Balancing act

Balancing act
Date:1 September 2011 Tags:

Everyone knows that Bikes in motion briefly stay upright without a rider. Now scientists finally understand why. By Alyson Sheppard

It’s a familiar phenomenon: push a riderless bicycle forward fast enough and it will briefly stay upright. An international team of researchers now says the established explanations of why the bike doesn’t immediately topple are misleading. Scientists have attributed a bike’s tendency to right itself to two effects. The first is gyroscopic torque, which steers the spinning front wheel in the direction of the fall when the bike leans. This brings the wheels back under the bike’s centre of mass, keeping it upright. The second, the caster effect, dictates that the front wheel aligns with the direction of travel.

But researchers in the US and the Netherlands have built a bike that doesn’t exhibit either effect, yet still self-balances (left). The engineers determined that the distribution of mass – low in front and high at the back – is actually what contributes to keeping the bike upright. After a riderless bike is pushed, the front dips faster, causing the front wheel to steer into the fall and briefly stay upright. The test bike is not built to ride, but researchers are hoping manufacturers will use the study’s findings to design folding or recumbent bikes that are easier to balance.

Andy Ruina, a mechanical engineering professor at Cornell University and co-creator of the bike, notes that the discovery could also aid makers of walking bipedal robots, which re-balance every time they take a step. “Steering right on a bike is almost mechanically identical to stepping right when you’re walking,” he says.

The bike’s wheels are small to minimise gyroscopic torque. It’s further cancelled by an upper set of wheels rotating in the opposite direction.

The forward wheel sits in front of the steering axis, so it cannot follow, or “trail”, the bike as it changes direction – eliminating the caster effect.