I'm picking up gear vibration....there is no explination.....
I’ve had a shimmy / vibration on my 7a since the first flight on Jan 3, 2023. I thought it was the nose wheel but after installing a vid cam under the wing and reviewing, I can plainly see both mains are bouncing forward and aft during those speeds. I’ve adjusted tire pressure and installed the wooden leg stiffeners but it’s still there. The shimmy is worse if I’m on the brakes during that speed window. If I stay off the brakes during that speed, it’s noticeably better but not 100% gone. I haven’t balanced the tires but I did put new ones on and made no difference. I’ve checked the wheel bearing pre-load too. It would seem to me that the leg stiffeners aren’t doing their job. I can send a link to the video if you want to see it. Johnbeilersrq at gmail.com. Thanks for any help.
This is a common problem and partly due to the design of the gear itself. It is likely more common with the round, tapered steel gear that angles aft but the flat gear has its problems as well. It is a harmonic vibration thing and, once it starts, can feed on itself. Gear leg stiffeners do help; that's why Van's recommended them so long ago. Other things may or may not contribute to the problem:
The wheel fairings: the older style were easier to balance than the newer style. I have the older and there is a sizeable amount of lead shot fixed in the nose with fiberglass resin to balance them. That helped mine. I bought a pair of the pressure-recovery pants from Van's just before the price went up $$
$$ and I will be interested to see how balanced those are out of the box.
Gear leg stiffening: I initially had no reinforcement on the legs and my vibration was inconsistent depending on I don't know what but more of an annoyance than anything. I recently fiberglassed 0.5" fiberglass rods on the front of the gear. I have not flight-tested those yet as it was on the list of a lot of other things I am in the middle of doing.
Tires: they vary significantly as to how well balanced they are from the factory. And on your airplane they will be coupled with your wheel/brake assembly and the entire unit being balanced may be better than trusting the 'red mark' on the tires. There has been much said about how to go about that. Some people have never balanced theirs and are perfectly content. My HF motorcycle wheel balancer does an excellent job for not much investment. I also use their wheel weights. Pressures: what is done is all over the map as far as personal preference. I would avoid the lower pressures. I have only pulled the stem off of a tube once from braking hard with a low pressure but that was enough. (It was in the Cub but still...) The consensus seems to be something north of 30psi. Trial-and-error will find the correct pressure for your airplane.
Brakes: if the vibration seems worse when you apply the brakes, the disk can be out-of-tolerance or brake pads might be wearing asymmetrically. I find there is less vibration when braking with new pads. A simple way to mic the disks is by going around with a "C" gage in multiple areas and making sure the thickness is consistent. Not as accurate as a dial gauge with the disk moving but adequate.
Acceptance: there will likely
ALWAYS be some vibration (see paragraph one) but minimizing it will make you happier and keep your passengers from giving you that 'what the heck was
THAT?'
look!
