if not ,why some and not others? Any thoughts or theories?
Chris
 
Thank you Paul, that is ringing endorsement enough for me. I'll do it. I'm glad you brought this mod to the 3 forum.
 
Gear leg stiffener clamps

I'm installing the aluminum bar stiffeners and have a couple questions for those of you smarter than I am with an engineering background (David Paule):
1. Does it matter if the Adel clamps are steel or aluminum?
2. Do the cushions have to be removed?
3. If so, why Gorilla tape rather than electrical tape or something else?
 
Stiffners

I used steel clamps
removed the cushion
Used gorilla tape in place of the cushions
I would have preferred the more common method of wood stiffners with two spiral wraps of glass tape with epoxy resin. However these would not fit under the upper cuff intersection fairing on my Wittman Tailwind.
 
I just finished installation of aluminum bar stiffeners on my 1999 RV-6. I used stainless 'Adel' clamps with the cushions, we'll see how they hold up. I have the Rocket gear fairings so there is ample room for the stiffeners.
 
1. No. Or rather, I don't think so.

2. Depends only on fit, since the leg fairings go over the clamps.

3. Probably a bit more durable, not critical.

Dave
 
Questions

I'm installing the aluminum bar stiffeners and have a couple questions for those of you smarter than I am with an engineering background (David Paule):
1. Does it matter if the Adel clamps are steel or aluminum?
2. Do the cushions have to be removed?
3. If so, why Gorilla tape rather than electrical tape or something else?

I just finished this project.
1. I bought two packages of padded clamps off Amazon. Cheap and stainless. Easy to cut to custom length and reshape. Seems like I posted something somewhere. Slept since then.
2. No way my gear leg fairings would fit over them with pads.
3. Gorilla is very tenacious stuff. No worries about slippage.
I also had to go with #10 screws and metal lock nuts. No room for standard bolts and lock nuts.
 
Larry,
I have been following your post re the adapting the stainless clamps from Amazon. I've made several and I'm interspersing them with regular Adel's and they seem to work fine. Mine seem to fit with the cushions on but they mount a lot stiffer without the pads. I guess it is stiffness and not dampening we are after. I may do some each way just to get the spacing the way I want it.
Thanks for posting the dimensions.
Steve
 
Stiffners

For those using wood I think the secret is a good epoxy primer on the gear leg. Use glass tape and saturate the tape with epoxy on the bench. Spiral wrap the wet tape and wrap with dacron fabric, paper towels, thin foam and duct tape in that order. In other words "redneck" vacuum bagging.
 
New build RV-3B
I Used oak half rounds fore & aft of gear legs, tapered to follow gear
taper, and fit gear fairing.
as much wood cross section as I could get away with.
Pro-sealed, cured then wrapped with wet fiberglass, sealed ends
painted - looks good.
With 25 psi no shimmy, thought 25 psi was low went to 30 psi -
got first shimmy.
My experience so far.

MG
 
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