If the wheel were to fall, a velocity component \(v\) develops at the far edge of the wheel, rotating the wheel, with the contact at the torus wall as pivot. This velocity component together with the circular velocity of the wheel (into the diagram) and \(g_B\), creates a force \(F_{gB}\) with a upward component that lifts the wheel. The wheel is thus prevented from falling down the center of torus.
