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Gear vibration

Greg Arehart

Greg Arehart

Well Known Member
I've looked through all the old threads (that I can find anyway) on gear leg vibration. Here's my problem: A year ago I converted my 9A to a 9, including adding the capability for larger wheels/tires. I flew for the past year or so with both small (standard) wheels/tires as well as ~60 hours last summer with 800x6 tires and had no issues. This spring I went to change out the standard tires for the larger ones and encountered horrible gear leg vibration. Upon swapping back to the small tires, the vibration goes away. No known change in the big tires - they've been mounted on the same rims as last year but just sat over the winter. I checked the balance (static) and they are still balanced. I looked at the roundness while they were on the axles and jacked up. Best I can tell, out-of-round is not an issue. I swapped out all the bearings just in case it was a bad bearing, and no change. So now I'm running short of options (that I can identify) and looking for suggestions on what to do next. I have considered adding stiffeners to see if that helps.

The conundrum for me is why these wheels/tires worked fine last year but seem to be a major problem this year.

Any comments/suggestions welcome. Meanwhile, I'm flying with the small tires that don't give me vibration issues.

Greg
 
Can you take them to a tire who to have them spin balanced or trued? I don't konw if a tire place has equipment to check plane wheels and tires. Maybe sitting caused them to have flat spots.
 
rockwoodrv9 said:
Can you take them to a tire who to have them spin balanced or trued? I don't konw if a tire place has equipment to check plane wheels and tires. Maybe sitting caused them to have flat spots.
Click to expand...

Already tried several phone calls to local places, and nobody seems able do this for airplane tires.
 
Spin them up yourself, it's easy to do. Find a suitable spot on the engine mount and jack one side until you have clearance under the tire. The gear leg needs to be free to vibrate should it choose to. Use a rotary buffer on the sidewall of the tire and spin it up. When the tire decelerates and reaches the critical speed, the tire will shake like a wet dog if out of balance dynamically is what's causing the problem.

Tony
 
Tony, that sounds like a great idea. I guess with some actual math, you could judge the speed. Maybe with a timing light?

I think I would try to make an adapter jig to the bolt pattern of a car and take it to Big O. Less math involved!
 
Wheel vibration

...We can balance & true them for you with the rotors installed. We can check the rotors and surface them as well if needed. Sometimes a glazed rotor or one that is not true can set up the vibration and once it starts it just keeps going.
...One other suggestion would be this, Lift the tail up to the landing configuration with a person in the plane to simulate landing weight. Roll the airplane a few feet forward to set the gear spread position. Set the tail on a stand at the height you are experiencing the vibrations. Now take two straight edges and straight edge the outer surface of the wheels and mark the floor 24" in front and behind the tires. Now measure the toe-in or toe-out at 48", any amount of in is unacceptable, and out should on more than 1/2". This can cause your vibrations to begin and once going they tend to get worse or at best go until you slow down. The taller tires amplify the problems associated with this issue and this could be why it only does it with them. If we can be of help just let us know. Regards, Allan...:D
 
Thanks for all the suggestions. I'll be working on these when I get back from traveling for the next couple weeks.

Cheers,
Greg
 
Try reducing the tire pressures. More tire contact with the ground seems to dampen the vibration.
 
This sounds like a good case for gear leg stiffeners if they aren't already installed. If you run your tires at 20 lbs as Sam suggests, and the shimmy goes away, you need stiffeners. If you decide to just run at a lower tire pressure, this will work fine but you will wear out tires really quick. (If you are landing on paved surfaces.) Good luck.
 
On the tire pressures, I've tried that (running anywhere from 35 down to 10 psi) and still have the problem. Yesterday afternoon I had a closer look at the tires and they appear out of round, so that may be part of (or all?) the problem. At least it's something that needs fixed before going forward. Also looks like the brakes may be dragging a bit so will be cleaning those up also. Stiffeners are another possibility if the first two things don't fix the problem.

Greg
 
Gear leg mount bolts

Also, while you are at it check your gear leg mounting bolts for tightness. I found mine loosened a bit in the first few years and the leg can develop a wiggle you can feel with your hands when off weighted. These bolts should be checked each annual for tightness. I had made special washers with a concave to match the tower and a flat side for the bolt. Seems to help.
 
John,

Indeed I had this problem on the R leg and just reamed the holes and put in new bolts. I thought that would fix the problem, but not the case. It did help but there is still far too much vibration after that. Thanks for the idea.

Cheers,
Greg
 
solution to one gear-leg shimmy problem

For the first 500 hours on my RV3, I used Goodyear tires that had a relatively flat-bottom cross-section. I always had gear shimmy at ~30mph. I reduced the tire pressure to <25 psi and that helped, but not much. I've got wood/fiberglass gear-leg stiffeners as well. I recently needed new tires and couldn't find the type I usually get, so I got Goodyear Flight Special II's. Big surprise: no gear shimmy at any speed. Pump up the tires to rated pressure (40-50psi), and still no shimmy. The only difference? A much rounder cross-section to the tire. I typically get ~1000 takeoff/landings on a set of tires, which means the toe-in/out is correct, and wear pattern is like all RV's, outside edge wears faster. The theory is that a flat-bottom tire is incompatible with the load on the rod type gear leg of the RVs.

- Steven
700+ RV3 hours
 
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