A Staff Report from the Straight Dope Science Advisory Board

Why is it easier to balance on a moving bike than a non-moving one (revisited)?

July 4, 2002

Dear Straight Dope:

Regarding the role that gyroscopic action plays in staying upright on a bicycle: You might want to have Karen Lingel take a look at the work of Professor Richard Klein (mechanical engineering) at my alma mater, the University of Illinois at Urbana-Champaign.  Professor Klein's particular interest is bicycles, and among many fascinating bikes that he has built are several that remove any gyroscopic effect from the wheels (each wheel has a duplicate wheel mounted above it, turning in the opposite direction). These bikes are just as easy to ride as a regular one, and so there is definitely something more at play. His bikes have been used as examples in studies of feedback-control systems.  He has built one bike that is so far impossible to ride; the rear wheel steers and it turns in the opposite direction given handlebar input. When I was in college there was reputedly a cash reward for the first person who could ride it the length of the ME building. I tried and fell off in about 5 feet as most folks did. Ride on!

SDStaff Karen replies:

Huh. Here I've been spending so much time trying to keep up with advances in supersymmetry and high temperature super conductors that I totally missed the latest revolutionary research in bicycle dynamics. (Maybe they should have called it bicycle superdynamics.) They've been teaching the same theory for hundreds of years and only now has someone done an actual experiment. That should be a lesson to everyone who believes all that stuff about string theory and 26 dimensions.

So, let's all get up-to-date with this hot-off-the-press research, which was only published in … huh … 1970. That is distressing, because my primary physics education took place in the 1980's.  Of course, you'd expect news of breakthrough research to take 10 years or more to reach a podunk school like my alma mater, the University of Illinois at Urbana-Champaign … um, wait, you're saying this UIUC professor in mechanical engineering knew all along and we never got wind of it in the physics department? Man, we really should have invited a wider circle of people to our weekly wine-and-cheese.

Groundbreaking research in bicycle dynamics was published in 1970 by David Jones. He mounted counter-rotating gyroscopes on bikes to counteract the gyroscopic effect of the wheels. The resulting bikes were quite rideable. So why do bikes stay up?

The answer is: trail. Trail is the difference between where the bike's front wheel contacts the ground and where the steering axis (drawn through the fork of the front wheel) meets the ground. Well-designed bicycles have negative trail — that is, the wheel contacts the ground behind where the steering axis meets the ground. When you tilt, the trail causes the wheel to turn, thus converting the tilting motion into a turning motion, etc., as per my original report. The acid test done by Dr. Jones is in creating bikes with positive trail. Even professional cyclists can't ride those very far.

Trail is evident in rolling office chairs and grocery carts, too. The wheel attachment axis is off-center, so that the wheels always "trail" behind the direction you are rolling or, in the case of shopping carts, attempting to roll.

So there you have it. My apologies, but on the bike path of progress, you have to expect the occasional bum steer.

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