bike friday's travel case comes with an "I" beam shaped support that prevents the hardcase from being compressed while in transit; this protects the derailler and other delicate bits from being damaged.




the black disc in the center of the image shows an elevation view of the "I" beam in the correct position.



there are other solutions fro protecting the derailler. if your bike has rear rack mounts you can screw in a protective "block" made from a sufficiently long machine screw and a piece of wood or plastic. or, you can buy a derailler protector. this solution is not as rigid, but it affords some significant degree of protection and can remain installed on your bike for general purpose insurance against derailler damage. plus, these doo-dads are cheap.