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06-06-2012, 09:17 PM
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Join Date: Apr 2011
Location: Wake Forest, NC
Posts: 96
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Gear Leg Shimmy
Quote:
Originally Posted by n601sc
For the first year or so with my RV-6, I operated with no damping device on the gear legs and, from time to time, experienced rather severe gear shimmy (forward and aft movement of the wheels at roughly 2 or 3 Hz at taxi speeds). Enough shaking to know, ?this is not good?. A change to Michelin tires appeared to help somewhat, but not cure the problem. I also experimented with varying inflation pressure, but I was not persistent enough with the ?test? to establish a best pressure. The fact that the problem would come and go didn?t help matters.
I did, however, have some good success by (literally) taping oak strips to the LE of the legs. To be specific, I visited my local Lowes, selected the most dense length of oak ?? cove molding (roughly the same radius as the steel leg), cut two pieces to 90% the length of the leg(s), then with 1? fiber reinforced strapping tape, bound the wood to the LE of the leg(s). Starting at the top, I just ran the roll of tape down like a candy-cane with a slight overlap with each turn. I used a fair amount of tension and tried to keep the tape smoothly wrapped all the way around. Very inexpensive, and should the need arise to inspect the legs for corrosion, simply remove the old tape and replace it with new. So far after about 2.5 years 200+ hours later, the tape is still looking pretty good and only rarely will the shimmy gremlin reappear. Note, with this size molding, I was able to still slip the fairing over the legs without issue.
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Has anyone else had any experience with this? My -7 will develop a (sometimes) pretty aggressive gear leg shimmy at LOW speeds after a landing. Not during taxi, not during take off, only when on a landing roll out, and only after the tail drops (on a wheel landing) and I am slowing to taxi speeds. I'll need to ride the brakes heavy for a few seconds to stop it, then once it stops, it will taxi fine. I would imagine if it was the tires causing the shimmy due to balancing, it would shimmy at higher speeds and on takeoff as well. How in the world will taping a piece of wood to a solid steel gear leg stop the shimmy? You would think the steel gear leg would be much stronger than a piece of wood??? Any advise from owners who have experienced this would be greatly appreciated. Thanks in advance!
__________________
Dennis Roberts
Wake Forest, NC
EAA 1114, Apex, NC
N713DR Super-7 IO/540  SOLD
http://www.drrv7.com
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06-06-2012, 11:28 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by DRBuilder
I would imagine if it was the tires causing the shimmy due to balancing, it would shimmy at higher speeds and on takeoff as well.
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My imagination has failed me more times than I can remember.
I think yours might be failing you. I suggest that you carefully check the tires for roundness. Replace if they are out very much. About .060 absolute max. (Preferably less).
Carefully balance both wheel/tire assemblies.
I bet one of these (or both together) is causing your problem when you hit a speed that excites a natural resonant interaction between the flexibility of the tire and the flexibility of the gear leg (I have seen it before).
In case these things wont solve it,
While you have the wheels off for balancing, check the alignment of the main gear axles to each other. They should have a small amount of toe in.
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06-07-2012, 06:33 AM
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Join Date: May 2005
Location: Florida
Posts: 774
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Vans's SK-75 instructions
I saw this post and had to give my two cents worth. Van's plans call for wood gear leg stiffeners for all round main gear legs. The reason for the stiffeners is that on some aircraft the round gearlegs may tend to oscillate in a 360 degree fashion upon taxi or landing. If the aircraft doesn't do this, then there is no reason to install stiffeners. To all those who posted answers on this forum, I ask two questions: What are your tire pressures? Low tire pressure will also eliminate oscillation because it produces more rearward drag on the gear legs. Tire wear increases significantly. Are you landing on grass? Same effect as low tire pressure. The stiffeners only allow the gearlegs to flex from side to side rather than all 360 degrees. The installation calls for 2" fiberglas tape to be spiral wrapped from top to bottom, 2 layers whetted out with epoxy resin. It is not a difficult installation taking perhaps an hour to install on 2 gearlegs. The side benefit is that if you accidentally flare a little high, ( I know, we never do this, lol) there is considerably less bounce effect. As most readers on the forums know, I manufacture the stiffeners and have sold them for the past 10 years or so to save about 3-4 hours of building them. I recommend contacting "Antisplat.com" for nose gear stiffeners. Contact: woodmanrog@comcast.net for details.
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06-07-2012, 12:49 PM
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Join Date: Mar 2007
Location: Sonoma County
Posts: 3,821
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Remember to use round tread. Flat tread will cause this problem.
I use Goodyear flight special II. And keep the pressure on the high side and add when they have dropped 10 lbs. No stiffener, no problem.
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VAF #897 Warren Moretti
2019 =VAF= Dues PAID
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06-07-2012, 01:06 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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One more data point...
When trying to solve a leg shimmy problem, builders should first confirm that all of the different possible causes are not inducing their problem.
Then reduced tire pressure can be used to reduce or eliminate the problem.
If that doesn't work, glassed on leg stiffeners can be used (but I would consider that a last resort)
Data point... all of the Van's Aircraft demonstrator airplanes have no leg stiffeners, and they do not have gear leg shimmy. The wheels are carefully balanced every time the tires are changed.
I solved a severe shimmy problem on my personal airplane by simply replacing the tires and carefully balancing the the wheels.
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06-07-2012, 08:07 PM
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been here awhile
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Join Date: Dec 2004
Location: North Alabama
Posts: 4,301
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Quote:
Originally Posted by woodmanrog
Low tire pressure will also eliminate oscillation because it produces more rearward drag on the gear legs. Tire wear increases significantly.
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I have found lower tire pressure (24-30 lbs) can reduce gear shimmy. I have not seen increased tire wear as a result of lower pressure (RV-6, thirteen years, 1150 hours).
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06-08-2012, 09:00 PM
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Join Date: Jan 2005
Location: Arkansas
Posts: 571
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Balancing is a big deal ....
Not sure if folks realize just how much weight it can take to balance the factory tires. We have a really good balancer on the airport and it took between 6 and 8 the small adhesive weights (sorry don't remember the oz) to get mine to really balance but it made a huge difference. I was shocked but the old hands were not!
I had called Gus at Vans directly about putting on the stiffners and he said not to do it until you had flown off the 25/40 and had done everything else. I took the advice and the balance job solved a reasonably noticable shimmy/shake problem.
Just my .02 but balancing the tires is the first thing to do... stiffeners later if that doens't work.
Bill S
7a GTG (got the grin)
Airplane at Glo-Custom for paint :-)
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06-09-2012, 04:49 AM
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Join Date: May 2005
Location: Florida
Posts: 774
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Question about rounding tires
I would like to know if shaving the tires to a more rounded profile is really a necessary item. It would seem to me that every landing must create a flat spot on a tire simply because it goes from 0-50 mph as soon as it touches. I believe there would have to be a certain amont of skidding before the tire actually begins to turn. Input?
Thanks
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06-09-2012, 08:27 AM
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Join Date: Dec 2009
Location: St Louis, MO
Posts: 957
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Wheel shimmy after landing
I had EXACTLY the same problem as Dennis on my -4. I tried changing the tire pressure up & down to no avail. I put new Goodyear Flight Custom tires on it but no help. I even contemplated putting the gear leg stiffners on it but didn't want the extra weight. This spring doing the annual I took the wheels off to repack the wheel bearings, put them back on when I was done and started tightening the axle nut. I was adjusting it slightly to fit the cotter pin and spinning it to make sure it wasn't too tight. Guess what- I noticed that the right wheel would spin freely about 3/4 of a turn then almost stop. After further investigation I noticed that the brake disc was ever so slightly warped. How it got that way I have no idea. Ordered a new one from Spruce and now it spins freely and shimmy is completely gone.
Worth a look,
Oly
__________________
RV-8, Flying
Previous airplanes: Corby Starlet, Citabria, Cessna 140, Vari-Viggen, RV-3, RV-4
RV grin every flight
"Sure is nice to have smart friends"
2018 dues paid
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06-09-2012, 09:34 AM
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Join Date: Jan 2009
Location: Salt Lake City, Utah
Posts: 554
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Stiffeners
I had the shimmy on the right gear leg in my RV-10. I checked toe-in, balance and pressures and finally ended up glassing in oak. About 6 months ago I first tried the strapping tape over a milled piece of oak just like N601SC suggests. This worked great so I ended up glassing it in. Much better now.
__________________
A&P RV-10 Flying 1000+ hours
SuperSTOL Flying 170 hours
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