What's new
Van's Air Force

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

Wheel/tire balancing - what could cause it to change?

plehrke

plehrke

Well Known Member
Patron
I have over 1000 hours on my 6A with never experiencing any main gear shimmy. Then last April I did a perfect greaser landing and held the nose up until it fell at about 40 kts and all of a sudden I got a violent left main gear shimmy. Only change I had done recently was increased tire pressure from my normal 32-35 psi to 38 psi. I have no gear leg stiffeners and always statically balanced my wheel assembly. I disassembled all the gear and brake parts to try and determine the cause. Nothing obvious. I took the opportunity to do Vans SB-00047 that replaced fluid with 83282 and updated to the high temp o-rings. I also rebuilt one of the master cylinders. Put it all back together and had no more shimmy. I did not rebalance the wheels as when I put on the tires 100 hours prior they did not require any weights to balance.

Fast forward to a few weeks ago, and 60 hours (~50 landings) of shimmy free flying, and all of a sudden I got some violent shimmy again. I had just finished my condition inspection and had put on new brake pads. I had not checked tire balance as again, I had been flying for several years with no change to tire, tube, or wheel (except tire pressure back to 32-35 psi) so assumed still balanced. I jacked up the plane, spun the wheels, and noticed the new brake pads were dragging so, per reading some threads here on VAF, I added a 0.025” shim to the caliper assuming somehow the brake dragging was causing the shimmy. Went for a flight and shimmy solved. Flew a couple more flights and no shimmy. Then a couple days ago, after doing a landing to full stop, I took off again and after a short flight landed and had some moderate shimmy. Shimmy only happens at landing and when slowed to 40 kts with or without application of brakes. Shimmy only stops when slowed to 10 kts. No shimmy when taxiing at any speeds I tried up to 25 kts.

Ok, back to wheels off and brakes checked. I decided to check the tire balance. Well, it turns out wheels were way out of balance and I would need to add about 6 oz at the stem to get the left side to balance. Weird. So I decided to spin the tire 180 degrees on the wheel and try balancing again. That showed I need 5 oz at 180 degrees from the valve to balance. So it seems somehow my tire has gotten way out of balance. Tire still has lots of tread on it after 3 years and ~150 landings. No flat spots, it runs true and round.

So I believe I need new tires as I think anything over 2 oz to balance is excessive. My question to the crowd (for those still reading): what would make a tire change balance so significantly? I am not assuming my earlier shimmy had the same cause as my current shimmy. Maybe the first shimmy several months ago was caused by tire pressure or something else and somehow unbalanced the tire causing the current shimmy. Do you agree the next thing I do is get new tires?
 
Last edited:
That is weird. Was there anything common for the flights?

Was it cold when you did both flights with the shimmy? I have equipment that if they sit for several days of cold the tires take a shape and are very bumpy until they warm up?

Very strange.
 
Tire balancing....

plehrke said:
Ok, back to wheels off and brakes checked. I decided to check the tire balance. Well, it turns out wheels were way out of balance and I would need to add about 6 oz at the stem to get the left side to balance. Weird. So I decided to spin the tire 180 degrees on the wheel and try balancing again. That showed I need 5 oz at 180 degrees from the valve to balance. So it seems somehow my tire has gotten way out of balance. Tire still has lots of tread on it after 3 years and ~150 landings. No flat spots, it runs true and round. So I believe I need new tires as I think anything over 2 oz to balance is excessive. My question to the crowd (for those still reading): what would make a tire change balance so significantly? I am not assuming my earlier shimmy had the same cause as my current shimmy. Maybe the first shimmy several months ago was caused by tire pressure or something else and somehow unbalanced the tire causing the current shimmy. Do you agree the next thing I do is get new tires?
Click to expand...

I kept reading! The first part of your story is mysterious, to be sure. The second part is about what tires do: get out-of-balance! If you had your car tires changed every 6K miles rather than just balanced and rotated, you would be getting Christmas, Birthday, Easter, Halloween, and New Year cards from your tire dealer as you would be one of their favorite people! ;)

Unless you mic-ed the tires, you are not going to see the out-of-round they have become. Your tires, when landing, start at speed ZERO and are suddenly going at least 65! Where the tire touches the runway at zero will where the wear is. And it is likely a different spot with each landing. That wear is going to shift the balance of the tire. I redo mine at every condition inspection or every 100 hours, (which is when I do a condition inspection if I fly over 100 hours in a year rather than just annually).

SO: my advice (with that and a $5 you could get a coffee...I remember when it was a dime....) is to rebalance your tires. Near or over two ounces is not a big deal: it is what it takes to balance it. Have you looked at the weights on your car tires lately? First thing they do is take all the old weights off, then rebalance it. I've seen tires go from a few small weights to several long ones after being redone. You likely don't need new tires. Unless that would make you more comfortable. Then send me your old ones and I'll see how many years I get out of them....:D:D:D IMHO; YMMV.....
 
I don't find it very surprising that these small tire change balance as they wear. They are quite small and we put odd wear patterns on them that are far from normal. Outside wears more than inside, we wear more excessively in the spot that touches first each time we land, etc.

Can't say if poor balance is causing the issue. I have 1100 hours on my 6A and have never had a main gear shimmy. I have never balanced a tire for my planes. It makes logical sense that poor balance can excite a harmonic that gives the shimmy, but have seen lots of posts from guys that balance new tires and still get shimmy.

Larry
 
How much weight is too much to balance a tire/wheel? 4-6 oz seems like a lot and is not practical to put on our small wheels (wraps around a large portion on wheel therefore requires more weight) and really don’t like to stack them 2 weights tall.

rockwoodrv9 said:
That is weird. Was there anything common for the flights?

Was it cold when you did both flights with the shimmy? I have equipment that if they sit for several days of cold the tires take a shape and are very bumpy until they warm up?

Very strange.
Click to expand...

Really nothing I can think of that was in common except for things that are common with all flights
I do get flat spots when sitting a while on cold floor but i would notice that pulling it out of hangar, on the taxi out, or just after lift off with a shudder.
 
Last edited:
Balance

Are you using any lubricant when you mount your tires? I had someone use silicon spray to help mount his car tires and then he complained about lack of brakes. When he pulled into my house with his brakes on, I saw the rim not turning! as the tire was. Your tire could be spinning a little bit in relation to the rim, which would affect the balance. I even wipe the excess talc off the mating surfaces before inflating a new tire. I also mark where the valve is (if no red mark) to confirm no slippage. -Ben
 
Check the break out force on the nose wheel. Went through something like that recently. Thought it was coming from the main wheel. I tightened the break out on the nose wheel to around 30 lbs and solved the problem. Had to drill a new hole location for the cotter pin to get the castle nut positioned for the correct break out force.
 
I recommend putting a camera on the wing to confirm what is shaking. I have a GoPro-knockoff in a case clamped to the wing tiedown ring. It helped me conclude my shimmy was a gear stiffener issue, not an out-of-balance tire, due to the way it shook. Without video, I'd still be guessing. A nosewheel shimmy can sometimes feel like the left main.
 
kenpilot said:
I recommend putting a camera on the wing to confirm what is shaking. I have a GoPro-knockoff in a case clamped to the wing tiedown ring. It helped me conclude my shimmy was a gear stiffener issue, not an out-of-balance tire, due to the way it shook. Without video, I'd still be guessing. A nosewheel shimmy can sometimes feel like the left main.
Click to expand...

Good idea. My GoPro is already mounted under the wing to capture video when I return to flight.
The first occurrence of the shimmy I was landing with a large crowd watching therefore positive it was the left main. They were there to observe the first flight of a newly restored Emeraude that had just taken off.
 
I too am experiencing a pilot's side main gear shimmy on my 7A.
I don't know if my issue is similar to yours, but I have not solved the problem yet.
Three different observers have confirmed it is the Pilot's side main gear.
The analogy they have used to describe the shimmy is similar to a wobbly shopping cart wheel with a rapid back-and-forth movement.
I have rebalanced the wheel, retightened all mounting bolts at the wheel end of the gear leg as well as the through bolt at the top of the leg, removed the gear leg fairing and inspected the integrity of the gear leg. Do not seem to have a brake dragging issue at rest with the gear jacked off the ground. Tire is at about 1/2 life.
Any thoughts?
 
Huh. A shopping cart wheel usually will go side-to-side like a fish tail rather than back-and-forth. I'm sure you have messed with the tire pressure. Have you taken the wheel fairing off to see if the shimmy is there without it on? Sometimes if that is out-of-balance it can cause the shimmy... Are your gear legs stiffened? 🤔
 
Shimmy and gear-walk. The RV main gear mainly exhibits gear-walk, but may exhibit some shimmy due to the torsional flexibility of the rod-type gear leg. The RV nose gear may exhibit either or a combination of both.

1706132399611.png
 
I have recently re-confirmed that the vibration is generated through the gear in the form of a "shimmy". It is definitely not a Gear Walk issue.
I intend to remove the wheel pants and see if that make as difference. If it continues, I plan to replace the tire which is currently at about mid-life with good tread still showing.
Thanks, guys, for the suggestions. I especially appreciate the graphic description differentiating shimmy from gear walk.
 
I believe 'shimmy' has been used as a generic term for what the gear feels like it is doing. I actually don't recall anyone being concerned about 'gear walk', although on the main gear of a taper Wittman-style gear, is likely the main culprit. But who knows without video confirmation? I would suspect the nose gear (comments withheld from a tail-wheeler! :ROFLMAO: ) is likely prone to both. Gear stiffening solutions address the gear walk aspect and wouldn't seem to have an effect on shimmy. My vocabulary has been expanded! :giggle:
 
You must log in or register to reply here.
Back
Top