By Gary Van Dyke
I’ve been fighting a shimmy on my Cardinal RG for a while. I had a fairly significant event on a trip that pushed solving that problem to the top of the list.
We were taking off from Texas Gulf Coast Regional Airport (KLBX) in the Houston area to Lakefront Airport (KNEW) in New Orleans. Just as we got to rotation speed, we experienced a violent shimmy that was so bad it was actually scrubbing speed. I had plenty of runway, so it came down to a decision on whether to abort or to go ahead and take off.
I thought the nose wheel had gone flat. Since we were at rotation speed, I decided to take off.
The takeoff went fine. Since KNEW has a good maintenance facility, I decided to make the trip and have the problem fixed once we were on the ground there.
I told my wife that I thought we had a flat tire and that things were likely to get bumpy as the plane slowed but that I was confident everything would be OK.
When we got to New Orleans, I let the tower know that I may have a flat tire and may need to be towed once we got the plane stopped. I did a soft field landing at minimum speed and held the nose wheel off as long as possible.
I had the yoke all the way back when the nose wheel finally touched down and was surprised that there was absolutely no shimmy.
We taxied to the FBO and I did a very thorough look at things once we shut down. With the tail pushed down to get the nose wheel off the ground, everything looked like it should.
We decided to check everything in the nose wheel and steering systems. We pulled the tail down and secured it to a ring I have installed in the floor of my hangar for that purpose. We then pulled the nose wheel and went through all the connectors and settings for the nose wheel and steering system. Everything we checked was in spec.
We had the tire static balanced when I first put it on during last year’s annual, but I thought something might have changed. I found a local motorcycle shop that had the equipment needed to dynamically balance the nose wheel, so I took the tire to them.
The mechanic they assigned to balance the tire owns and flies a Maule, so he had a good understanding of my issues. He took a quick look at the tire and said something didn’t look right but mounted it to the balancer and started to spin it up. He stopped it quickly and got me to come over to look. Once it was spinning it looked like there was a lump on one part of the tire.
To verify that, he got it spinning then took a grease pencil and lightly placed the tip on the center of the tread. When we stopped the tire, you could clearly see gaps in the mark the pencil left showing low spots.
He went through the balance procedure. It took a full ounce of weight on a spot opposite the high spot to get the tire in balance.
That was causing the tire to bounce at high speed when lightly loaded (my shimmy).
My new friend pointed out that the tire was so lightly worn that it still looked original in most places. I told him I would replace it anyway, then he showed me a process they use on motorcycle tires to get them back in round and suggested I try that first.
With the tire off the ground, use a belt sander to get it spinning, then carefully touch the tip of a grease pencil onto the tread. Ideally you mark three or four areas across the width of the tread.
Once the tire stops, spin it slowly by hand. If the tire is round, the marks will go around the entire tire. If it’s out of round, you’ll see gaps in the marks on the low spots.
If there are low spots, get the tire spinning with the sander again and then turn the sander 45 degrees to the tire. Sand a little, check progress, and repeat until you get the tire round, or you decide you need to just replace the tire.
I found 120 grit sandpaper worked best. The coarser grits don’t let the tire slip enough to sand the rubber. I had a small scattering of rubber on the floor when I was done but still had very little wear on the tire.
I took the tire back to the motorcycle shop and this time it was in balance with no balance weights applied. I put it back on the plane and haven’t felt any shimmy since. Next time I will start by checking that the tire is round and in balance before doing anything else.