The balance of a bicycle is maintained through a combination of factors such as the rider's body position, the design of the frame and wheels, and the gyroscopic effect of the spinning wheels. When the rider steers the handlebars, the bike's front wheel will turn in the direction of the lean, helping to stabilize the bike. This is known as the "trail" effect. Additionally, the spinning wheels create a gyroscopic force that helps to keep the bike upright.
The main forces that affect the motion of a bicycle are friction, gravity, and air resistance. Friction between the tires and the ground allows the bike to move forward, while gravity pulls the bike and rider downward. Air resistance, or drag, is the force that opposes the forward motion of the bike and can be affected by factors such as wind speed and aerodynamics.
Most bicycles use rim brakes, which work by pressing rubber pads against the rim of the wheel. When the rider squeezes the brake lever, a cable is pulled, causing the brake pads to press against the rim and slow the bike down. Some bikes also use disc brakes, which work by pressing brake pads against a metal disc attached to the wheel hub.
The gear system on a bicycle allows the rider to change the ratio between the pedals and the wheel, making it easier or harder to pedal. This is achieved through a system of gears and chains. When the rider shifts gears, the chain moves to a different sized gear, changing the gear ratio and making it either easier or harder to pedal depending on the terrain and the rider's preference.
The weight of a rider can affect the performance of a bicycle in various ways. A heavier rider will require more force to move the bike forward and may experience more drag from air resistance. The weight distribution of the rider can also affect the balance and handling of the bike. However, the weight of the rider can also provide more stability and momentum, especially when going downhill. Ultimately, the impact of a rider's weight on a bicycle's performance will depend on various other factors such as the design of the bike and the terrain being ridden on.