A bicycle is surprisingly stable for an upright, two-wheeled vehicle that needs to be propped against a wall when it’s not moving. But perhaps a bigger surprise is that no consensus exists on why the bike is as stable as it is. For such a simple design, which almost anyone can understand, this seems crazy. After all, we live in a world of self-driving cars and safe passenger airplanes. Surely the bike can’t still hold any physics or engineering mysteries?
At the heart of the puzzle is something we’ve all observed. If you push a riderless bicycle, it balances itself, steering automatically to correct for any wobbles, until it slows down and finally falls flat on its side.
There are two theories as to how the bike keeps itself upright. One is the gyroscopic theory, where the spinning wheel provides enough stability to stop the bike from falling. You can try this for yourself if you have a bike handy. Remove a wheel (the front is easier and probably cleaner) and—holding the axle—give it a spin. Now, try to twist the wheel by moving the axle. You’ll see it resists you. Now, with the wheel still spinning, crook a finger under one side of that axle, and let go of the other side. Magically, it stays there, like somebody invisible is holding the other side up.
Striking as this effect is, it doesn’t account for the bike’s self-balancing ability. By mounting a second wheel that spins counter to the first, the gyroscopic effect can be canceled out. Way back in 1970, science writer David Jones did exactly this. "One bike that Jones built had a counter-rotating wheel on its front end," says Scientific American, "that would effectively cancel out the gyroscopic effect. But he had little problem riding it hands-free."
Jones proposed an alternate theory, which came to be the second major explanation for the bike’s self-balancing ability. The "caster theory" considers the bike wheel to be like the caster on a shopping cart. On a shopping cart, the caster touches the floor behind the steering axis. In this case, the steering axis is the spindle that connects it to the rest of the cart. This, as you know, lets the caster automatically align itself to the direction of travel.
On a bike, the steering axis runs down the fork. If you imagine a line that continues out the end of the slanted fork, it actually hits the ground ahead of where the tire touches the ground. That is, the steering axis is ahead of the contact point, just like on a shopping cart. The distance between these two points is called the "trail." Jones, says Scientific American, found that a long trail makes a bike more stable, whereas a short trail makes it harder to ride.
Jones was so pleased with his discovery that he was still crowing about it 40 years later. In his memoir, he wrote: "I am now hailed as the father of modern bicycle theory."
The problem is, he was wrong. While caster trail does determine how easy a bike is to ride, and the gyroscopic effect does help stability, neither is responsible for the self-balancing effect of the bike. Engineer Jim Papadopoulos, the subject of Scientific American’s feature, demonstrated that a bike with significant negative trail can be ridden, as long as it has a weight jutting out front. That weight could, in theory, come from cargo on a front rack.
So, just how does a bike stay up by itself? It’s still a mystery, although one which might finally be solved thanks to a new research center in (where else?) the Netherlands, which now employs Papadopoulos. In their lab, researchers build all kinds of crazy bike designs to investigate how physics and cycling interact. The aim is to better understand how the simple-seeming parts of today’s bikes work together, and to improve rideability, or discover new ways of doing things. Who knows? Perhaps today’s ubiquitous diamond-shaped frame, with equally sized wheels, isn’t the best way to build a bike after all.
Have something to say about this article? You can email us and let us know. If it's interesting and thoughtful, we may publish your response.