This interesting look at what actually causes tailwheel shimmy was written by Gilbert Pierce of Pierce Aero.

Tail Wheel Shimmy

I noticed when flying my Piper Clipper heavily loaded I experience tail wheel shimmy on my Scott 3200 when landing on a hard surface such as concrete or asphalt. Several years ago I was parked at the landing end of runway 36L at Oshkosh. I always get tail wheel shimmy when landing there. This afforded me the opportunity to observe many landings as I lounged under my wing. It was here that I made the discovery that about 50% of the tail wheel airplanes landing on 36L experienced tail wheel shimmy. I believe the grooved runway exacerbates the problem. Anyway those tail wheels were not just shaking side to side, they were rotating around their pivot axis 360 degrees and doing so violently. On my recent trip to Alaska I had tail wheel shimmy on almost every landing unless I really greased it on. On my return I vowed to solve the problem.

I started the quest for a solution on the Internet. I was told that if you raised your tail wheel tire air pressure it would assure the tail wheel would shimmy no more. I was told to reduce the tail wheel air pressure. I was told I had too much grease in the tail wheel. I was told that if the tail wheel didn’t spit grease at you when walked by it, it did not have enough grease hence the shimmy. I was told to loosen my steering springs, I was told to tighten my springs. I was told that the pivot axis must be absolutely vertical so that the surface the tail wheel swivels on is parallel with the ground hence the pivot bolt would be vertical. Mine was. I was told the pivot bolt must face forward at the top, I was told the pivot bolt must face aft at the top. So what did I do? I took the tail wheel apart and made sure it was mechanically in top-notch condition and adjusted to the manufactures specifications-again. It was. Then I tried each and every remedy listed above except changing the angle of the pivot bolt, no help. The Scott 2000 tail wheel does require some tension on the steering springs to control the unlock tension and hence shimmy. The Scott 3200 installation instructions say that chain tension is not required or recommended.

Next I got out an old 1950’s auto repair manual that explained king pin front wheel suspension systems and steering castor angle. If you have ever pushed a grocery cart through the supermarket with one of the front wheels shaking side to side you have experienced wheel shimmy and improper castor angle. What I learned from the chapter on steering alignment was basic steering geometry. To measure your steering geometry, drop a line drawn parallel to and through the pivot axis and extend it to the floor and make a mark on the floor where this line hits or use a straight edge parallel to the steering axis shaft. Next drop a line vertically from your tail wheel axle to the floor or again use a straight edge and make a mark on the floor. This will also be where your tail wheel contacts the floor. Now move this line or straight edge that passed from the axle to the wheel/floor contact point horizontally until intersects the pivot axis line at the pivot axis midpoint The line that is parallel to the steering axis must hit the floor ahead of the line dropped vertically from the wheel axle. The angle formed by these two lines is your castor angle. The larger the castor angle the better as far as tail wheel shimmy is concerned. In other words, the farther ahead of the tail wheel that the steering axis line hits the floor the greater the castor angle and the less likely that will have shimmy. To put it another way, the steering axis pin or bolt must be vertical or tilted with the top pointing behind or to the rear of the airplane when the airplane is fully loaded. Emphasis on fully loaded.

When my airplane was empty the steering axis bolt was vertical. When I loaded the airplane, the tail wheel spring compressed and the top of the steering axis bolt was pointing to the front of the airplane. This would put the extension of a line drawn through the steering axis behind the tail wheel contact point. Bad news-it will now shimmy. You don’t want the castor angle to be too large because it will make steering on the ground more difficult. The large castor angle will tend to lift the rear of the airplane slightly as you turn the aircraft. This is called self-centering effect. Having the steering axis bolt vertical or inclined slightly with the top pointing back when fully loaded should be sufficient.

So how do you correct this angle? There are two easy solutions. If your airplane is like most, the spring is bolted at the front to the airframe with a bolt that passes through the spring leaves. The spring then rests on a pad several inches behind the point through which the through bolt passes. Usually the spring is clamped to the pad at this point. You can add a shim between the pad and the spring to increase your steering angle. Or you can take the route I took. I took the spring off and laid it on a piece of poster board and traced out it’s arch. Then I took the spring to a spring shop and had them re-bend the spring until the tail wheel end of it was about 1-1/2 inches below the original. In other words I increased the arch slightly. Walla-no more shimmy when loaded.

One other point. You should carry sufficient air pressure in your tail wheel to keep the tire firmly attached to the rim when it hits the pavement on landing. Due to the small diameter of the tail wheel, it accelerates very rapidly on contact. If you have insufficient pressure in the tire it will slip on the rim and cut the valve stem. You now have a flat tail wheel tire. I know- it’s happened to me twice. I now carry a minimum of 45 pounds of pressure in my Scott tail wheel. The same thing can happen to your main tires but with more surface contact area around the rim it is less likely to happen unless your plane lands at very high speeds.

Tail Wheel with Positive and Negative Castor Angles – exaggerated The terms positive and negative are simply the naming the convention I choose to use as they agree with my text book references.

Gilbert Pierce
Technical Counselor
EAA Chapter 182

Nice article Zane. However, I will cloud the issue a little and welcome anyones input.
I had a tremendous problem with shimmy. I had the Maule tailwheel to begin with. Over inflated, under inflated, didn't make a difference. I was eating up tail wheel tires at the rate of about 50 hours. So, I succumbed to pressure and installed a 3200. Worked really good for about 75-100 hours and lo and behold, shimmy. Just as bad as the Maule wheel. I met up with Wup and Bill at JC for the fly-in at JC and wound up buying a new tail wheel spring. Wow! What a difference in handling on the ground but still got an occasional shimmy. Got to looking at the Scott, really dirty from you know, landing off field. Went through it and cleaned it all up and aired it to about 55psi. No shimmy. Now when I detect even the slightest shimmy, I tear it down and get all of the crap out of it. (once found what appeared to be a little fur in it, that was a bit disturbing) and the shimmy is gone. I have never made any other changes to it and it clears up.
So, what say you all?

The usual culprits are turntable(scott) or swivelpin(maule) angle with the ground, or grease in the wrong department. The Scott shimmy dampner is above the bearing surface but too much grease can find its way up to the friction disc. It must be dry to work. The Maule shimmy dampner is at the bottom and beneath the bearings and here too much or too thin a grease when spring warmtime arrives can migrate past the felt seal to the friction disc. It must be clean and dry to work, so an apropriate heavy or sticky grease is required. With plane jacked up, move the rudder by hand and the steering springs should stretch(scott) or compress(maule) before the tailwheel turns. Unlock the tailwheel to the side beyond 12.5deg and see if there is drag to make it swivel, there should be. Tyre pressure should be min 45lbs or 55 on heavier craft. The leaf spring must be clamped sufficiently tight to prevent roll in longtitudonal axis. I still use Maule tailwheels on my Maules aqfter 31 yrs ops.
my 2 Jeremy

All good information, and right on, in my experience.

The other issue you'll run into with the 170, Zane, is that the stock tailspring main leaf is essentially too wimpy for the size of the airplane. Start out with proper geometry on your tailwheel, as you noted, and load the airplane to gross weight, and with many of the main leaves I've seen, the main leaf will have flexed to the point where it'll shimmy.

My answer was to install a tail spring main leaf from an L-19. The main leaf is about 1.5 times the thickness of the stock main leaf. We were able to field approve it a few years ago. It's a standard part, available from Univair, I believe. Works like a charm. Get in touch with me if you need particulars.

By the way, shimmy is VERY destructive, and not just to the tailwheel.

If you've ever sat, as you suggest, next to a runway where a tailwheel (or a nose wheel on an amphib) shimmy'd, you can't believe how much movement there is there. The stresses imparted to the airframe are significant, and this characteristic MUST NOT be allowed to continue.

I've also found tailsprings that were bent in a rotational manner, I assume from landing heavily tail first, with the tailwheel cocked some, with rudder deflection. In this case, the tail spring was bent in a rotational direction, and the tailwheel was cocked to one side, as viewed from behind. Shimmy City!!. That was a much abused Cub.

Wheel landings are the nicest things you can do for your tailwheel. As noted, that little wheel really spools up fast when it touches. Why put that poor little guy down at landing speed?

Practice wheel landings, and use them regularly, and your tailwheel will thank you by being virtually maintenance free.

Land tailwheel first a lot, as I see many people do while attempting three point landings, and your mechanic will be working on that tailwheel a lot more than necessary.

MTV

MTV,
You are right on with the destruction from shimmying. when I had a 170B it had alot of hours on it and one shimmy to many. The pulleys on the bulk head that coordinate the tailwheel and rudder ripped off the bulkhead and we had to fabricate a bulkhead doubler to repair it. $$$ if you know what I mean. It took a sheet metal guy and the A&P to just figure it out, then since I was the smallest I got sent back in the fuselage to buck the rivets while he did the repairs. Hot summer day about 400f in that crawl space. A moment in aviation I'll never forget.

DB

That article of Gil's is the best I've ever found. He and his son posted it on Shortwing Piper Club's site a long time ago. He's got it nailed! The other advice on here is all good too... the most prevalant thing I see, besides wrong angles, is too much lube. One thing we did to a few shortwings here that really helped is the GarAero tundra tailwheel, and a Pawnee tailwheel spring.
JH