Posted for "Lowell Fitt" <lcfitt@sbcglobal.net>:

 This might ba apples and oranges, but the most common cause in shimmy in a
tail wheel airplane is incorrect pivot axis angle.  One way we can confirm
that to be the problem is to remove some pressure on the tail wheel spring by
giving a little nose down elevator.  This will usually cause the shimmy to
stop as it allows the spring to straighten a bit which gives a more aft
leaning  pivot axis.   Remember the shopping cart that shimmies - pivot angle.
 
 We just mounted the nosewheel strut on our IV and it called for  a .5 - 1
degree forward divergence from vertical.  With the tailwheel shimmy in mind,
the 1 degree would be preferable. It places the tire contact with the ground
further aft of the pivot axis.
 
 Lowell