What's new
Van's Air Force

The definitive Van's Aircraft support community! Buying, building or flying an RV? Join our exclusive family of mentors and enthusiasts!

Shimmy before turnout and before brakes

J

Jesse23

Well Known Member
I knew it was too good to last. Did AntiSplat bearing mod on nose and mains. I was enjoying smooth taxi and no fuss roll outs for 2 years. Changed nose wheel to retread with what looked like double tread depth. Now, just before making the turn after landing @25-30 and before braking, I get a shimmy that lasts for 5-10 secs, then I brake, still a bit of shimmy, then it goes away and I taxi back. I recently sent the nose back to Alan for dynamic balancing and a skim of the OD. I have the same shimmy at 25-30.

Played with tire pressure, checked break out (little high @34 Lbs). Not worth drilling extra holes to get the spec). A little higher works, since it always decreases over time. Checked landing gear SBs (no cracks).

Any hidden diagnosis from the collective? Natural frequency at that speed?

Current:
Nose 35 PSI
Mains 50 PSI (68 PSI max GY III)

Thanks,
 
Usually shimy is from the mains. However, if you had zero issues for two years until changing the nose wheel tire, then your source seems pretty apparent. Don’t know what alan does, but doubt it is robust enough to address all balance issues.

Maybe take it to a tire shop or get a balance rig at HF. Could also try a new tire. Did you get these from dresser? In my limited experience, they are NOT the same company they were prior to the acquisition. I will no longer buy from them. Suggest trying wilkerson.
 
Last edited:
lr172 said:
Usually shimy is from the mains. However, if you had zero issues for two years until changing the nose wheel tire, then your source seems pretty apparent. Don’t know what alan does, but doubt it is robust enough to address all balance issues.

Maybe take it to a tire shop or get a balance rig at HF. Could also try a new tire. Did you get these from dresser? In my limited experience, they are NOT the same company they were prior to the acquisition. I will no longer buy from them. Suggest trying wilkerson.
Click to expand...
I flew a 172 with a bad shimmy damper. Soon as I touched down I put full back pressure on the yoke. Shimmy was non existent. Have you tried full stick back on roll out? If not it might help to pinpoint the cause a little better. Just a simple idea...
danny
 
Certain engine/prop rpm & particular taxi / roll out speed seem to set up harmonics that feed the nose wheel shimmy on my 6A. Same thing happened on the 7A when I had it.
My solution is to either change engine rpm or tap one brake & do a bit of a S turn to stop the shimmy harmonic.
 
Make sure the nose wheel has the proper brake out force. I thought I had a shimmy coming from the main, but it was from the nose wheel. If the break out is less than 26 lbs, you can get the shimmy. I had to drill new holes for the cotter pin to get the load set correct.
 
togaflyer said:
Make sure the nose wheel has the proper brake out force. I thought I had a shimmy coming from the main, but it was from the nose wheel. If the break out is less than 26 lbs, you can get the shimmy. I had to drill new holes for the cotter pin to get the load set correct.
Click to expand...
From post # 1, 34 lbs. is fine.

"Played with tire pressure, checked break out (little high @34 Lbs). Not worth drilling extra holes to get the spec). A little higher works, since it always decreases over time. Checked landing gear SBs (no cracks)."
 
We started to get some shimmy just before we started to turn off the runway. I suspected the nose tire was out of round or unbalanced. The next flight I was waiting near the turnoff with a camera and my wife flew it for a quick landing. As she slowed for turnoff, I was surprised to just visually see the left main with a ton of vibration.
I went back to the Van’s construction manual in the area of gear leg vibration. In short, the easier the tire rolls, the more tendency for vibration in round gear legs. Their solution, wood gear leg stiffeners.
Background: I just had the anti splat bearing job done a few months prior. We had just put air in the tires prior to the first vibration. Anything you do that decreases the rolling resistance of the tires increases the tendency for vibration of round gear legs. In our case, it was the anti splat bearing mod but if we had been using higher tire pressure, we would have gotten the vibration anyway. Since the builder had elected to not install gear leg stiffeners, that was our solution. I got the stiffeners from fly boys and wrapped them with two layers of carbon fiber. There is not much room under the gear leg fairing. Shorten the stiffeners if necessary by cutting off the bigger end. It solved our problem.
 
You must log in or register to reply here.
Back
Top