RV-8A Main Gear Shimmy on a 10 year old RV

I completed my RV8a in 2002 and have always had some shimmy when i taxied to fast and aways assumed it was the nose wheel. Now it has become quite violent on fast taxi, take off and landings. This has just happened the last few months. Thanks to GoPro and iMovie I have documented it for analysis. So any one willing to give me their 2 cents worth I would appreciate.
If you go to http://www.youtube.com/watch?v=_HrcTKt3zBc

there is a 4 min. clip of the mail gear in various stages in regular speed and 1/4 speed.
I am running Good Year Tires at about 45 lbs. psi I haven't done anything lately but put some air in the tires about 3 months ago.

Any Suggestions ??

Dane
[email protected]

I do not have an 8, so no empirical knowledge, but I seem to remember prior threads about having to re-tighten the gear mount bolts often, and some folks have had mounting bolts break.

Have you tried to do a search?

Check the mounting bolts first and then consider gearleg stiffeners if you have round gearlegs. Yours looks to be a classic example of gear leg oscillation. If you need stiffeners, contact me:
[email protected]

Less pressure

Try 30 PSI, If I ran 45 I would have the same problem, some people run as little as 22 psi.

I am not flying yet but I remember hearing that high tire presures cause this. I just installed the Aero Classic tires from Van's and they are recommended @ 25#.

Gear movement is fore and aft as much as horizontal - the gear is in a state of unstable chaos. Why are the tires not aimed straight ahead and staying that way? The imaging from aft indicates the tires are trying to go in or out, not straight ahead. The slowed down side image of high speed taxi shows a definite fore and aft movement which may be induced by a considerable side load while the tires are not aimed straight ahead.

I would check all lower gear fasteners for security. I believe the upper leg is fastened with one bolt like all other RV's, it is not the flat plate leg of the 8. I doubt that is a factor.

I would check alignment. Are the wheels aimed straight ahead?

I would check tire wear.

I would check the brakes for release and/or possible dragging.

I might install quality new 6 ply tires and tubes.

Whatever is happening, it is not good and has to be corrected. The loads imposed on the tires and wheel mounting system can not go on indefinitely without undesirable consequences.

Shimmy

Since there has always been some shimmy I would really check the toe-in/toe-out. It's possible to that over time it's gotten worse due to wear on the gear hardware.

Out of Round ?

Looks like shimmy on both main gear. Poor quality tires may be the culprit.
I would jack it up and check for out of round tires, and if OK and lots of tread on them maybe have the wheel assemblies balanced at a motorcycle shop.

Have you ever balanced the tires/wheels and checked them for roundness?

I have seen severe shimmy issues solved by installing new tires and then balancing them carefully (and verify they are not out of round while turning them on the balancer).

Only after doing this would I investigate some of the other things mentioned.

Also, high air pressure magnifies the problem. 45 PSI is way more pressure than needed for an airplane the weight of two seat RVs. Somewhere around 35 is more typical.

Second on bad tires. Are they failing or delaminating?

Watched video. Very interesting! Looks to me like the right gear is exciting the left. How about warped brake disk and you're riding the brakes. You have one of the -8 brake pedal mods?

shimmy

I would take a look at the brake rotors and check them for warp age...
my 2C

Shimmy - wheel bearings?

Just wondering if you have found the problem? I would check the wheel bearings for smoothness at all speeds. Please let us know what you find out.

amazing slow mo

that's an amazing slow motion video. In one shot it looked as if all the gear were moving with the main gear frequency 2X the nose gear. it seems as if the main gear are the prime stimulus but I wonder about effect of the nose gear stimulus on the mains. any changes to the nose gear recently?