Comments on: Riderless-Bike Research

By: Isegoria

Isegoria — Wed, 20 Apr 2011 18:11:31 +0000

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.)

By: T M Colon

T M Colon — Tue, 19 Apr 2011 20:32:36 +0000

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.

By: Isegoria

Isegoria — Mon, 18 Apr 2011 22:01:58 +0000

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:

In the theoretical and experimental TMS designs, the front assembly mass is forward of the steering axis and lower than the rear-frame mass. When the TMS bicycle falls, the lower steering-mass would, on its own, fall faster than the higher frame-mass for the same reason that a short pencil balanced on end (an inverted pendulum) falls faster than a tall broomstick (a slower inverted pendulum). Because the frames are hinged together, the tendency for the front steering-assembly mass to fall faster causes steering in the fall direction.

By: T M Colon

T M Colon — Mon, 18 Apr 2011 20:21:54 +0000

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.