The International Cessna 170 Association

My tailwheel shimmy went away when I replaced the old tire and tube with new ones. However, for those that might still have the problem, I found this in the Cessna Service Information Summary ( 1946 thru 1962 )
Date: 10-24-50
"We have had some reports on Scott pneumatic tailwheels shimmying. The Scott 8" pneumatic tailwheel is designed with a friction type shimmy dampenenr. This friction is regulated by 3 small compression springs inside the upper casting. If the main king pin nut becomes too loose the shimmy dampener effect will be lost. When the shimmy occurs, it is recommended that the large nut be tightened in increments of 1/6 of a turn until the tailwheel starts to bind when rotated by hand. Then back off 1/6 of a turn for correct tension on the shimmy dampener. The king pin nut is on the underneath side of the lower housing directly above the tire. This procedure will definitely assure that compression springs in the dampener are acting against the friction dampener disc. We recommend a tire pressure of 30 pounds on both the 140 and 170 for the 8" pneumatic tailwheel. When connecting the springs and chains on this assembly, it is recommended that the stretch in the springs be from 1/8 to 1/4". The springs used with the 3200 assembly have a high tension rate and if stretched to any extent on assembly the mechanism may not release easily. No slack should be left in the chain and spring hook-up when both sides are connected."

This adjustment advice sounds right on the money to me,at least with regards to the kingpin nut. However,when hooking up your steering chains & springs,check the tension both with the tailwheel on the ground & off. Jack up the tail of the plane by putting your jack under the tailspring bracket,there's a bolt there forward of the leaf spring. My chains are a little slack when the tail's on the ground,but they're snug (not tight) when the tail is off the ground & the leaf spring is not under tension.In my opinion,if the chains are tight there's gonna be some unnecesary wear going on. Tight chains do make for real good tailwheel steering,though!
Remember,put plenty of grease in that tailwheel!

Eric

The article did not specify whether their recommendations were for the tailwheel to be off the ground or not, thanks Eric

Another thing to watch for: make sure the axle nut is tightened properly. Sounds obvious,but I had a bad tailwheel shimmy when I first bought my plane--I started tinkering with the tailwheel & discovered that the axle nut was loose enough that I had a fair amount of sideplay in the tailwheel itself. Tightened the axle nut & alot of the shimmy went away. The kingpin nut was a little loose too, it took some trial & error adjustments to get that right.
Sometimes,I still get a shimmy on landing if I have an aft CG & I do a hard three-pointer,this can be stopped if I give it a little forward stick to unload the tailwheel.

Eric

N1478D wrote:The article did not specify whether their recommendations were for the tailwheel to be off the ground or not, thanks Eric

The only time you're interested in either shimmy or steering is while the tailwheel is on the ground.

zero.one.victor wrote: Remember,put plenty of grease in that tailwheel!

Eric

I was told many years ago to be careful to NOT overgrease the tailwheel. I've always followed this advice and have not had shimmy problems (yet).

Dale

gahorn wrote:

N1478D wrote:The article did not specify whether their recommendations were for the tailwheel to be off the ground or not, thanks Eric

The only time you're interested in either shimmy or steering is while the tailwheel is on the ground.

But, you could make adjustments either way - Eric cleared that up

George,I wanted to emphasize that the chain tension should be checked in both taxi & inflight conditions. I didn't do this when I first started tinkering with the tailwheel,nobody had mentioned it,& I adjusted the chain length to where they were nicely snug with the tail on the ground. For whatever reason,I later happened to check them with the tail jacked off the ground--they were SO tight you could get a high C note by plucking them! I didn't feel that much stress on the parts was a good thing.
Grease: there's 2 schools of thought on this,we've had that debate a couple times. I knew I'd get a reaction if I mentioned it! Obviously,I belong to the "grease 'em up" camp. Why would Scott have put a grease fitting on the tailwheel if they didn't want it greasy? On the old club site,someone posted results of a talk they had with the Scott people about this. Anybody here remember what they said?

Eric

Scott says grease'up. I've greased Scott tailwheels for years until I can see clean grease coming out and have never had a problem with shimmy.

I pump grease until is ozzes out. Same for the tailwheel axel.
Then wipe it clean.
No shimmey.
Bob

Just wondering how to reach tech support for Scott? I have the installation instructions for mine and am trying to determine the method for chain adjustment. Any input on this topic as well as a source for new chains. Thank you.

Mark N9268A 170A

Mark,

There is no method of chain adjustment other than cutting off links. You might find the perfect length this way and that is to have no slack and I would prefer as little tension as possible but some tension is permissible. This adjustment procedure has been discussed at length here : http://www.cessna170.org/forums/viewtop ... f=2&t=7028

As for were to get the chain? It is available through several sources like Spruce and Univair. You also might want to take a sample down to your closest well stocked hardware store and compare your chain to window sash chain. You might find an exact match of both the length of links and material with the material used being more critical. You wouldn't want something softer. This chain is a good example of common hardware stuff found in older airplanes. If it quacks like a duck, it might just be a duck.

Bruce,

Thanks for your input. The current chain is so tight that wearing apparent on the rudder attachment horn, the holes are oval now. This item is now at the top of the list.

Mark

I might understand wear on the rudder horn tabs if someone has installed the springs in the wrong place and had the smaller clips at the rudder horn. But if you have the springs attached directly to the rudder horn as they should be then the chain and the clips at the tail wheel end, I can't see how the rudder horn tabs get worn. I suppose they could wear but at a much slower rate with the fatter spring rather than the thinner clip which acts more like a saw.

To ease installation and removal I've used a AN 115 shackle and pin to connect the spring to the rudder horn tab. The spring can easily be stretched to insert the pin through the shackle and rudder horn and out the other side of the shackle. The pin like the spring is thicker and wear will be reduced if not eliminated.

http://www.aircraftspruce.com/catalog/appages/an115.php