Everyone knows that a moving bike tends to stay upright, but a stopped bike falls right over, and most people who’ve thought about it have assumed that two effects were responsible — gyroscopic effects and caster effects — but a new riderless bike takes those effects out of play without falling over:
The theory of gyroscopic precession holds that when a bike leans to the right or to the left, the spinning front wheel forces the bike to turn into the lean, effectively keeping it upright. Further, the caster effect likens the wheels of bicycles to those on shopping carts.
Next time you go to the grocery store, notice how the point of contact for the cart’s wheels are just behind the steering axis, which is the same imaginary line that extends downward from the forks of the bike. That makes wheels on casters self-righting: As soon as they start to tip, they turn into the direction of the fall, straightening themselves out again.
To debunk the theory, Papadopoulous and colleagues built a bike that eliminates both effects. The steering axis of their model lies behind the front wheel, canceling out the caster effect. And the addition of wheels situated above the front and back wheels, spinning in the opposite direction of each, counters the effect of gyroscopic precession.
They gave it a push and, you guessed it, the bike stayed balanced on its own.
The researchers found it has to do with the bike’s higher distribution of mass in the rear versus lower in the front. As the front of the bike will try to fall faster than the rear, the front is forced to steer into the fall and pull the bike out of tipping over.
I’m doubtful of their explanation of stability. One thing I don’t like is “As the front of the bike will try to fall faster than the rear…” Inanimate objects don’t “try” to do anything. They have no mind of their own. They react to forces applied.
A tall object takes longer to topple than a short one. Take a fence post and a telephone pole. Let them fall over. The fence post completes its fall first, but it has not moved faster than the telephone pole. The top end of the telephone pole has much farther to travel before it hits the ground. At the same acceleration the pole takes longer, but it doesn’t move slower than the fence post.
Connect the post and pole together with a hard mechanical connection so they are a single unit and they fall together and the tops hit the ground at the same time. A bike is one object rolling at one rate on two points, not two separate objects rolling at different rates.
That the fence post will complete its fall faster than the telephone pole does not mean it’s more stable. Unless your definition of stable is how long it takes to topple. I don’t see that as a useful definition.
The idea that one end of the bike is falling faster than the other sounds like nonsense.
The phrasing from the original paper explicitly notes that a shorter inverted pendulum falls faster than a longer one — and this causes the bike to steer into the fall, which is how it remains stable:
Well, if this bike has a frame hinged in a way a regular bike does not, it’s a flawed test if they’re intending to study bikes. Bike frames are a single unit with one roll rate.
Still, for the bike to be more stable the front end assembly has to turn into the fall. What force rotates the front end assembly into the fall? How does any roll of the frame do that? How does a difference in the roll rates create a rotational torque on the front end assembly? It doesn’t make sense to me.
Enjoy the site, BTW.
Their language can be a bit confusing. What we would call the frame, they call the rear frame. The hinge they mention is the connection between the rear frame and the front frame, which comprises the fork and handlebars.
(Glad you enjoy the site. Thanks.)