Why bicycle does not go backward after reversing the pedal?

Bicycles do not go backward after reversing the pedal because of the freewheel mechanism. The freewheel is a crucial component of the bicycle's drivetrain that allows the rear wheel to rotate freely when the rider stops pedaling or pedals in the reverse direction.

Here's how the freewheel mechanism works:

1. Pawls and Ratchet: The freewheel consists of several pawls (usually three) that engage with a ratchet mechanism in the hub of the rear wheel. When the rider pedals forward, the pawls catch and push against the ratchet teeth, causing the wheel to rotate.

2. Non-Ratcheting: When the rider stops pedaling or pedals backward, the pawls disengage from the ratchet, allowing the rear wheel to spin freely without engaging the drivetrain. This means that you can coast forward without moving the pedals or pedal backward without the bicycle moving backward.

3. Return to Forward Motion: As the rider begins to pedal forward again, the pawls re-engage with the ratchet, allowing the wheel to be driven by the pedaling motion once more.

The freewheel mechanism ensures that when the rider stops pedaling or pedals in the reverse direction, the rear wheel can rotate independently, preventing the bicycle from going backward. This is an essential safety feature, especially when coasting downhill or in stop-and-go traffic.

However, it's worth noting that bicycles can still skid backward if the brakes are applied while the pedals are being reversed, so riders should exercise caution when using this feature.