How Bicycles Work
Bicycles use ball bearings to reduce friction. You can find ball bearings in:
The front and rear hubs for the wheels
The bottom bracket, where an axle connects the two pedal cranks together
The fork tube, where the handlebars are allowed to turn
The freewheel, where they do double-duty (In the freewheel, they also help provide the one-directional feature.)
The bearings in the fork tube are typical and are shown in the following figure:
The ball bearings (yellow) ride in a cup (red). The cones (dark blue) screw onto the light blue tube that is attached to the fork. The cones are adjusted to be tight enough that there is no play in the fork, but not so tight that they squeeze the ball bearings and cause them to bind. The wheel hubs and pedals work exactly the same way, with the cones providing the adjustment. In the crank axle, one of the cups provides the adjustment instead of the cones. A little bit of grease in the bearings makes them even smoother.
Periodically, you have to disassemble the bearings on a bicycle to clean out the dirt and put in fresh grease. Some more expensive bicycles have sealed bearing cartridges that never need adjustment or lubrication.
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