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Bicycle bearings support the wheel hubs on the axles. Bearings require a certain amount of pressure, otherwise referred to as preload. The specific amount of preload determines the number of bearings that support the wheel hubs at any given time. Cone flats thread onto the axles and press bearing cones against the bearings. Adjusting bearing cone preload prolongs the life of the bearings. With proper preload, light drag should be noticed when spinning the wheels.

Wheels with quick-release levers present a challenge. Closing the lever alters the preload. Adjusting bearings on wheels that have quick-release levers may take several attempts.

The wheel bearings sit between a cone screwed to the wheel axle and the cup in the wheel hub. They must be properly adjusted and greased. To find out if they need greasing without taking everything apart, simply remove the wheel from the bike, support the axle horizontally with your fingers, and turn the wheel slowly--try not to tilt the axle as it rotates. If the wheel does not turn smoothly or you can feel tiny bumps in your fingers, then you must dismantle the bearing assembly and service the skf bearings.

Bicycle bearings should last for years and provide you with thousands of miles of travel---but they won't last forever. In some cases they even fail completely if they haven't been maintained correctly. Part of a maintenance schedule should include replacement of wheel bearings before they fail. The procedure isn't that difficult if you have about an hour and a few small tools. Front and back wheels are similar in procedure to change iko bearings and only require slightly different tooling.

Remove the wheel from the bike. If you are doing the back wheel, remove the gear cassette freewheel first by turning it counterclockwise. If it's too tight, apply the freewheel puller. Hook it on, loosen the freewheel and lift it off.

Lay the wheel on its right side, and work on the left side of the wheel. Take a screwdriver and pry off the rubber seal. Slide a 15 mm cone wrench onto the cone flats on the bottom for the back wheel, a 13 mm wrench for the front. Now place the 17mm open-end wrench on the nut on top and loosen it by turning it counterclockwise against the cone wrench.

Stand up the wheel. Using both hands from both sides, unscrew the locknut, cone and spacers. Now slide out the axle from the right side. Using a small magnet, remove all the bearingsfrom both sides and count them. Clean off all the old grease.Liberally apply new grease to the inside of the hub where the bearings ride on both sides and to the cone that you just took out. Now carefully push the new ball bearingsdown into the grease one at a time---the front hub using 3/16-inch [bicycle bearings] , and the rear hub using 1/4-inch bearings. Now cover all the bearings with grease on both sides, letting the grease hold them in place. Grease the axle and slide it back in the hub.

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