from Aircraft Landing Gear Design: Principles and Practices by Norman S. Currey:
9.5 Castering Nose Wheels ....... Configurations a and b in Fig 9.24 are statically neutral and dynamically stable in both forward and aft movement. In the latter configiration, the wheel swivels 180 deg for aft movement. A shimmy damper is resuired for both configurations unless corotating wheels are used. Configuration c is statically and dynamically stable in both forward and reverse and also requires shimmy damping except, posibly, when t = R-1.2R. Configuration d is statically unstable and dynamically stable. It is unstable in reverse and the gear must be locked ot steered for this operation. Shimmy damping is required. This configuration is often used for tail wheels, using friction to provide shimmy damping. illustration at: http://www.mykr2.com/90437.pdf jg
