Sorry to hear of another nose wheel shimmy problem.  We have been over this ground many times.  I have a further thought to add to the comments already contributed.

 

The nose wheel shimmy is controlled by internal damping using the oleo hydraulic fluid bleeding through an orifice as the strut rotates from side to side.  Too little fluid or too much clearance, too little damping, and destructive oscillations set in.

 

Consider: The viscosity of the strut oil varies dramatically with temperature.  When retracted, the nose gear gets heat soaked in the hot air under the engine which is roughly 150F above ambient in normal cruise conditions.  The strut and oil get hot, and the oil viscosity drops – a LOT.

 

Then you drop the gear and get a nice (comparatively) cold blast across the strut that cools it and the oil inside.  The oil is in the annular space between piston and cylinder, and probably cools fairly rapidly as the external surface is exposed to the air blast.   Without doing the heat transfer calculations for flow around the strut, my guess is that the time to cool the oil in a 100 knot air blast is a few minutes. 

 

So here is the thought: if the nose strut is truly heat soaked, and the gear are extended only 1-2 minutes prior to touchdown, the oil may still be warm to hot, and the ability to damp shimmy is therefore substantially reduced compared to a cold damping test in the hangar.

 

So here is the proposition: shimmy may well correlate with time between gear extension and touchdown.  If in doubt, lower the gear early, and extend downwind.

 

This is pure supposition.  Other thoughts?

 

Fred Moreno