The “gyroscope” theory of bicycle stability was debunked over 40 years ago by physicist David Jones. Since then, hobbyists and bike nuts (including Yours Truly) thought bicycles stayed upright via something called the caster effect. Physicists from Cornell, the University of Wisconsin-Stout, Delft University of Technology in the Netherlands and University of Twente in the Netherlands found they can build a “two mass skate” bicycle with no gyroscropic effect and with no trail (for no caster effect) that can stay upright.
As a child, I was taught that bikes stayed upright because the spinning wheels give enough gyroscopic force to impart stability, but experimentalists learned years ago that there’s not nearly enough mass in a bicycle wheel to counteract the mass of a leaning bike and its rider. Physicist David Jones put theory to the test by building a bicycle with counter-rotating wheels to eliminate the gyroscopic effect and empirically determined the gyro is unnecessary for balance.
Since then, the conventional wisdom has been that trail — the fact that the bicycle wheel touches the ground behind the steering axis — creates a caster effect to keep bicycles (and motorcycles) upright. Caster is what keeps the front wheel from wobbling wildly around like you might see on some shopping carts with a broken caster wheel.
This fun video from Science Friday explains some of the background of how the caster effect works, but basically, when a bike begins to tip over, the bike steers itself into lean to automatically bring it upright. As long as the bike is moving at a sufficient speed, the bike will “ghost ride” itself.
These experimenters did the math on bicycle dynamics and learned that the caster effect may not be necessary as well. Traditional models of bicycle stability require second order differential equations to solve, so these physicists created a “two mass model” (TMS) bicycle to simplify the dynamics of bicycle stability. With this model, they learned they could completely eliminate the gyroscopic effect and the caster effect and still have a stable bicycle.
With this model, they place a mass far forward of the front wheel to create their self stable bike. That’s how triathletes ride and might explain why their bikes are so difficult to steer — they’re made for going fast in straight lines.
If you’re interested in the details, PDF of the full paper is available here: A bicycle can be self-stable without gyroscopic or caster effects, by J. D. G. Kooijman (Delft University of Technology, Delft, The Netherlands), J. P. Meijaard (University of Twente, Enschede, The Netherlands), Jim M. Papadopoulos (University of Wisconsin-Stout, WI, USA), Andy Ruina (Cornell University, Ithaca, NY), and A. L. Schwab (Delft University of Technology).
Photos by Richard Masoner. The young woman in the bottom photo is Kimberly Capriotti. She’s a professional fashion photographer in Chicago who takes the lovely pinup photos for the Thought You Knew Us cheesecake calendar project..