Nicola Rutzou
For many years, the university boffins who sought to discover what kept bicycles upright agreed that two forces were responsible – gyroscopic and caster (also called trail) effects. But more recent research has found that it’s more complicated than that and includes other factors, including the brain.
Hugh Hunt of the University of Cambridge says that our brain compensates for the wobbling of the bicycle, and we learn to smooth the ride.
Hunt says that gyroscopic forces are not important for the stability of a bicycle, but they help us to control the bike when riding with no hands.
The gyroscopic effect
The gyroscopic effect occurs because a spinning wheel wants to stay spinning about its axis, just as a spinning top stays aligned to its spin axes. If a bicycle did stay upright because of the gyroscopic effect then any novice getting on a bike could just push off and ride the bike, and wouldn’t need to learn how to ride.
Hugh Hunt says that the trail effect is important to stabilise the bike. “The front wheel makes contact with the pavement at a point that lies behind where the steering axis intersects with the pavement – and the distance between these is called the trail. The trail works to stabilise a bike in much the same way as castors work on a shopping trolley.”
Actively controlling the steering
But more importantly, Hugh Hunt says that we stay upright on a moving bike by actively controlling the steering.
“This is why we have to learn to ride a bike. If, as learners, we find ourselves falling over to the left, then we learn to steer the bike to the left, which generates forces that tilt us back upright again, thereby putting the wheels back under our centre of gravity.
“Beginners are very wobbly, but as we become expert, the corrections become smaller, and we can ride in a straight line. The faster we ride, the smaller the steering adjustment needs to be, simply because the bike moves much further in a given time. When riding very slowly the steering adjustments required are very large. When completely at rest, active steering can do nothing for us,” said Hunt.
Gyro and caster forces
Another group of researchers from Cornell University, the University of Wisconsin-Stout and Delft The Netherlands, including Jim Papadopoulos, Andy Ruina and Arend Schwabhave, also established that a bike will be stable even if gyro and caster forces aren’t present. Papadopoulos believes that two well-accepted studies from the 20th century on the self-stability of bikes contain mistakes.
To prove that gyro and caster effects were not needed, the researchers built a riderless bicycle with two small wheels. Each wheel was 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 rolling at more than about 8 km/h per hour, the research bike still balanced itself, and if knocked slightly to one side, it straightened itself back upright.
Centre of mass
One key area that Papadopoulos and colleagues highlighted was centre of mass. This is demonstrated by fitting a long stem to a bike that is much easier to ride no-handed because the weight of the handlebars is further ahead of the front wheel.
“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 titled, “Bicycles can be self-stable without gyroscopic or caster effects.”
“We have found that almost any self-stable bicycle can be made unstable by misadjusting either the trail, the front-wheel gyro or the front-assembly, centre-of-mass position,” the researchers explained in their paper.
“Conversely, many unstable bicycles can be made stable by appropriately adjusting any one of these three design variables.”
Potential improvements in bicycle design
While their work was intended to gain insight into the nature of bicycle balance, the researchers say their analysis may lead to further improvements in bicycle design.
“If you look at the bicycles of today,” says Professor Arend Schwab, one of the report’s co-authors and a biomechanical engineer at the Delft University of Technology in the Netherlands, “they look almost identical to the ones made around 1890 when scientists first looked at the self-stability of the bike.”
Schwab says there are two main reasons for the slow rate of development in bicycle design. Firstly, the double-triangle frames (which have one triangle in the front composed of the top tube, head tube, down tube and seat tube, and one in the back from the chain and seat stays) are very strong compared to any other kind of frame. Secondly, cycling’s governing body, the UCI, has established that road race bikes must be manufactured this way, so the market has largely followed suit.
“For the general purpose of cycling, it’s a good design, and it works. It’s the human size, it’s practical, not too big, not too heavy. But there is room for improvement and the family of bicycles is bigger than what we have seen so far,” said Schwab.
Despite the team’s enthusiasm for alternate bike designs, its findings are not well accepted by the cycling establishment.
“Manufacturers are only rarely innovators, in any meaningful sense. At least, that is my perspective from outside the industry. They want to make a profit. And we are not showing them a working product, but only ideas,” said Papadopoulos.
