Which mechanism makes the bicycle to move forward when we cycle forward but when we cycle backwards cycle doesn't move back?

Which mechanism makes the bicycle to move forward when we cycle forward but when we cycle backwards cycle doesn't move back?

Ans: Freewheel Mechanism



The reason for this, is the presence of a simple but beautiful mechanism called the Freewheel, which is attached to the rear wheel hub. To understand the working, see the figure below.



 A sprocket is fitted over this freewheel, which is pulled/rotated by the chain.

Now when you have to move forward (counter-clockwise in above figure), the red color link - called pawl, acts like a hook and gets locked with the teeth - called ratchet and transmits the torque. The complete mechanism is called ratchet and pawl mechanism.

But when you reverse pedal, it falls back and becomes "free". The yellow color link is actually a spring that prevents it from falling permanently. This is the reason why you hear the distinct "click-click" sound when you reverse pedal. Also, there are multiple "pawls" placed along the circumference too.

The figure above is very simple diagram for the freewheel. The actual part diagram is shown below with the names of each.

This is what it looks from inside in real.





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