[hgerhardt]

Had a strange vibration last week-- coming back downhill from Big Bear, fairly high airspeed (180 kts true), I felt a vibration that can be described as feeling like a twin with the engines slightly out of sync. Tried changing rpm and that changed the harmonic slightly, but it never went away. Removed the cowl and looked for a cracked engine mount and anything else that didn't look right. Nada. Next day, I realized it had to come from something spinning, like the wheels. Maybe the tires are turning fast enough at high airspeed to cause a vibration. Flew it and hit the brake pedals. No change. Had to be the tailwheel. Put in a fair amount of rudder and the vibration changed. Bingo! Then I remembered I had replaced the tire on my Matco pneumatic tailwheel a few weeks earlier. Jacked up the back end and spun up the wheel with a drill motor with a 3" hole saw as the turning tool. Around 2000 wheel rpm it vibrated like crazy. Hard to believe the thing would spin that quickly in the air, but evidently it does. Let the air out and rotated the tire on the rim a bit until the vibration went away. All is well now.