Why do bicycles stay " up"? Physics, that's why. Bicycles rely on the concepts of angular momentum and precession to prevent tipping over when. When a wheel is spinning fast, it tends to resist changes in it's angular momentum. This is called 'precession', and is an important concept in bicycles and unicycles alike. When gravity tries to tilt a wheel, it is effectively trying to alter the angular momentum of the wheel. In reaction, the wheel will turn only slightly, and gain a slight 'wobble' as one side of the wheel juts out above the rest, which is usually resolved by dampening forces 'n' stuff. So bikes work because of physics, and that's pretty cray.