Why is it easier to balance on a moving bike than a non-moving one? | The Straight Dope
" The answer is: trail. Trail is the difference between where the bike's front wheel contacts the ground and where the steering axis (drawn through the fork of the front wheel) meets the ground. Well-designed bicycles have negative trail--that is, the wheel contacts the ground behind where the steering axis meets the ground. When you tilt, the trail causes the wheel to turn, thus converting the tilting motion into a turning motion" Read the author's original (and incorrect) response . Read more about P rofessor Richard Klein's Zero-Gyroscopic bikes . [Read more on straightdope.com]