Tailwheel Shimmy

Check the arch in the tailwheel mounting spring. If the spring gets too flat, shimmy will likely ocur. Check older posting for more info.

I did a search, but didn't come up with much on this site. I went over to the Yahoo Groups and found a good bit of info.
I've also changed the bolt, it was a little worn, but it wasn't real bad.
I'll check the caster (spring arch) this afternoon when I get to the airport.

Thanks for the info, I'll let you know what I find.

Looks like my spring is sprung a bit. Here's a few pics, what do you think?
Also, are these the right springs?

The information you need is on the ISC CD. In your photos it looks like the tail spring is flattened so that the tail wheel pivot is tilted back from 90 degrees. With the tail spring off the airplane and on the bench, the arch should be 2-2 1/8 inches. The fix is to either buy a new tail spring from Univair, or have a spring shop reshape your spring and heat treat it to Rockwell hardness of 42.

Thanks Logan. I figured that was the problem after looking at it closely, just wanted a second opinion.
I'll be ordering the CD soon.

From the picture, you do not have the correct springs, but your shimmy is more likely caused by the flattened spring arch. The proper tailwheel steering springs are scott P/n 3239. They are much larger and heavier springs. They should be installed with minimal slack but not tight to where they exert continuous pressure on the rudder horn. They give much more positive steering and may help on the shimmy too.

Larry Wheelock

Thanks again Larry.

Check tire pressure on tail wheel. Too low and it will also induce shimmy particularly on rough runway.

Doug,
There is a small bushing that should be installed in the large vertical hole in the tailspring, where the vertical bolt that holds the tailwheel housing to the spring passes through. If that is missing, it could contribute to your problem, although a properly arched spring, and the Scott 3239 steering springs are probably the biggest contributors.

The bushing is readily available from Univair.

Good luck!

-Bill

Forgot to get back to you guys.
Had my spring re-arched last summer and installed the correct springs.

All better now!

Thanks 8)

I have a 108-1 and recently replaced the tail leaf spring which mounts to the fuselage, UNIVAIR had the part in stock and it was an easy fix. Unfortunately my tailwheel continued to shimmy because the previous owner had installed a Scott 3450. This tailwheel was not only to big, but the new spring did not have enough bend to allow the wheel to free caster or trail properly which resulted in large shimmy's at high speeds and excessive braking to turn on the ground. After much debate I contacted Alaska Bush Wheels and spoke with Wup Winn and eventually bought a brand new Scott 3200. Alaska Bush Wheel is a great resource and if you have any questions regarding tailwheel replacement, maintenance, or use they are a fantastic resource and are very customer oriented and friendly people.