The International Cessna^® 170 Association

Tain Wheel Shimmey I found some interesting information on this subject. It can be found at: pierceaero.net

Ralph

Thanks, Ralph, for posting. This is a reoccuring problem with tailwheels.
That site is a commercial site hosted by an EAA member. EAA has some fantastic maintenance expertise within their community. But keep in mind they are largely concerned with advising owner/builders of experimental aircraft, therefore they frequently do not make the distinction between altering an experimental versus a certificated aircraft.
While the article mentions a J-3, it suggests a couple of unapproved methods of correcting shimmy. One suggestion is altering the assembly by introduction of a non-approved part, a "pad" beneath the mainspring to alter the angle of the "kingpin". The other suggestion is to bend the mainspring at an automotive/spring shop. The latter suggestion does not address the issue of unduly/permanently stressing the mainspring without proper data. Neither method is approved for certificated aircraft.
To it's credit, the article mentions the "kingpin" angle or caster. This has been addressed several times here at our forums.
The correct caster or angle of the "kingpin" (the shaft/bolt around which the tailwheel actually pivots) is clearly illustrated in the Scott assy drawings found in several places including The 170 Book, but I really like the mentioned article's illustration. (Thanks again, Ralph, for posting the reference.) The top of the "kingpin" should be aft of the bottom.
CAUTION: THIS IS NOT A CESSNA 170 INSTALLATION in the picture. Ignore the tailwheel mainspring shape/curve/geometry. This seems to be a Piper product installation. BUT THE SCOTT 3200 ANGLES AND SETUP ARE THE SAME AS FOR A 170 and the tailwheel should be at similar caster angles. YES! This will cause the tailwheel to lay over slightly on it's side during ground handling when the tailwheel is deflected 90 degrees.



Other message posts regarding shimmy can be found here at:
http://cessna170.org/phpBB2/viewtopic.php?p=2734#2734
http://cessna170.org/phpBB2/viewtopic.php?p=7580#7580

George, can you show me where it states that rearching a tail wheel spring is not an accepted practice. If that is unaccepted then why can a main gear be rearched. Bending or creasing I can go along with, but rearching is another matter. Jon

Jon, I don't mean to say it cannot be done. It just cannot be done by the average guy in his garage with a hammer and a vise.
If bending a paper clip weakens it, then bending a tempered spring to a new shape definitely is beyond it's elasticity. I don't know what many of our friends mean by a "spring" shop. I don't believe the local tractor supply or truck repair shop qualifies for aircraft repairs.
Many owners seem to think it's OK for them to redesign the angle of the spring because they have some "better" idea or special knowledge of how Cessna really must have meant for things to be. I don't want to encourage anyone to take it upon themselves to simply re-bend their springs 1- because they think it'll work better or 2-without realizing it will weaken the spring...an already weak design, in my opinion.
New tailwheel mainsprings are too inexpensive to risk a damaged rudder.

Go to a spring shop and watch them arch springs. They do it a little at a time moving back and forth with a large press and a good radius. We have had them rearched with excellent results on Pipers.