# Gyroscopic Wheels Don't Keep Bikes Upright? Back to the Drawing Board…

By Veronique Greenwood | April 18, 2011 3:28 pm

Take your hands off the bicycle handlebars and your bike won’t notice. Hop off and give it a shove, and chances are it’ll keep skimming along all on its own (as long as you don’t push it over en route to your faceplant). Ever since bicycles were invented in the 1860s, people have been wondering: what makes bikes so spookily stable?

Popular explanations are that the spinning wheels behave like gyroscopes or that the front wheel making contact with the ground just behind the steering axis stabilizes the bike. But take both of those properties away, researchers reporting in Science (\$) have found, and the bike still rolls merrily onward.

The team built their own bicycle with extra wheels that rotated in the opposite direction to cancel out the gyroscopic effect and with a steering axis that’s behind the front wheel, and found that even that was not enough to knock the bike off balance. When graduate students tried to tip it over, it steered into a turn and recovered.

So what is keeping the bike up? A great sum-up at ArsTechnica explains that the answer is: we’re still not really sure. While the math is clear, how it connects to physical reality isn’t.

What their math can’t apparently tell them is why so many different bike designs tend to stay upright…The best they can surmise is that the stability is related to the ability of the bike to steer into a fall if it starts to lean, and that there are multiple ways of constructing a bike that does this.

But here’s an idea for next time: taking a bike that falls over all the time and tweaking it until it stays up.

Video credit: Jacques Tati, Jour de Fete (1949)

• Angus

First of all, wouldn’t wheels spinning the opposite direction but about the same axis still resist falling over sideways? They would independently, so why would spinning them around the same axle remove that effect? And second, bikes are more stable at high speeds. The only thing changing at high speeds is the speed of rotation of the wheels, and the only thing that can explain additional stability from a wheel rotating at higher speed is the gyroscopic effect. I’m unconvinced.

• Veronique Greenwood

@Angus, actually, I *believe* the extra wheels have their own axes–it’s pretty funny looking. If you can, check out the paper for a description of how the experimental bike was built. I’ll see if I can dig up a pic to post, too.

• TerryS.

I agree Angus. I did my bachelors thesis in 1981 in Mechanical Engineering on the stability of two-wheeled vehicles. At that time, there was an large volume of empirical evidence regarding rake angle, trail, and the moment of inertia of the fromt wheel on the stability (in acceleration, constant velocity, and deceleration) of a two-wheeled vehicle. This was backed up with theoretical 3-dimensional models. I would suspect that these researchers are missing something, not us.

• andy ruina

For those who want the whole story, the www site below has a preprint of the paper as
well as links to 90 pages of backup material. Also photos and videos.
http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/stablebicycle/

About the gyros above: two counterspinning gyros attached actually do cancel. Its not intuitive
that they do, if you think of gyros as just being stiffening against rotation. But that’s not what
gyros are. They have a directionality and that actually does cancel.

Of course we, the co-authors of the paper, could have missed a lot. We are only human. But as you will see if you look
through the material on the www site above, this was not a quick casual study that did not pay due to other past work.

-Andy

• Mike

The other gyroscopes don’t offset the wheels because they are not in contact with the earth. Instead of thinking of the wheels as the gyroscopes, pretend that the bicycle is actually stationary and the earth is the gyroscope, moving underneath, but in contact with the bicycle.

• Veronique Greenwood

@Andy, thanks for posting the paper and backup–that article about the history of gyros and bikes is very interesting. Where did you dig up the two-mass-skate bike idea?

• Andy

All the wheels seem to have a curved road contacting surface. As the wheel tips to the side, the contacting circumference becomes smaller. Assuming the bike moves at the same forward velocity, the wheel is forced to spin faster. In addition, the contacting surface has a smaller circumference away from the center of the wheel than it does at the center. Much like a foam or paper cup, the wheel will want to turn in a circle. With the high center of gravity of the bike frame, it creates what motorcyclists call a high-centering situation. the top of the bike, which was leaning into the turn, is forced by its forward momentum back toward the 90 degree angle. This angle brings the wheel surface back to its center position where the surface is relatively flat, causing the bike to again travel in a straight line.

The physics of bicycle turning with diagrams can be found in the book, Proficient Motorcycling by David L. Hough.

• actor212

Then how come I keep falling off my bike if it’s so bloody stable?

• DG

I think it has to do with the momentum and the low coefficient of friction on the axle and some gyroscopic effects. If you took out the wheels and replaced them with low friction skids, I don’t think you’d have the same luck.

The velocity of the bottom of the wheel relative to the point of contact on the road is essentially zero. So minor imperfections in the surface or tire would have little effect. Skids, on the other hand, have a relative velocity of their contact points and the road equal to the forward velocity. Small imperfections across this larger area and the additional friction will slow the vehicle down and probably cause it to swerve and fall.

Finally, if the bike is fairly well balanced, then all the gyroscopic effect really has to do is keep the front wheel heading in a constant direction. The rear wheel will follow the front. Changing the rigid frame to a flexible frame would probably reduce the distance the “bicycle” would travel given an equivalent push.

• billy

Gyros smyros.
It’s MAGIC… jeez, mailorder degrees.

• http://physics.asu.edu Rob

You can not counteract a gyroscope with a 2nd gyroscope. Method used is invalid.

• Kevin

I thought it was just a part of “Bodies in motion stay in motion” thing.
Bikes have good balance on their own because they are mostly symmetrical and have a low point for their center of gravity. If you took a minute of your time you can get a bike to stand alone on both wheels when it is at a dead stop.
The bike is moving foward on wheels so it keeps going foward until the force of gravity slows it down, and then the small imblances of the bike take over and it falls to its side.
And why it corrects itself from a sideways force.. maybe a pendulum effect with the bike tryign to swing around its low center of gravity.

• http://ruina.tam.cornell.edu andy ruina

Veronica (#6 above):

Andy Ruina here again, co-author of the paper. In answer to your question (above). The TMS idea was due to Jim Papadopoulos when he was at Cornell in about 1987.
He started by getting and confirming good differential equaitons for a bicycle. These were too complicated to make sense of,
with 25 length and mass parameters. So he set 17 of them to zero. Amongst those zeroed out were the trail and gyro terms.
He found that a bike could then still balance itself. We tried to build one like that at Cornell and failed. Then 20 years later the clever people in Delft (Schwabb and Kooijman) did it carefully and got the TMS bike, a real physical one, to work.

If you want to know more, as posted above, all manner of information is here:

http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/stablebicycle/

%%%%%%%%%%%%%%%%
Kevin (#12 above):
Have a look at video 4 on this www page:

http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/yellowbicycle/index.htm
There you will see that motion itself doesn’t balance a bicycle. You have to have the steering.
And that steering has to work just the right way. Have a look and tell me what you think.

-Andy

NEW ON DISCOVER
OPEN
CITIZEN SCIENCE