When the moving parts of the bicycle are clean, well lubricated and adjusted properly, your bicycle will feel smooth and solid. The chain is the easiest of these moving parts to service. The others, however, do require attention. Ball bearing systems are what make the bicycle go round. They are the guts of the headset, bottom bracket, hubs and even pedals. If you learn about them in theory, you can apply the same knowledge to all four systems.
Bearings, small metal balls, come in many different sizes. They can be loose or in a cage, and some are cylindrical in shape or come in sealed units.
So how do these systems work? Like a peanut butter and jelly sandwich, of course. Each system has two sets of ball bearings inside. These bearings will be enclosed by two races (bead) and covered in lubricating grease (jelly). The space in between the two races should be exactly the size of one ball bearing. If that is the case then there will be no friction so the bearings (peanut butter) can roll around with ease.
If the races are too close then they will crush the bearings into the races and it will not spin freely and will feel crunchy. If the races are too far apart, then the bearings will jingle around and bash into the cups causing pits. Both too tight and too loose will result in the same style of destruction of the bearing running surfaces, the races.
The major responsibility of the cyclist is to know when the bearings need maintenance. Bearings require clean lubrication. Lack of lubrication, bearing play or dirt or water can damage these vital parts.
Diagnosing Bearing Wear
A properly adjusted hub will feel solid when you rock your wheel from side to side. Remove it from the bike and spin the axle between your fingers. It should be smooth. If you feel grit or binding, the hub needs to be adjusted. Only service the hubs yourself if you feel comfortable doing so. While hubs may not be very expensive, damaging a hub could mean replacing the wheel.
Rotate slowly to check for binding. Pedals should turn freely. Try to wiggle to check for play. While not all pedal bearings can be serviced, it is important to understand when the lack of lubrication begins to affect the operation of the pedals.