Video: The physics of better bikes

This weird contraption is called a two-mass-skate bike (Image: Sam Rentmeester/FMAX)

If the usual ideas of how bicycles balance themselves are right, this weird contraption ought to quickly topple over. In fact it stays upright, an observation that might lead to a rethink of bicycle dynamics – and perhaps to better bike designs.

Push a riderless bike fast enough, and it stays upright for a surprising distance. When it starts tipping to the right or left, the front wheel steers into the fall, correcting the tilt.

Earlier studies attributed this in part to the gyroscopic effect of the spinning wheels. But this model, built by Andy Ruina of Cornell University in Ithaca, New York, and colleagues, nullifies the gyroscopic effect with discs that spin in the opposite direction to the wheels. Also, its front wheel is mounted forward from the usual place – a change expected to make the bike topple.


The key to its stability seems to be that the centre of gravity of the handlebars is lower than the frame’s and forward of the steering axis. So when the handlebars start to fall, they do so faster than the frame. This twists the hinge to the front wheel, steering it into a fall.

Journal reference: Science, DOI: 10.1126/science.1201959