The correct answer includes both points of view expressed above. Namely, you can ride your bike without your hands due to the fact that when moving, your bike has a vertical position as it's most stable equilibrium. This is both due to the fact that fast spinning wheels have rotational inertia - meaning that they would much rather stay spinning in the same plane than tilt, and due to the fact that bikes usually have a front wheel 'fork' designed in such a way as to reinforce this vertical equilibrium. Discs get lift and stabilization from Bernoulli's principle the wing of a disc acts the same as a jets.
