Tag Archives: bicycle

Phys.org: How the bicycle got its spokes

A recent news article on Phys.org tries to present some of the history of the technical development of bicycles. It goes off the rails, however, when it touches on balance:

Then comes the phenomenally complicated bit: not falling off. “Only a few people really understand how balancing a bicycle works,” says Philip Garsed, a PhD student in electronic engineering whose passion for bicycles developed into his recent talk at the Cambridge Science Festival, titled How the Bicycle Got Its Spokes. “There are lots of effects interacting with each other. One of the most interesting is the gyroscopic effect. If you have a wheel spinning around an axle and then try to tilt the axle from side to side, you get this weird effect that makes it resist that change. On a bike, that tends to keep you upright and for quite a long time it was thought that this was the reason why a bike can be balanced. It was then proved that it was not actually necessary – someone stuck a flywheel that rotated in the opposite direction on to the wheel and eliminated the effect, but the bike was still rideable. I have a book that explains the details of bicycle balance. It’s hundreds of pages thick and it helps to have a physics or engineering degree to get your head around it.”

1. It is not phenomenally complicated: bikes balance by being steered in the direction they are leaning.

2. Anyone who has read the second paragraph of the Wikipedia article knows this.

3. Spinning bicycle wheels do not resist change, they merely change in a way unlike non-spinning bicycle wheels. They cannot keep a bicycle upright unless at least one of them is allowed to steer.

It would seem that if you thought something was phenomenally complicated, you would avoid speaking publicly about it, or at least check with someone who does understand the topic before you do.

I posted a comment at the bottom of the article and emailed Philip Garsed at the University of Cambridge, but have received no reply.

The Straight Dope: Why is it easier to balance on a moving bike than a non-moving one (original and revisited)

The original response to this question was just terrible.

Because modern bicycles are equipped with a pair of  gyroscopic stabilization devices that require the motion of the bike in order to operate. These devices are known as “wheels.”

It implies that the reader is a dummy for not knowing this obvious “fact” and that non-modern bicycles maybe didn’t have a pair of wheels.

The revised response is better but it merely discards one misconception for another. After citing the David Jones Physics Today article from 1970 , the author writes:

So why do bikes stay up? The answer is: trail.

While it is true that both trail and gyroscopic effects can contribute to the self-stability of a bike, it has been demonstrated, by Kooijman, et al., in their 2011 Science article,  that neither trait is necessary nor sufficient by themselves for self-stability.

Instead, the simple answer is that when a bike is moving forward, it can be steered to keep the wheels under the center of mass. More correctly, by steering in the direction of a lean, the tire contact patches accelerate in the direction of the lean causing a moment about the center of mass to counter the moment created by gravity.

The steering torque necessary to steer in the direction of the lean can be provided by the rider, or, in some cases, by some combination of mass distribution, geometry, gyroscopic effects, and tire properties.

The revised response ends with

On the bike path of progress, you have to expect the occasional bum steer. 

They have provided two bum steers so far. Perhaps they can get it right on the third try.