The freewheel makes a characteristic tick-tick-tick sound as the pawls spring into place behind ratchet teeth. When the cyclist coasts, the pawls slip forward (counterclockwise in the photo) over the rachet. The ratchet is the circular stepwise arrangement near the center of the photo. The photo below shows the outer body from the back (left) side. Small wire springs, visible in the photo, rotate the pawls outward. The red arrow points to one of the two pawls of this freewheel. The outer body (the part that holds the sprockets) has been removed. The photo below shows the inner body (the part that threads onto the hub) of a freewheel. Most freewheels achieve this by means of pawls which engage a ratchet. The outer body of a freewheel can turn freely in one direction relative to the inner body, but not in the other direction.
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