Making a unicycle with Box2D

Question

I'm fairly new to Box2D and trying to figure out the best way to make a unicycle. The unicycle essentially is in two pieces, the wheel and the stem (with seat post etc). I've tried attaching the two with a revolute joint and using a motor for the wheel, which works well except that the stem is then subject to forces from the movement of the wheel. I want to be able to directly control the rotation of the stem (via the accelerometer on the iPhone), and have it unaffected by the movement of the wheel, except to maintain its position based on the position of the wheel.

What is the best way to do this? How do you control rotation of b2Body's? Should I be using a distance joint instead? Any help would be appreciated.

Answer 1

I see several routes, depending on your needs. Which way is preferable is up to you and your game.

1. Fix the stem's rotation

For the bodyDef for the stem set the fixedRotation-flag to true. This prevents any rotation of the stem (be it by forces from the motor joint, (de)acceleration or collisions.

Than you'd have to manually set the rotation each tick. This is easy if it's purely based on the iPhone's position. If you still want to calculate other factors into it, things might get from a bit more complicated (eg adding rotation if stem leans too far in one direction) to somewhat painful (have collisions affect the rotation).

2. Constantly apply balancing forces to the stem Each tick read the stems angular velocity and apply countering forces to balance the stem.

While this would probably be more complicated to implement correctly (always find the right force to apply etc.) it may result in a more realistic behaviour as the fixed rotation obviously removes most reactions stem movement would have and how the stem itself is affected by the world.

3. Don't actually use a wheel While your layout is the obvious choice for a unicycle (and seems to be a somewhat popular choice for all kind of characters) it might not be the best choice from a gameplay point of view.

Instead you could combine the stem and wheel fixtures in a single body (or attach them with a prismatic joint) and create all movement by applying forces to this body. A sensor at the bottom can inform you of ground contact to determine if movement forces are to be applied.

This way you'd get rid of all the forces a wheel creates (forces to the stem may not be the only unwelcome ones in gameplay) and still have it react to all external forces.