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No hands!

Growing up in hilly country, I rode a bicycle for many years but I was leery of riding 'without hands' -- that serene, vertical stance in which the bike seems to steer itself. I certainly tried. Relinquishing control in careful steps, first by sliding my hands to the center of the handlebar, then releasing one hand, and finally lifting one finger after another, I could not remove the last touch of my fingertips. In the sure cool metallic touch, control seemed to exist. Evidence was to the contrary. Kids did it all the time. In the face of such an appalling lack of skill, I came up with an excuse: Much of the biking I did was on back roads with innumerable rocks and pot holes; roads swooped up and down hills, around sharp corners. Constant vigilance was required!

I now live an area with big roads, wide shoulders and bike paths. I have no excuse. After studying physics, I should be confident that I can steer very nicely without using my hands.

A physical understanding develops by locating essential elements. For a bicycle, this is the wheel. An isolated, spinning wheel continues to rotate, just as it is. The wheel's axle always points in the same direction. We call this property conservation of angular momentum. Hiding in this use of the word momentum is a length and a direction. A nice way to visualize this is to imagine an arrow or vector parallel to the axle of the wheel. When a rolling wheel turns, the arrow's direction changes. When a wheel rotates faster, the arrow's length grows.

Playing with gyroscopes I can learn how wheel and arrow react to force. A gyroscope may be an obtuse character, but at its heart it is a simple, spinning wheel with a (mostly) unchanging angular momentum. Gyroscopes don't turn in the direction they are pushed! The wheel turns perpendicular to a push. This is how my bicycle turns.

When I lean to my left, my weight pushes down on the left side of the axle and lifts up the right side of the axle. The wheel doesn't just tilt. (If it did I would fall over!) But because of the way in which angular momentum changes, the wheel turns perpendicular to my downward push turning the front wheel left -- just what I want!

OK, that is the theory. I choose a mostly empty access road. I coast down the slight incline at a good clip. The arrow representing the wheel's angular momentum points directly to my left. Releasing my hands, first one then the other, until just my fingertips touch the handlebar, I sit upright. I'm riding without hands! I try leaning to the left. Shifting weight pushes the left side of the axle down. Instead of the arrow moving down, due to the change in angular momentum, the arrow shifts towards the rear so that it points a little behind; the wheel turns! The bicycle's miracle is a conversion of tilt into turn catching my fall. Oops, the road isnít this wide! I lean to the right pushing down on the right side of the axle. The change moves the arrow forward, straightening out the wheel. I ride on vertical, not yet serene, as I learn to make the make little adjustments of weight.

© S. Major 1993-2004 Last modified 11 April 2004

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