A bicycle in zero gravity is unrideable (The bricycle).

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Author: Noopi Grewal

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Owen Dong, Christopher Graham, Anoop Grewal, Caitlin Parrucci and Andy Ruina Mechanical Engineering Cornell University, http://ruina.tam.cornell.edu A bricycle can be adjusted to be a bicycle, a tricycle, or anything in between. The spring mechanism in the rear provides a restoring force proportional to lean. Using a so called 'zero rest-length spring', the effective spring constant can be adjusted by moving the attachment point of one end of the spring. Clamping the spring, infinite stiffness, makes a tricycle. Detaching the spring, zero stiffnes, makes a bicycle. A person can balance and steer a normal bicycle, navigating the course and staying upright, counter-steering to start a turn and leaning into turns. When we detach the spring of the bricycle it is in bike mode and can be balanced and steered like a bicycle, counter-steering, and leaning into turns. If we clamp the spring, effectively making the bricycle into a tricycle, it is held upright and can also be steered, with the centripetal acceleration balanced by the outer 'training' wheel. At some intermediate value of stiffness, the spring restoring torque cancels the gravity capsizing torque and the bricycle is, for balance purposes, effectively in zero gravity; when not going forwards it is in neutral equilibrium for leaning. But this vehicle, a bricycle, a bicycle in zero gravity, cannot be both balanced and steered. Attempts at steering the bicycle, tip it. It can't be righted and also steered in a desired direction. And steady turns are impossible as there is no torque available to balance the centripetal acceleration. As best we can tell, nobody can steer the bricycle around the simple obstacle course. The first attempt a steering causes a lean which can't be corrected, at least not while trying to steer in a desired direction. A controlled bicycle is really just an inverted pendulum, it is balanced with sideways accelerations of the support (on a forwards-moving bicycle this happens because of steering) which, if coordinated can also control the position of the base while balance is maintained. However, if there is no gravity, as for a horizontal pendulum, the sideways displacement and the pendulum angle cannot be controlled independently, they change in proportion to each other. In fact, there is a point on the pendulum stick which can't be moved at all. The bricycle is really the same as the gravity-free pendulum. Assuming friction and so on are negligible, if we start from an upright position, the lean and the sideways displacement of the ground contact point are always in proportion to each other. So changing direction would cause both an ever-growing distance for the original line of travel, and an ever-growing lean angle. The riders don't tolerate this. Instead, they maintain balance and thus are stuck going about straight. So gravity, superficially the thing that makes it hard to balance a bicycle, is the thing that allows you to steer it.

Comments

What confuses me is that the lean of a bicycle in a turn (when countersteering) is not based on the rider or any external forces, but on the mechanical constraints of the bike. The rake and trail create a situation where the turning of the front wheel describes a circle tangentially connected the ground that must lower the bike to stay in contact as the arc naturally takes the contact point away from the surface by its arc. So, a stationary bike with or without rider, when the handlebars are turned, will lean it self. This is because the act of turning moves the front contact patch out of alignment with the rear contact patch and lowers the front of the bike. The simultaneous lowering of the front and moving (left or right) of the front is combined as 'lean.' This effect is independent of gravity as it can still be seen in your bike when the springs are set to null gravity. It seems then, that countersteering in a null gravity (or always balanced) environment would still work since balance is sustained by forward motion after about 5mi/hr. Is this not correct in some way? does your experiment hold at speeds where countersteering would normally be the sole method of turning?

Thanks for the great evidence for the explanation I give in Bicycle Stability 101. Of course, if you transfer the weight to the outriggers, you will eliminate the lateral forces at the bicycle tires and control as well. The TMS gadget proved that a bicycle stays upright because of the contention of camber and slip angle forces and now this. Thank you.

This led me to wonder: would you be able to go round corners on a cycle in artificial gravity? I mean like a torus type habitat spinning for artificial gravity? Does it depend on the radius, and spin rate, or can you cycle at any combination of those values?

Hello, according to this principle would you be able to build a sytem to remove the gyroscopic couple on a paramotor?

Eh first, the title is wildly wrong since this has nothing to do with zero gravity.
And second, the last configuration is easily steerable. Slow down a bit and lean your body into the turn. Come on people.
ET is waiting for us to wake up and realize we are not alone, we can't be tripped by simple things like this.

Not only is it an 'uncontrollable' bicycle, it's also a wheeled vehicle that can be forced to travel in a straight line. That could be a useful tool for some

fantastic explanation ! Thanks.

Wonderful experiment!

I must try this!

Estaria bueno utilizar una

Interesting zero-G simulation - but not "real life" since any bike or trike being ridden in a zero-G environment would soon find itself floating above the surface after the first surface irregularity.... One could keep it on the surface by having the surface curve up, even keeping on rising until it had curved over to meet itself again, when centripetal force would ensure contact (as long as it keeps moving) - but it would then become steerable again, since (as Einstein pointed out) gravitational attraction and acceleration are equivalent.