I think the answer tothe question of Contracts for Riders in Speedway, as in Football, is that that they are only valid as long as both Parties are in agreement.
If one side or the other gets fed up or wants more money etc.. Contracts don't seem to matter a damn.
An example of this in Football is a Player playing badly for his Club in order to engineer a move. That, in effect, is breaking your Contract.
Not sure if that happens in Speedway , though I do have my suspicions.