Staudacher gear failure. Operated off grass strip approximately 30 flights before failure. During most take offs and landings, the gear would 'shimmy' or vibrate fairly severely. On landing, it appeared that the vibrations would begin during brake application. Observed skid marks on the grass revealed that the gear would hop from one side to the other, locking brakes from side to side. (see diagram).
My theory is that there are 2 factors that caused this. (a theory from a non-engineering or physics educated individual). First, the spring gear on this airplane has an extremely wide wheel base. This staudacher had about 90" between tire centers. To give some perspective, a Pitts S1S has about 52" and a much larger Waco Taperwing has 82". Both airplanes have rigid gear. Torque is a product of Force X Distance. The added force of the extended gear adds stresses that are exponential as the wheelbase grows. The second theoretical factor is resonance in the spring gear. I believe that this gear develops a vibration that resonates until all forces inputted are eliminated or reduced significantly: Liftoff or a speed slow enough on landing that reduces the forces enough to stop vibration. The gear appeared to have been vibrating and flexing rearward, creating a rotational moment on the gear legs that rotated the leading edge of the gear in a downward direction at the bracket, shearing the bolt and nut.
http://cnx.org/content/m13544/latest/
http://en.wikipedia.org/wiki/Torque

I believe that the spring gear on this airplane is too wide.

Just a theory.

Mike