Join Date: May 2004
Location: San Diego, CA
Bikes: old peugot frankenbike
I wrote this for physics a while ago, and it's bike related, so I might as well post it here.
Observation Report 1
My observation report is on what I observed about the forces and properties of mass involved in a bicyle rider popping a wheelie.
The forces acting on the system of the bicycle and its rider when the system is at rest are gravity, and contact force (of the ground pushing up against both wheels of the bike). When the bike is in motion, the same forces are present, as well as the force of the back wheel pushing against the ground, the ground pushing in the opposite direction against that wheel, and the friction/rolling resistance of the tires contacting the ground (these are all contact forces)
The ability to pop a wheelie while riding a bike relates to the force of the back wheel pushing against the ground, and the position of the center of gravity which includes both the rider and the bike.
While the rider is sitting on the bike, with both feet on the pedals, the center of gravity is slightly behind the physical center point between the areas where the wheels touch the ground.
It (the center of gravity)'s position moves forward or backward if the rider either leans forward or backward, and raises when the rider stands up on the pedals. In order to get the front wheel off the ground, the rider will have to at least lean backward, and probably stand up, which makes the center of gravity shift to the back wheel. Then, when the pedals are turned, they push against the chain, which in turn pushes against the chain wheel connected to the back wheel, and causes the back wheel to push against the ground. Since pushing against the ground is finally the resistance needed from an object external to the system of the bike (for the system to move), the wheel turns, and the wheel will move under the rest of the bike, which will initially remain almost stationary and tip backwards, rather than move forward with the wheel, because the mass of the rider and the rest of the bike combined will want to remain still, as Newton's first law states.
If the rider continues to pedal, the back wheel will continue moving under the frame until it tips backward, unless the rider leans forward, to get the center of gravity slightly in front of the wheel.
If the wheel continues to turn, and the ground is solid enough, the ground will continue to push against the wheel, and the bike will stay in motion until either the rider stops pedaling/leans forward, in which case the front wheel will return to the ground, or pedals too hard/leans backward, when the rider would fall off the back of the bike.