Hmm, ok.. so the wheel should spin and move the unicycle or no?
The frame should only change its rotation, but not affect the wheel?
If thats the case then maybe dont use the physic behavior on the frame but pin it.
if you want physic on the frame hmmm idk ... maybe set the frames density to nearly 0 or make the wheel super havy so it doesnt get affected so much?!?!
The thing is as long as both are physic objects and they are attached to each other they will always interact with each other... that's the point of the physics behavior :/
Maybe you could pin a sprite with physic behavior to the wheel, set immovable to yes, make it invisible and attach the frame with an revolute joint to that invisible sprite?!?!