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Gear Leg Shimmy
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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. |
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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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. |
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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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 :-) |
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?
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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 |
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.
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