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Landing Gear Shake

fredbauerjr

Well Known Member
:confused:

We previously built an RV4 and have finished building our second RV which is a new RV3B, (s/n 11366) and we are proceeding with making it an everyday airworthy aircraft. We have discovered in our aircraft an unacceptable landing gear shimmy that sometimes borders on violent.
We are well aware of shimmy issues that affect some aircraft and yet are not present in others.
We know well and have done some of the, (let me call them "patches") that builders try such as: very low tire air pressure, gear leg stiffeners, balancing tires, balancing wheel pants, out of round tires, etc. etc. etc. in an effort to eliminate gear leg shimmy.

We have a destructive shimmy in our right side gear leg and it is busting up the trailing end of our right wheel pant. Only the right gear leg shimmys, the left gear leg seems to be operating OK. We are at this time adding wood gear leg stiffeners to both of our gear legs but have yet to get this finished and tested.

I don't want to get all the Van's crazy faithfull upset over this post, BUT --------------------- I feel we might have a heat treating or a heat treating quality control problem with our gear legs.

That is why, I believe, some gear legs work and others do not.

Has anyone ever taken their gear legs and had the heat treatment checked to see if BOTH gear legs had the SAME heat treatment? or to see what the heat treatment was? or to heat treat them to a harder treatment?

Does anyone know if the Harmon Rocket with titanium gear legs suffers from the shimmy we are experiencing?

Thoughts anyone?
 
Fred;
I have had pretty consistent shimmying as my speed slows to around 20 - 25 mph. After 140 hours the original Airhawks were replaced with Dresser Monster Retreads. The sidewalls are significantly stiffer on the retreads. After a few landings it dawned on me that there was no shimmy. None. End of story.

Bill Newkirk
 
titanium landing gear

Yes titanium gear do shake. They shake more than steel gear if the tires are out of balance or on rough ground. Titanium gear is a lot less ridged, and A LOT more flexible. I think on steel gear that shake, it's either out of round tires, or out of balance tires. What else could cause it?

Steve
 
geometry ??

My experience is limited to a 9A with pre drilled gear but is there any way you have a toe in / toe out issue? Is there any way the geometry is off?

If the geometry is off, you should see the tread scrub one way or the other.

Can you post a picture of the tire?

Barry
Tucson
 
As I recall the titanium geart legs have a flat mounting surface for the axle and standard Cessna wheel shims can be used to fix any alignment issues. They will shake more than the steel legs. Have you checked the lower fuselage inboard engine mounts? Look for hairline cracks or worse radiating from the 90 degree bends.
Good luck
Tom
RV3 978TM
 
Check the geometry

My experience is limited to a 9A with pre drilled gear but is there any way you have a toe in / toe out issue? Is there any way the geometry is off?

It is more likely that the geometry is off then the heat treat is bad. Resently witnessed a guy at our airport doing first taxi (not and RV) and had a wicked shimmy. Taxied around for several weeks trying to determine the problem. Finally made some shims to toe the wheels in (or out, do not remember which) and it all went away.
 
Can you swap wheel assemblies from side to side, that should confirm if its a tire/out of round situation if the right side starts the shimmy.
 
I've had 4 airplanes with this gear(2 built,2 bought). Lowering tire pressure helped but I agree its a patch. The last one built and a freind's Tailwind, we shimmed the outboard of the axle fwd about .065 for a little toe-in when we drilled the motor mount. No shimmy on these two.
Seems to me we drill the mount with no weight on the gear then loaded the gear flexs outwards and rearward which toes the wheels out, the tires scrub, drag, toe out more and sets up the oscillation. My first Tailwind only did it when loaded heavy. Just a theory but I'm doing the same on the -3 I'm building now.
FWIW, Its possible to adjust the toe-in/toe out by elongating the holes in the mount and tig welding washers on front and back. Never done it with the engine on.
Mike
 
Don't think heat treat is a factor

:confused:
I don't want to get all the Van's crazy faithfull upset over this post, BUT --------------------- I feel we might have a heat treating or a heat treating quality control problem with our gear legs.

That is why, I believe, some gear legs work and others do not.
Thoughts anyone?

Fred,

I'm doubtful that a change in heat treatment would affect the presence of shimmy or not. The reason is that the "springiness" (modulus of elasticity) of steel is essentially the same regardless of the heat treat. Heat treatment does affect the yield, ultimate, and fatigue strengths of steel, but unless the shimmy has caused your gear leg to take on a permanent bend, a non-heat treated gear leg would behave the same as a heat treated one.

Good luck with your problem.
 
Wheel shimmy

Fred;

Just to let you know, I've had the same problem for about a year now.:eek: I just brake alittle harder when I reach the range of the shimmy, I have talked to Vans, and they have said to check the wheel balance, which I did and found the set of tires were out of balance by 5oz!:eek: properly balancing the tires and rims helped, but it is still there. Mine has been flying for 20 years, if you find the solution, please let me know!! By the way, one day the shimmy was just "there".:confused:

Mike Bauer
Bartow, Fl.

RV-3 N87LB flying
RV-4 N742MC building
 
shimmy

to all
upon replacing a bent gear on my 4 (ground loop, I replaced both gear and the motermount) I started going through a set of tires in a month and also it had a great deal of shimmy. I set the airframe on large table, clamped a pair of levels (w/blocks) to the wheels and found over 4 inches of tow in.
I locked the gear in line w a 8ft piece of 4 x4 angle iron, drilled new holes then reamed them for brown&sharp tapered pins..... end of tire wear, end of shimmy the aircraft has over a thousand hours and flying great..

mtnflyer
 
Go to retreads?

Fred,

I have shimmy on my -3B also. I did NOT install gear leg stiffeners, rather I did everything else including balancing the PR wheel pants fore/aft. At 30 psi, which is what I had when I first flew the plane, the shimmy was significant. I started lowering tire pressure based on the advice/testing of others and that helped signficantly. Through testing I ended up running 18-20 psi and at this level it only shimmies about one in three landings and even then it's not too bad. I've found that once the shimmy starts not even hard braking will stop it, slowing down is the only thing that works.

I've started paying attention to taxi speed using my GPS and my shimmy zone is 14-18 mph with the lower pressures. Other builders have tested and found that raising the tire pressure raises the speed where it shimmies.

I replaced the Vans-supplied Aero Hawks with Michelin Airs, story here, thinking they had runout, imbalance, or high hysterisis that was causing the problem -- unfortunately not much improvement.

Did everyone read what Bill Newkirk reported above? I'd love to have one other person try changing to the retreads with the stiffer sidewalls as confirmation to see if that works. I will definitely order a set if that fixes it. Bill, still no shimmy?
 
Just to throw an odd curve into the mix, my airplane has had occasional shimmy. It doesn't have gear leg stiffeners, but the wheel pants are balanced.

Anyway, I started out with 28-30 lbs of pressure in the tires, and over the years dropped it down to about 22 lb trying to find a magic pressure that would reduce the shimmy.

For whatever reason, one time I said "what the heck" and put 35 lb of air in both tires. Surprisingly, the shimmy got better, not worse. I've been running 32-35 lbs ever since and think that is an improvement for me.
 
For whatever reason, one time I said "what the heck" and put 35 lb of air in both tires. Surprisingly, the shimmy got better, not worse. I've been running 32-35 lbs ever since and think that is an improvement for me.

I run 32 lbs. also. It seems about right.

L.Adamson --- RV6A
 
Sorry for the hijack back to Freds original question. I think if you have "destructive shimmy" its not an issue of tire pressure. I'm guessing toe in/out, camber issues. My gear legs from Vans were not even close to spec for camber. I had to have them rebent. I still have a little of the RV shakes but totaly manageable with tire pressure adjustments. Btw tire wear is now close to perfect.
Ymmv
Tom
RV3 x 2
 
shimmy

glassing oak wood strips to the legs stopped the shimmy on my -6. tire pressure is important, I use 40-42psi. My -6, N16DD is rather heavy with IO-360 and 3 blade. goes like stink tho.
 
Retreads

Randy, to answer your question, still no shimmy, BUT one fact I failed to include is that I have been flying without wheelpants. The slightly larger Monster retreads wore a hole in the top of one wheelpant. Perhaps it is the lack of wheelpants, not the retreads, that is responsible for the improvement. I'll keep you posted.

Bill Newkirk
 
what an impressive bunch of -3 owners

Everyone, Thanks for all the great input, insight and general concern.

Like Randy's RV3 our shimmy can not be stopped with braking, you just have to ride it out until you reach a low enough speed for it to stop. (this is about the same with tire pressures of 20 to 38 psi)
When we built our -3B we did install just a little "toe-in" to our landing gear, maybe 2 to 3 degrees. The reason we did this was:
#1 my past experience showed that when we built our -4 and aligned the gear "straight on" after a little usage and some harder landings our gear took a set with a little toe-out causing inside tire wear and we wanted to end up with zero toe-in/toe-out on our gear, #2 any weight on the gear caused slight toe-out, #3 any drag through braking or otherwise caused slight toe-out, #4 for what it is worth any snowmobile with the tracks set "straight on" wander all over the place (they need a slight toe-in to run straight).

We built a Thorp T18 with the 2" longer gear legs and all the while we owned and flew the T18 we NEVER EVER had a shimmy problem or a problem of any kind with the gear, (the T18 gear are swept back very much like the -3 and -4).
The T18 gear was made using a larger 4130 tube (at the lower engine mount attach point) with a smaller 4130 tube inserted "inside" the larger tube and continuing down to a weldment at the axle attach point. The larger tube only extended to about 1/2 way between the lower firewall attach point and the axle. I am seriously considering contacting Ken Brock Sport Aircraft to see if he will make me some gear legs like the T18 uses. The only critical spot would be the attachment to the existing Van's engine mount and if the new gear tube does not nest inside the existing RV engine mount attach tube maybe it could be attached using a machined steel insert between the engine mount and the gear leg.
Anyone have any poor experience with the T18 landing gear?

OK ---- What say you guys?:confused:
 
Fred,

I have the shimmy in my -3 as well. The sure fix is to fly it off grass, which I do about 98% of the time. I've gotten it to a manageable level on asphalt by running 18-20 psi and watching the taxi speed as Randy does.

I'm just thinking out loud here, so don't start throwing rocks. If there's any toe-in/toe-out in the normal three point rolling attitude, the tires will be driven together or apart which will load the gear legs. At some point the force the gear leg exerts on the tire will exceed the friction between the tire and pavement and the tire will break loose. Increase the rate and you have what seems to be a lateral vibration.

If your tires have a periodic squeak when you roll the aircraft forward on a smooth hangar floor, I'm guessing you have too much toe-in/toe-out. A better test might be to remove the wheel pants and clamp a couple of 1"x2"x8'
to the pant brackets, then roll forward and see what happens. Give that a try and see what you get.

Tony
 
I've been trying to get rid of an RV-3 landing gear shimmy with mixed results.
The wood stiffeners helped the most.
Lowering the tire pressure helped some.
Balancing the wheel pants helped some.
Balancing the tires helped a little.
Switching to Cleveland wheels and brakes (5.00/5s) helped too.
In my case, the gear alignment is just fine and tire wear is normal.
Also, I spoke with Van about it, and he said the shimmy is a sympathetic harmonic vibration that's a common characteristic on tapered-rod gear.
Best,

Dave

P.S. Try altering your main gear tire pressure (one at the high end of the range, the other at the low end). This seems to help, too.
 
I know that excessive drag on the wheels (axle nut being too tight) can set up a shimmy situation. Ask me how I know. :eek::)
 
I am just thinking out loud here but has anyone thought about installing something to "oppose" the natural gear leg frequency? Something maybe inside the wheel pant that would dampen the shake?

I am thinking that when the gear leg moves forward or aft to start the fore and aft movement something inside the wheel pant would resist moving forward or aft at the same speed and would then almost arrive at its most forward or aft position later meeting the gear leg that is already moving in the opposing direction so it would deaden the gear leg shake frequency kind of like how a marracca works? Something we could bond into our wheel pants.

Oh Well! maybe just another dumb idea.

I still believe the T18 gear legs with a steel insert at the top end to attach them to our existing Van's engine mounts is the answer. Thereby eliminating entirely the Van's tapered rod gear legs, that way we all could use our existing engine mounts by pinning the steel insert with the very same 1/4" bolt we now use to hold the existing tapered rod legs in place. Besides, we could then "shim" the T18 bolt-on axles at any time to adjust our toe-in, toe-out and camber as needed.
 
For whatever reason, one time I said "what the heck" and put 35 lb of air in both tires. Surprisingly, the shimmy got better, not worse. I've been running 32-35 lbs ever since and think that is an improvement for me.

Since I've never tried anything over 30 psi, and since others have reported that high pressures helped them, I plan to put 42 psi in and try it just in the interest eliminating that possibility. I'll report back once I do.

Suspecting a gross toe in/out issue is good thinking and may be the cause in some cases, but I'm pretty sure I have no toe in or out issues as my tires are wearing very evenly.
 
but I'm pretty sure I have no toe in or out issues as my tires are wearing very evenly.

Randy,

Is toe in/out what causes uneven tire wear? If the elliptical tire footprint is centered left/right on the tire, then it would seem that rotating it slightly one way or the other would still scrub rubber mostly from the center of the tire. Adding pressure would make the footprint smaller, decreasing the wear band width. Don't know the answer, just asking the question.

Tony
 
Randy,
Is toe in/out what causes uneven tire wear? If the elliptical tire footprint is centered left/right on the tire, then it would seem that rotating it slightly one way or the other would still scrub rubber mostly from the center of the tire. Adding pressure would make the footprint smaller, decreasing the wear band width. Don't know the answer, just asking the question.
Tony
I would think toe in or out certainly could cause tire wear just like in a car. If the tire is forced to roll down the road (runway) at an angle it will scrub. The scrubbing will necessarily wear whichever side is the leading edge. Set an excessive toe-in condition on a car and it wears the outside edges of the tires, set it to an excessive toe-out condition and it will wear the inside edges of the tires. Wouldn't this apply to our mainwheels also?

With aircraft landing gear we also have camber to think about which introduces yet another variable and potential cause of uneven tire wear.
 
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We need a better study of the geometry

I find it interesting that some RV's have the shakes, and some just dont. I was lucky and mine does not have any shimmy that I have ever been aware of or anyone else has noticed.
My tires wear slightly more on the outside. This makes sense to me as that part of the tire hits first until the gear is loaded. However, there may be a bit of toe in too, I just dont know.
So the question, would you not want a slight amount of toe in for better tracking? I would think perfectly aligned wheels could allow for oscillation to start. If you have a bit of toe in, that resistance would load the gear slightly. As noted, cars have both, toe in and camber. Just a thought.
While we are thinking out loud, keep in mind we are hanging more weight over the gear than they used to also. But, that is true of my 6 too.
My 6 was predrilled. I will need to drill my 3 so this thread is of great interest to me.
So, Fred and Randy, did you drill yours with any toe in? Do you think it might make a difference?

PS- Randy, I have seen you land many times and have never noticed it shimmy for what that is worth. Usually perfect greasers too.
 
When we built our -3B we did install just a little "toe-in" to our landing gear, maybe 2 to 3 degrees. The reason we did this was:
#1 my past experience showed that when we built our -4 and aligned the gear "straight on" after a little usage and some harder landings our gear took a set with a little toe-out causing inside tire wear and we wanted to end up with zero toe-in/toe-out on our gear, #2 any weight on the gear caused slight toe-out, #3 any drag through braking or otherwise caused slight toe-out, #4 for what it is worth any snowmobile with the tracks set "straight on" wander all over the place (they need a slight toe-in to run straight).
 
When we built our -3B we did install just a little "toe-in" to our landing gear, maybe 2 to 3 degrees. The reason we did this was:
#1 my past experience showed that when we built our -4 and aligned the gear "straight on" after a little usage and some harder landings our gear took a set with a little toe-out causing inside tire wear and we wanted to end up with zero toe-in/toe-out on our gear, #2 any weight on the gear caused slight toe-out, #3 any drag through braking or otherwise caused slight toe-out, #4 for what it is worth any snowmobile with the tracks set "straight on" wander all over the place (they need a slight toe-in to run straight).
And I drilled mine dead straight with the axles clamped solidly to a dead straight board. My belief is that a slight touch of toe-out is desirable and I figured all the friction and braking forces would provide
 
Fred,

Just remembered that I had an incident with gear shake on my Sonex, which has constant-diameter rod titanium gear legs. It was so bad I thought it would knock the flaps off the wings. I was able to duplicate it in the hangar by jacking the wheel off the ground with the gear leg unsupported, then spinning the tire up using a rotary buffer on the tire sidewall. Spun it up as fast as the buffer would go, then watched as it decelerated. At a certain speed, it would shake violently for a second, then stop as it slowed below the critical speed. Flying off grass, the tires weren't worn. Unable to figure out what was causing the problem, I replaced the 11.4.00x5 tires with new ones. Problem solved. Never did check the balance. I plan to check the balance on my -3 tires at the next opportunity.

Tony
 
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Toe in Toe out

Ok, we have one drilled Straight with a tendency to Toe out and one drilled Toe in with a tendancy to go Straight. I am beginning to think the problem lies elsewhere. Fred, can you really measure one or two degree's accurately on that small of a tire? Even so, is one or two degress enough or is it possible that you are toeing out when weighted as Randy would suggest?
Randy, I am surprised by your comment that toe out was a good thing to try to achieve. That goes against traditional thinking in regard to tracking and stability. I know you track straight so it must not be an issue in that regard.

Now I am just as confused about this whole shake issue as everyone else I guess. I really like Tony's idea of testing the wheel off the ground. If you dont have your own, I have wing jacks and a buffer if you want to give that a try some time. Fly on over and let's give it a "spin" and see what happens.
 
Randy, I am surprised by your comment that toe out was a good thing to try to achieve. That goes against traditional thinking in regard to tracking and stability. I know you track straight so it must not be an issue in that regard.

Now I am just as confused about this whole shake issue as everyone else I guess. I really like Tony's idea of testing the wheel off the ground. If you dont have your own, I have wing jacks and a buffer if you want to give that a try some time. Fly on over and let's give it a "spin" and see what happens.

Tailwheel aircraft are very different than cars, the same alignment principles don't apply. There are indeed differing opinions about whether toe-in or toe-out is desired in a taildragger but based on what I've read, and on my understanding of the forces at work, I'm a believer that just a touch of toe-out is the way to go. Do some research, you'll see. Now I wish I'd kept a copy of some of the articles I've read.

JJ, our chapter has that wheel balancer we bought from Desser, we should be using it. Though I've balanced by wheel pants I nave NOT balanced my wheels yet, shame on me. Will do that at annual.
606-7283.jpg


Tony, doing the test you did before and after balancing the wheels would be most informative.
 
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Sent this to Randy earlier this morning to get his take on it before posting on VAF, but since I'm hitting the road for a week in a few minutes, I'll go ahead and post it here:

The more I think about this gear shake problem, the more I think it's tied to tire imbalance/out of round. I'm guessing that because the -3 has the lowest gross weight of all the RV's, it also has the gear legs with the smallest cross-section. But it still carries the same weight as all the other RV's at the wheel end.

Here are a couple of photos using a gocart wheel balancer. Scroll down to page 22:

http://www.jimslightning.com/Lightning_Newsletter_2-3

Also, go here and have a look at the static balancing ways:

http://www.mcmaster.com/#static-balancing-ways/=ub9ax

We had those at Corning when I worked there. Mighty expensive, but sensitive enough to check the balance on a shaft. I think I can duplicate in aluminum for just a few bucks.

And Google tire shaving.

Tony
 
I'm a firm believer in toe-in. The Bakersfield guys went thru this years ago. The toe-in shuld be measured in a static (no wieght on wheels) condition. when the gear deflects because of the geometry it also goes toe -out. Lower the tail and the toe-out increases. I forget the the exact recommendations but it was something like placing a .040 shim at each axle end. then squaring the axles with a 2x4 before drilling the gear/mount holes. If you weenies :p are building quick builds then you might not have this option.
YMMV
Tom
 
Ok, we have one drilled Straight with a tendency to Toe out and one drilled Toe in with a tendancy to go Straight. I am beginning to think the problem lies elsewhere. Fred, can you really measure one or two degree's accurately on that small of a tire? Even so, is one or two degress enough or is it possible that you are toeing out when weighted as Randy would suggest?

I don't know how to copy the previous post and display it so hang in there with me until I get it figured out.:)

Jon,

We will get some accurate measurements on our -3B gear legs and post it for everyone soon.
It was pretty easy for us to measure the 2 to 3 degrees because we did so BEFORE mounting the engine mount to the airframe. We therefore could easily extend the alignment lines and work with a larger picture.
 
I'm a firm believer in toe-in. The Bakersfield guys went thru this years ago. The toe-in shuld be measured in a static (no wieght on wheels) condition. when the gear deflects because of the geometry it also goes toe -out. Lower the tail and the toe-out increases. I forget the the exact recommendations but it was something like placing a .040 shim at each axle end. then squaring the axles with a 2x4 before drilling the gear/mount holes. If you weenies :p are building quick builds then you might not have this option.
YMMV
Tom

You will not find any weenies in the RV3 group!:D
 
For what it's worth...

I did quite a lot of research on the toe-in vs. toe-out on tail wheel aircraft when I designed the conventional gear for the Moni Motorglider. I started with a slight amount of toe-in. The airplane was slightly difficult to handle on the ground. Then I read an article that explained it nicely. Think of it this way; If you have toe-in and start a swerve to the left, this puts more weight on the right wheel. Now because of the toe-in, this wheel is pointing left pulling the airplane more into the left swerve. Now with the same scenario, but with toe-out, the right wheel is now pointing right pulling the airplane straight.
After reading this, I modified my gear to have a very slight amount of toe-out and it became a pussy cat on ground handling.
Now, the design of the RV gear calls for the wheels to be straight with no load. As you load the gear it toes out slightly.
BTW, my Moni gear was straight aluminum so load did not change toe-in/toe-out.

Just my $.02 as usual. You get what you pay for!
 
Sent this to Randy earlier this morning to get his take on it before posting on VAF, but since I'm hitting the road for a week in a few minutes, I'll go ahead and post it here:

The more I think about this gear shake problem, the more I think it's tied to tire imbalance/out of round. I'm guessing that because the -3 has the lowest gross weight of all the RV's, it also has the gear legs with the smallest cross-section. But it still carries the same weight as all the other RV's at the wheel end.

Here are a couple of photos using a gocart wheel balancer. Scroll down to page 22:

http://www.jimslightning.com/Lightning_Newsletter_2-3

Also, go here and have a look at the static balancing ways:

http://www.mcmaster.com/#static-balancing-ways/=ub9ax

We had those at Corning when I worked there. Mighty expensive, but sensitive enough to check the balance on a shaft. I think I can duplicate in aluminum for just a few bucks.

And Google tire shaving.

Tony

I found an excellent wheel balancer. It is designed for motorcycle tires and comes complete with smooth running bearings on each end of a shaft with cone shaped sleeves that insert into the wheel center. Very well made and works on various wheel axle sizes. All you need is a flat surface to set the assembly with the wheel onto. I will find out where we got it on google.
 
Agree with Mike regarding heat treat.

The geometry of a swept tapered leg is complex. Toe-in or toe-out changes with aircraft attitude (level or three-point) if there is any camber in the set-up. And to make it more interesting, camber changes with load.

BTW, designing an airplane with rearward-swept legs introduces an overlooked item of geometry...negative trail. Any torsional deflection of the gear leg is divergent. Toe-in or toe-out would be a driver.



I suspect the wood strip "dampers" would be much more effective if the wrap was kevlar at a 45 degree bias. No data.

Tony's very good observation regarding wheel balance and a critical speed is an issue separate from geometry. The critical speed is found where the natural frequency of the gear leg is matched by the rotational frequency of the out-of-balance tire. Balancing the tire doesn't change the natural frequency, but it does greatly reduce the driving power of the exciting frequency. Perfect balance makes it zero.
 
I didn't build my -3 so I can't say how the gear was set with regards to toe and camber. It has the wood damper strips on the legs. As long as I keep the tires inflated to above >22# psi it does not shimmy. I've been airing my tires to 38# and if I get shimmy, I know it is time for more air. It's like a low tire pressure warning system.
Bill
 
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I found an excellent wheel balancer. It is designed for motorcycle tires and comes complete with smooth running bearings on each end of a shaft with cone shaped sleeves that insert into the wheel center. Very well made and works on various wheel axle sizes. All you need is a flat surface to set the assembly with the wheel onto. I will find out where we got it on google.

Here is the tire/wheel balancer we purchased. www.marcparnes.com.

Great service from this gentleman.
http://www.marcparnes.com/Moto_Guzzi_Motorcycle_Wheel_Balancer.htm
 
I didn't build my -3 so I can't say how the gear was set with regards to toe and camber. It has the wood damper strips on the legs. As long as I keep the tires inflated to above >22# psi it does not shimmy. I've been airing my tires to 38# and if I get shimmy, I know it is time for more air. It's like a low tire pressure warning system.
Bill
Good info, now I REALLY need to get out and try 42 psi and see if there's any change. Will do and report back when it stops raining around here.
 
Tried the high pressure route

Ok, the sun peeked it's head out today and I was available so I filled my tires up to 42 psi and went flying. Wow, it sure does taxi faster! I could tell just taxiing though that it wasn't going to work. I flew to a nearby airport (SPB where Mike Seager does his thing) and did some landings will full taxi back and there's no question that it is worse, wanting to shimmy at least a little at the slightest provocation. I took my tire gauge and valve stem extension with me, reset them to 20 psi and went up again... nice! It just feels better all the way around with the tires soft, only the faintest a hint of shimmy, the low pressure just damps things out. It also provides more rolling friction which better controls taxi speed and requires less (or no) braking on the runway. For many of the larger RVs 20 psi wouldn't work, the tire would deform too much, but with our relatively light RV-3s is seems to work just fine.

I will make a point of using our chapter's tire balancer at my condition inspection and report back then also. I suspect that will help the little shimmy I have left, but we'll see. I would still love to hear from someone with shimmy who then balanced their wheels/tires to see if there was a change.

Man, it sure felt good to get back in the plane! There is just nothing like the RV-3 you guys, very sweet flying bird, truly an extension of your body. Going out on a sunny day with an airplane that you know so well, and with nothing on your sqwauk list, it's just hard to describe the feeling as you motor along in complete harmony with the plane and the environment. Slide into a close-in downwind, fly a tight pattern turning base just after the end of the runway, keep it turning it tight rolling out on very short final with the airspeed nailed, bring in a scosche of power as you finese it down the last few feet with ever-so-subtle pressures on the stick, feel the mains just kiss the runway in a nice wheel landing, hold the tail up down the runway because it's just so easy to do, let it down at just the right rate so it looks like you know what you're doing, and exit -- it just doesn't get any better guys. They really are the best flying member of the RV family, just no denying it.
 
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wood stiffeners added

We installed some wood (maple) stiffeners to the trailing edges of our gear legs. Our wood stiffeners start at the engine mount socket and end just above the gear leg curve to the axle. These wood stiffeners were wrapped with 2 layers of normal fiberglass cloth over 1 heavier layer of bi-directional fiberglass cloth applied using the WEST System epoxy and using fiberglass cloth purchased at a boating store.
We can only report our findings on 3 flights so far and the three wheel type landings we made, but we have found only a very very slight "quiver" not even approaching or close to the violent shake we had originally. We only notice the slight quiver when landing and slowing through 10 to 20 mph.
We will report any other findings (both good AND bad) as we add more flights and 3 point type landings to our testing.
We have our tires at 25 psi.
 
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LANDING GEAR SHIMMY

The flat spring and round spring gear were both designed by the late Steve Wittman. He held the patent on both and sold the flat gear rights to Cessna in the mid 40's and the round gear rights at a later date. The first production Cessna to use the flat gear was the 120/140 in 1946. Having been involved with the Wittman Tailwind since the early 60's I have collected quite a bit of information on the round version of this gear. The current Tailwind plans call for 1/16" toe in and 3/8" camber, both measured in the length of the axle. The toe in "goes away" at some point as the gross weight is increased. Also momentarily on hard landings. Any toe out is very undesireable. The stiffners definitely help. Something that I have not seen mentioned is an adjustable clamp on the lower end of the socket tube. This can be a short piece of heavy wall tubing, 5/16" i.d. welded to the forward portion of the socket tube. After welding the tube is slit down the center, thru the socket tube and a bit above the clamp bushing. The bolt can be adjusted periodically to keep the lower portion of the gear leg tight. If there is ANY play in the upper end the bolt can be tightened enough to slightly deform the socket and eliminate this play. The Beech Model 58 Baron has a 500 x 5 nosewheel. At forward cg it is very hard to hold the nose off on landing, and the Baron is prone to moderate to severe nosewheel shimmy. I found this can be eliminated completely by useing the most expensive Goodyear tires and static balancing the wheel/tire assembly. The Michelins should be equivalent to the best Goodyears. I believe some of the early RV's were 3/4" diameter gear legs at the smallest diameters. Increasing this to 7/8" should help to reduce shimmy. Any thing less than 25# tire pressure is very marginal on all but the lightest airplanes. At very low tire pressures it is possible to spin the tire on the wheel with heavy braking, and a hard landing could deform the tire until the wheel is damaged. The prototype Wittman Tailwind, which is on display at Pioneer Airport at Oshkosh, has flats milled on the sides of the round gear on one gear leg only. I know nothing else about this. In theory it would seem that this modification would cause the gear to move more in and out more than it does fore and aft.
 
Shimmy

Fred,

Mine was had a shimmy on the Left side. I took the wheels off and saw two things. The tires are out of round for sure and secondly the left tire was way out of balance. I think cheapy tires out of balance are a big part of the story. As previous posts have said try swapping your wheels and see if it follows the wheel. Easy enough.
 
Been following this thread for awhile; do the planes that shake their gear do it on both take off and landing or on landing only or on take off only? Mine only did it on landing. Tire pressure and a new set of skins took mine away. Any comments on landing only shakes.
 
Mine was wheel balance

Been following this thread for awhile; do the planes that shake their gear do it on both take off and landing or on landing only or on take off only? Mine only did it on landing. Tire pressure and a new set of skins took mine away. Any comments on landing only shakes.
My 7A developed a pretty bad shake on landing, never noticed anything on takeoff. The tires were getting pretty worn on one side, so I took them off to flip the tires, and checked the balance while I was at it. The balance was way off. The rotated and balanced tires are smooth as silk now!
 
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