a moment of the same sign. CW if the wheel is rotating one way and CCW
if it is rotating the other.
These moments sum to a net moment on the wheel as a whole, i.e. the
combined inner and outer wheels.
All other vectors must add to zero since the wheel as a whole has no net
movement up or down or to one side or the other.
Therefore the wheel must rotate with the combined moment.
In effect the slicing of a single whole wheel into individual inner and outer
wheels has released the internal shear strain energy between the inner and
outer parts of the whole wheel.
