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6A Vibrating Main Wheel pants

Planecrazy232

Well Known Member
See the video below. I'm not an editing guru so please forgive the video length and quality.

I was getting some severe vibration and noise during taxi so I mounted a GoPro under the wing to see what was going on. I was surprised to see how much the mains were vibrating. I had balanced the nose wheel but not the mains.

I balanced the mains and although it felt much better inside the plane, they still moved around alot. The pants are mounted securely.

I was told that a possible fix for this is to install about 2 pounds of lead into the nose of the wheelpants to balance them. I'd like to hear the consensus on that.

Look how much the exhaust pipes are moving during cruise flight. Normal?

There are a few takeoffs and landings in the video also.


https://youtu.be/G7rqQa1VIwI

Thanks,

Anthony
 
You may find

as others have, that wood stiffeners, even if only installed on the opposite side of the brake lines, as opposed to both sides as some have, will provide significant dampening to the motion you uncovered in the video. I put stiffeners on the mains and nose gear on my 6A - fairly easy to do and they work well. There is some discussion, as I recall, in the Van's 6 manual.
 
Rumor has it that by dropping the main gear tire pressure to 26 PSI, you'll get no shimmy. This comes from a fellow that built a 6 more than 25 years ago. He still owns and flys regularly. Also, it has no gear leg stiffeners.
Lorne
 
Rumor has it that by dropping the main gear tire pressure to 26 PSI, you'll get no shimmy. This comes from a fellow that built a 6 more than 25 years ago. He still owns and flys regularly. Also, it has no gear leg stiffeners.
Lorne

That was the wisdom years ago and may be true.

That said, I have no vibration these days (I run 35-40 PSI), but had a lot of shimmy when I ran 25 PSI.

Go figure.
 
There is a bunch of different factors that influence the tendency for main gear leg shimmy.

- out of round tire
- out of balance tire
- tire air pressure

This is a lot of the reason some people have had the problem and later don't, or sometimes go for years not having it, and then suddenly do.
 
An old thread but .....

RVbuilder2002.

I just read your comment about wheel balance, pressure etc. Im just researching this problem and have tried different pressure and flipping the tire on the wheel to get the better portion of tread and to change the wheel from left side to right side (not balancing yet).

Id though the shimmy would switch sides or the non-shimmy wheel would stop the shimmy when moved over to the shimmying leg. Since moving the left wheel to the right side and Visa Versa, didn't change my shimmy it makes me want to disagree with any balancing, roundness etc comments.

I think that if the wheel was causing the shimmy it would have moved to the other side when i switched the wheels.

I've been doing a lot of reading tonight and seen a lot of comments where balancing, tire pressure and or new tires have fixed the shimmy. And others that have done these things and it didn't fix the shimmy.

I didn't build my airplane. Its got roughly 1100 hr and the shimmy only started two landings ago. Before that it was great.

This is a very old thread so I will probable post a new one tomorrow. Id decided tonight was just going to be research.

Thanks
 
There is a bunch of different factors that influence the tendency for main gear leg shimmy.

- out of round tire
- out of balance tire
- tire air pressure

This is a lot of the reason some people have had the problem and later don't, or sometimes go for years not having it, and then suddenly do.

I've been doing a lot of reading tonight and seen a lot of comments where balancing, tire pressure and or new tires have fixed the shimmy. And others that have done these things and it didn't fix the shimmy.

I never meant the above list to be consider comprehensive (note, I said there are a "bunch of different factors that can influence shimmy"). Sorry if it made that impression.

These are some of the common causes that can easily be checked and rectified if needed.

Another shimmy cause is incorrect axle alignment. This can be from incorrect alignment when the airplane was built or a slightly bent gear leg.

Even though you have swapped from one side to the other it it didn't change, I recommend you balance the wheels before doing anything else (it is possible that something with the one side is more sensitive to imbalance).
If that doesn't help, then start looking at gear leg and wheel alignment. One quick check for that is to confirm that the airplane still sits level on the gear (measure the height of each wing tip).
 
No apology necessary.

I’ll apologize if I implied there was just three possibilities :)

I am not a builder and I’m working my way through a strange occurrence. Mechanically, when something starts right out of the blue it’s generally something that has just changed. I’ve been working my way through it.

The one WTF moment was when I switched the wheels side to side and the one side still wobbled.

My tires are old and have not been balanced. Good advice to start there. I’m going to order a couple new tires balance them and start again.

I also need to pull that leg, clean it up and tighten it up. It has a very small amount of motion and squeaks when moved so it could be contributing factor.

Good thing it’s raining and not sunny.

Thanks for your input I appreciate it.
 
More food for thought:

Some clues for possible axle misalignment:
- uneven tire wear- do both main tires have the same wear pattern? does one tire have uneven wear, flat spots, fish scale, more wear on one edge than the other edge (compared to the other main wheel?
- harder to push into the hangar than it is to pull it out of the hangar, or the other way around. (most evident on a nice epoxy floor, does it squeal more pushing it compared to pulling it out of the hangar). This is symptom of toe in/ toe out issues.
- more black tire powder in one wheel pant compared to the other.

Other potential cause of shimmy.
- wheel bearing pre-load setting, too tight could cause resistance in roll out.
- brake pad dragging, uneven pressure release, maybe needing master cylinder return spring.
- brake pad dragging, caliper slider pins wearing miss aligning pads, pads uneven wear.
 
Jack your main tire off the ground and lay a 2 x4 in front of it close to the tire but not touching. Spin the tire to see if it is out of round.
 
ok now Im up tooooooo

More food for thought:

Some clues for possible axle misalignment:
- uneven tire wear- do both main tires have the same wear pattern? does one tire have uneven wear, flat spots, fish scale, more wear on one edge than the other edge (compared to the other main wheel?
- harder to push into the hangar than it is to pull it out of the hangar, or the other way around. (most evident on a nice epoxy floor, does it squeal more pushing it compared to pulling it out of the hangar). This is symptom of toe in/ toe out issues.
- more black tire powder in one wheel pant compared to the other.

Other potential cause of shimmy.
- wheel bearing pre-load setting, too tight could cause resistance in roll out.
- brake pad dragging, uneven pressure release, maybe needing master cylinder return spring.
- brake pad dragging, caliper slider pins wearing miss aligning pads, pads uneven wear.

A whole bunch of stuff to check. Because this is a new issue that just started two landings ago and from the advice I've received here i think I am going to

#1
Take a careful look at the tires to see if there are any wear pattern clues

#2
Check the torque on the right gear leg attachment bolt (the only one shuddering). Pull the leg, clean grease and put in a new bolt if necessary. This leg is squeaking and has a tinny amount of movement.

#3
Order some new tires (i had just flipped these around as part of the trouble shooting and to wear off the unworn side). It made no difference in the shuddering but ....

#3
Balance them well.

#4
Have someone check my preload and tightening technique (even though I have not had problems before).

#5 ????

BTW, I have read about either going one size bigger on the gear attachment bolt or using a "close fit" bolt? Some of the logic to me is to just go to the oversize bolt. However, drilling in that location over reeming in that location might help in choosing which method to use. Thoughts?

Thanks
 
#5 ????

BTW, I have read about either going one size bigger on the gear attachment bolt or using a "close fit" bolt? Some of the logic to me is to just go to the oversize bolt. However, drilling in that location over reeming in that location might help in choosing which method to use. Thoughts?

Thanks[/QUOTE]

Consider epoxy like JB Weld to fill any low spots in an oversize bolt hole.

RC680 loctite bearing retaining compound on the gear leg & socket . It is NOT thread locker, but 4000PSI shear strength for cylindrical parts bonding.

You can check the wheel alignment with straight edges on the brake discs and adjust if the leg can be rotated slightly before the RC680 sets up.
 
I have a -6A which i had been flying without a shimmy for some time, then I changed the tires. No changes in tire pressure would eliminate the problem. I fought the problem for a year with no luch. I balanced the mains and the problem went completely away. I always keep my tires balanced now and no problem.
 
Every time I've gotten into a shimmy problem it was due to air pressure (I keep the mains at 34 psi) and/or tire issues like being flat spotted.
 
I would also check that the bearing preload isn't too loose. A loose taper bearing will drag and release underload, probably more of a cause than a tight bearing.

I keep my mains at 50psi with no main-gear shimmy.

On a side note I do get a bit of nose-gear shimmy at high taxi speeds. I believe this is a mass/harmonics issue. for the following reasons:
1) I have sealed ball bearings in the nose wheel and they run very smooth, had them since day1
2) During much of phase 1 I did not have any shimmy at any speed, at the time, the nosewheel pant was not installed. The shimmy started immediately after the pant was installed (next flight).
3) Per video evidence, the shimmy is fore/aft flex of the gear leg, not the caster.

The pant mass must have changed the resonant frequency of the gear leg assembly. I need to test with some added mass to see the impact, on my to-do list.
 
Great discussion. Thanks

I'm unable to work on the plane for a couple weeks but i will come back with an update of what i did and the results.

Some time back i build a wheel balancer for RV tires and made it high enough to straighten bicycle wheels too. It works pretty good but i've been looking for a better way. Initially, i couldn't find anyone (within a reasonable amount of time) to balance a wheel for me. Hence building my own.

However, i now have a contact in a local collage that runs a motorcycle course. They have a wheel balancer and I will be able to get in to try it out. Ill also let you know how well their system works. I'm planning on balancing it with mine first then put it on theirs. :)
 
JB Weld

WHY is JB weld being considered here for an oversize hole repair?

Adding material with a compressive/tensile strength of 3.9 KSI compared to the parent material strength of 81 KSI (4130 steel) makes no sense at all.

The best that will do is stabilize the joint temporarily and then fail, leaving you with the impression that the problem is fixed.

Not to bash, but the surfaces designed to bear the loads were designed with the parent material in mind.
 
Every time I've gotten into a shimmy problem it was due to air pressure (I keep the mains at 34 psi) and/or tire issues like being flat spotted.

My experience exactly. Anything pushing the high 30's in psi on the mains will cause trouble.

SNIP

I keep my mains at 50psi with no main-gear shimmy.

SNIP

I'd love to run 50psi on the mains, as the plane is so easy to push in/out of the hangar, but it doesn't work on my plane. The internal friction and rolling friction of the tires are the only damping in the spring-mass system (unless one installs some wood behind the gear leg).

It is interesting that this property seems to vary so much from plane to plane, even though the gear legs are the same (in this case 6A to 6A).

Tire brands also make a difference.
 
It is interesting that this property seems to vary so much from plane to plane, even though the gear legs are the same (in this case 6A to 6A).

Tire brands also make a difference.

I think much of the variation maybe driven by toe-in/toe-out. Also, camber. My left main wears more on the outside than the right. When building, I noticed that the bend in the gear leg was not perfect between the two. The left had slightly less bend.

Also, I run a bit higher bearing preload, not enough to make it hard to roll, but the rollers are well seated, all this may contribute to the variation from one aircraft to another
 
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I think much of the vaiation maybe driven by toe-in/toe-out. Also, camber. My left main wears more on the outside than the right. When building, I noticed that the bend in the gear leg was not perfect between the two. The left had slightly less bend.

Also, I run a bit higher bearing preload, not enough to make it hard to roll, but the rollers are well seated, all this may contribute to the variation fron one aircraft to another

Colin, very good observations, and quite likely a large (dominant?) the source of variation. I've been threatening to add wood stiffeners for 19 years, one of these days I might... However, if it is the caster that is energizing the resonance, they might not help. You've given me food for further thought.
 
Colin, very good observations, and quite likely a large (dominant?) the source of variation. I've been threatening to add wood stiffeners for 19 years, one of these days I might... However, if it is the caster that is energizing the resonance, they might not help. You've given me food for further thought.


You can always do a test. Attach the stiffeners with a couple layers of fiber-reinforced or duct tape, see if it helps before you attach with fiberglass. Someone else wrapped their leg with carbon fiber (without the wood) and said it helped.
 
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You can always do a test. Attach the stiffeners with a couple layers of fiber-reinforced or ducts tape, see if it helps before you attach with fiberglass. Someone else wrapped their leg with carbon fiber (without the wood) and said it helped.

This is a good idea - maybe strapping tape and some oak. The temporary fastening will likely not prevent shear movement between the gear rod and the wood strip, which might be a large component of the damping of fore/aft vibrations.

Will ponder this, thanks.
 
This is a good idea - maybe strapping tape and some oak. The temporary fastening will likely not prevent shear movement between the gear rod and the wood strip, which might be a large component of the damping of fore/aft vibrations.

Will ponder this, thanks.


The strapping tape is permanent enough. I've got wood stiffeners in place with strapping tape for 3 years now, and still in good shape. Easily changed out if necessary. No reason to glass them.
 
There is a bunch of different factors that influence the tendency for main gear leg shimmy.

- out of round tire
- out of balance tire
- tire air pressure

This is a lot of the reason some people have had the problem and later don't, or sometimes go for years not having it, and then suddenly do.

... This is all spot-on, and is most likely the source of the issue. One other possibility, and method of checking for it is this.
If the problem gets worse as you slow down to exit the runway, or as you apply braking? This is an indicator of a potato
chipped, glassed, or hot spotted brake rotor / rotors. easy to check and verify! Thanks, Allan...:D
 
I’m back :)

Just caught up on the posts. I’m going to start from the beginning and tell you where I’m at and the results. First.

Shimmy started after the annual. I had violent shimmy so I checked the front breakout, pressure etc etc. Btw I have the Antisplat bearings on the front and wish they were here to do balancing and rounding out tires. Everything looked good. High speed taxi still shaking. I had a friend watch while taxing and he said the right wheel was flopping all over the place.

Tires are old and ready to flip next annual. So I flipped the tread with the tire on the same side but new tread to the ground. No difference.

Flipped the tires from right gear to left gear etc. No difference. While looking for problems on the leg, but figuring it was a tire problem, we noticed a very small amount of motion in the right leg. 1 or 2 thousandths (purely a guess).

So it wasn’t the tires and had to be the slight motion in the leg. Solution?

I bought new tires, tubes, balanced them and put in 34lb of air. Recommendation from basically everyone and I needed them eventually anyways.

Next I researched getting rid of the small amount of play in the leg. There is a post here where a gentleman who used locktight ?? 680 ?? To fix the movement. I called locktight and permatex for advice. What it came down to was the more gap filling and strength you wanted the more heat or hammering you would need to remove the leg. And what product was available in my area. One of the local Suppliers in Vancouver wouldn’t talk to me because I wasn’t Boeing”. One said they could get what I wanted in 4 to 6 weeks.

I’ll try to remember to attach a word doc I made with some characteristics of their products.

So to make a long story short. I used locktight 690 (not as strong nor as gap filling as the 860) You have 4 minutes to get it into place. Then you need to leave it for 24 hours. I got my wife into the jacked up, leg less aircraft. We practiced getting the leg into place the bolt in and ready to get onto the ground. Then we did it. And, of corse the wife got stressed out about the four minute working time and had trouble getting the bolt in. But she did, I put the wheel on, dropped the airplane onto the ground, moved it around so the legs could “centralize (for the sake of a word) then we left it for 24hours.

Flight test two days later you ask? Went great. No shimmy everything is back to normal. So what was the fix?

Old tires, flat spots on the tires, air pressure? Switching the tires from side to side should have moved the shimmy to the left side and it didn’t. I think it was the little amount of movement in the leg. Doing the permatex fix was the solution. I can’t see any other reason for the shimmy to be gone.

Hope this helps The next person and thanks for the advice and thoughts I really appreciate it.
 
I forgot.

The comments on toe in/out. The aircraft has been perfect to this time so I didn’t go into it. There is no room in the upper mounting bot to change the location of the toe in /out. If the locktight, tires etc hadn’t done the job I’d be looking at toe in/out next
 
Locktight info

Attached are two pictures of my comparison screen for Loctite :)) I've been spelling it wrong for a while) materials. I used the 609 because it was available. I'd have proffered to use the 638 or 680 because of their gap filling characteristics. However, they are both stronger and i think it would be harder to remove them later.

On the flip side of that last sentence is how would you remove the leg later? Well 609, 638 and 680 need heat to release them. But if you look at the third picture the shear strengths are very different. Because the leg is tapered, the surface areas that contact are only approx. 1/4 " and because you have great access to the top of the leg (access for a brass drift and a hammer), i think any of these could be broken apart without heat. I have no experience or chemical knowledge to back that up so take it for what its worth.

We'll, that's my two cents. New tires, well balanced and 34lb of air might have fix it. But there was a little play and in articles I had been reading there were a lot of comments about something, an action, starting the shuddering process. I believe the term was resonance/harmonics (??).
 

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