Tag Archives: bike

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.

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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.

University of Wisconsin-Madison Department of Physics: The Bicycle Wheel Gyroscope

Under the banner of “The Wonders of Physics

The article does a nice job of describing a system, the demonstrator and bicycle wheel, that conserves angular momentum about a vertical axis because it is free to rotate about a vertical axis.

It ends, however with a comment that

A motorcycle turns around corners this way. When the rider leans to one side, the whole motorcycle turns around a corner.

Motorcycles do not turn this way. Motorcycles are not free to rotate about a vertical axis because of the two tire contact patches. These generate a couple that easily alters the angular momentum of the vehicle about a vertical axis. For a similar reason, the demonstrator does not begin rotating about a horizontal axis when he tilts the bicycle wheel. His feet create a couple that prevents him from rotating about a horizontal axis and so alters the angular momentum of the system about a horizontal axis.

The actual role of gyroscopic effects in motorcycle and bicycle behavior is due instead to torque-induced precession, and this can occur in a couple of ways when the bike is rolling forward:

1. Applying a torque to the handlebars about the steering axis causes a rolling moment. For example, steering left causes a roll moment to the right. Vittore Cossalter, in his Motorcycle Dynamics​ book, describes how this can be useful in motorcycle racing.

2. Leaning the bike causes the front wheel to steer in the direction of the lean. This can contribute to, but has been shown to be not necessary​ for, the self-stability of the bike. Kooijman, et al., demonstrated this in their 2011 Science article.

The rear wheel is prevented from precessing as the front wheel does by friction of the tires on the ground, and so continues to lean as though it were not spinning at all. Hence gyroscopic forces do not provide any resistance to tipping.

I have sent an email message to the address listed at the bottom of the article, randall@physics.wisc.edu, but have not heard back.

The wonders of physics indeed. I wonder who comes up with this stuff?