headset looseness is _not_ the key to shimmy, but it is the key control system that can eliminate shimmy due to adjustment. and yes, i realize that over tightening a headset will cause it to wear out faster. overtightening will also MASK the shimmy by damping it down to nothing, quickly.
my email suggested that shimmy was either from too little trail (e.g. unstable steering system), or something in the steering system that results in "two local minima" for the steering column. when there are 2 or more local minima, it's likely that the system can oscillate between the two local minima. more trail (which, like headset tightness) will make the "go in the direction of trail": local minima more dominant, but you haven't done anything to address the other local minima.
what's the other local minima ?? I don't know, but a badly faced head tube or misaligned steering column, or a tire with the wrong profile (or possibly unevenly manufactured tread). Any other guesses?
- Don Gillies
San Diego, CA