Bicycle design: back to the drawing board

How a bicycle stays upright while moving has always been something of a mystery to science, with a vague cocktail of gyroscopic effects being the accepted explanation. Now, however, scientists from Cornell University and the Delft University of Technology have determined the complex interplay of design characteristics that make a bike stay upright. The findings, which may catalyze radically new bike designs, are published in Science.

Traditionally, the consensus within the scientific community was that the stability was very closely related to gyroscopic effect and “trail.” Trail (also known as the caster effect) is the distance by which the contact point of the front wheel trails behind the steering axis. The new study shows both these factors to be relatively unimportant.

“It’s all about how bicycle leaning automatically causes steering, which can bring the wheels back under a falling bike,” said Andy Ruina, professor of mechanics at Cornell and a co-author on the paper.

To prove that gyro and caster effects were not needed, the researchers built a riderless bicycle with two small wheels, each matched with a counter-rotating disk to eliminate the gyro effects, and with the front wheel contact point slightly ahead of the steering axis, giving it a negative caster effect. When launched at more than about 5 mph, the research bike – like many bicycles – still balanced itself. If you knock it slightly to one side, it straightens itself back upright.

All up, the researchers found some 25 different factors influence bicycle stability, but Dr Arend Schwab, from Delft, said the critical factor was mass distribution, especially the location of the centre of mass of the bicycle’s steering mechanism. “For a bicycle to be stable, the steering mechanism has to be unstable; if the bike falls, the steering should fall even more quickly,” he explained.

The researchers add that bike design is rather conservative and there is nothing about the basic schema of the bicycle that has changed since the end of the nineteenth century. “Manufacturers can use our model to make directed modifications to the stability of their bicycles. That may be of particular interest for unusual designs,” concluded Schwab.

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Source: Cornell University, Delft University of Technology

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