Science News

Self-righting bicycle

Sun, 17th Apr 2011

Diana O'Carroll

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A team working in the USA and Holland have this week come one step closer to working out how a rider-less bicycle remains upright. As you know, this only happens when the bike is moving, so there must be some interesting mechanical forces at work keeping it from toppling over.

The self-stable bike modelWhat happens when the bike coasts is that, each time it starts to fall to one side, the front wheel will steer into the fall and so correct it – making the bike upright again.

For some time the conventional wisdom has been that two principal forces cause this corrective steering to occur. The first of these is a sort of precession – otherwise known as gyroscopic torque. This is when something spinning in a particular direction provides a force that acts on the axis of the spin. For example, if you have one of those spinning tops where you pull a piece of string to make it spin, the spinning motion keeps it upright as gyroscopic forces pull on the axis to create a sort of self-correcting balance. When the top is not spinning, it simply topples over on one side. For a long time it’s been thought that the same effect happens on the bicycle wheel.

The second effect is the caster effect. You’ll see this on shopping trolley wheels where, when you steer it around corners, the wheels appear to get dragged around behind the direction you‘re steering it in. And this is because they’re set on posts which are (if viewed from the side) in front of the position where the wheel meets the ground. The same thing is effectively true of a bike, because the place where the wheel meets the ground is actually slightly behind the steering axis.

Publishing in Science, this team built a bike that counteracted these two effects, to see if it would still stay upright when moving. Jodi Kooijman <ka-oy-mun> and colleagues from Cornell and Delft University of Technology constructed a frame with wheels that spun in the opposite direction to those on the ground – counteracting the gyroscopic torque. And they moved the steering axis so that it sat directly above the front wheel; preventing the caster effect.

But... when they let this construction roll alone, it still managed to balance upright. Further, it didn’t lose its balance as it went faster – unlike normal bikes.

The researchers think that there must be something else going on, possibly related to mass distribution. They also argue that, until we fully understand the mechanics behind them, bikes today could be falling well-short of their design potential.

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What would happen if you introduced a disc into the bike frame that spins very fast on the same axis as the wheels?  The gyroscopic torque may be increased and if there is a connection the stability of a moving cycle.

Could it also be possible to use gyroscopic torque suspension by having something spinning horizontally?

Would it make it harder to accelerate and decelerate if you had something spinning 90° to the angle of movement?

Aaron_Thomas, Tue, 19th Apr 2011

It's a bit hard to tell from the photo, but it looks to me as if the axis of the steering is not actually vertical.

Consequently, gravity acts to center the steering. When the steering turns, the mass of the bike is raised slightly, so as the steering attempts to turn, it encouners a force that tends to restore it to the straight ahead position.

I think they will find that if the steering axis is truly vertical, the bike will fall over.

(Hey - it's an interesting theory at least  ) Geezer, Tue, 19th Apr 2011

I think I'll have to retract that! I don't think there is any tendency to elevate the mass.

Instead, as the bike tends to fall in one direction, a small torque is created that turns the steering so that the bike turns towards the direction of the fall. If the steering repositions the bike's tipping fulcrum to the "other side" of a vertical line through the bike's center of mass, the bike will start to fall in the opposite direction.

This process will continue indefinitely if the "gain" of the system is adjusted correctly, and it will appear as though the bike is travelling in a straight line, although it is actually executing a continuous series of very slight S-curves. The gain can be adjusted by positioning the height of the center of mass of the bike. The higher it is, the greater the moment of inertia. This will slow the rate of the fall, and give the steering more time to correct.

It's probably a lot more complicated than that, but I think that's the basic principle. It's really a mechanical analogue of an electronic control system that is stabilized with negative feedback. Geezer, Tue, 19th Apr 2011

This is interesting because all the literature states that the bicycle corrects because it steers. does it? What if someone asked an engineer as this is what engineers do. apply science. Cal, Fri, 25th Jan 2013

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