Wheel bearings last longest when they are properly adjusted. The purpose of your hub bearings is to allow your wheel to spin freely as you pedal, while preventing the wheel moving from side to side in the frame.
The first part is obvious – hubs that bind instead of spinning will obviously slow you down and sap your energy. But play in your bearings will slow you down as well – if your wheel can move from side to side in the frame, your braking surface (the brake rotor for disc brakes, or the rim for V-brakes) gets constantly dragged against the brake pads or blocks. Loose bearings will make themselves felt when you ride as well, with wheels rocking within the frame rather than tracking your movement neatly around tight twists and turns. Your bike will feel uncertain, with small unnerving pauses, before it follows your directions.
Checking for loose bearings is the same procedure for front and back wheels. Hold onto your rim, near where it passes between the forks or the frame. Pull the rim gently toward the frame. If the bearings are loose, the rim will rock toward you – you will feel, and maybe even hear, it shifting on its bearings. It’s OK if the rim flexes a little, but it shouldn’t knock at all. Loose bearings need to be adjusted straight away – as well as affecting your ride, they will wear quickly. If the bearings are allowed to bang onto the bearing surface, instead of rolling smoothly across it, they will create pits there. Check for tight bearings at the same time. Pick up each wheel in turn and spin it. The wheel should continue to rotate freely with just the gentlest encouragement. If it slows down prematurely, check the brakes first. If the brakes aren’t the problem, your bearings are too tight. Use the steps below to adjust them. Front-wheel bearings are easier to adjust than rear-wheel bearings because both sides are accessible. With rear wheels, the right-hand cone and locknut are buried under the cassette.
Adjusting your bearings
Remove the wheel from the frame and remove your skewer – it just gets in the way. Spin the end of the axle between your fingers, then rock it from side to side across the wheel. If you found your bearings were too tight when you checked the wheel in your frame, the axle will feel gritty now – it may not move at all. If it felt loose in the frame, you will feel a slight rocking as you move the axle from side to side across the wheel. Since the axle runs through the centre of the wheel, you only need to work on one side of the axle to adjust both sides of the bearing. The right-hand cones on the rear wheel are concealed by the cassette, forcing you to adjust from the left side. The front can be adjusted from either side, so just follow the instructions for adjusting the back axle. Check that the right-hand locknut is locked securely onto the axle before you start. If this side shifts about as you work on the other side, you’ll not be able to set the critical distance between the two sides accurately. Hold the short stub of threaded axle that protrudes out from the middle of the locknut and try to turn the locknut. If it moves easily with your fingers, you really need to service rather than adjust the hub. Dirt and water will have been drawn into the hub as the loose cones shifted on the axle. The pictures below show the hub on its own, without spokes, so that you can see both sides at once. When you’re doing this for real, the hub will be attached to the rest of your wheel.
Step 1: Turn the wheel so that the left-hand side of the hub faces you. Remove any black rubber seals, so that you can see the locknut nearest the end of the axle and the cone behind the locknut. There may be a washer, or washers, between the cone and locknut. Slide a thin cone spanner onto the cone and hold the cone still. Use a spanner to undo the locknut one turn anticlockwise.
Step 2: The locknut and cone on the right-hand side of the hub are locked together, clamped firmly onto the axle, so you can hold the axle still by transferring your locknut spanner onto the right locknut. Leave the cone spanner on the left-hand cone and turn to adjust the bearings – clockwise to tighten, anticlockwise to loosen.
Step 3: Holding the cone still, transfer the wrench back to the left-hand side of the hub and tighten firmly onto the cone in its new position. Check the bearing adjustment again – it can take several goes to get the cone position right. Refit seals, skewer and back wheel. Check the bearing adjustment once you’ve got the wheel back in the frame – it’s annoying, but you can often feel only slightly loose bearings at this point.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Fitting and spacing hubs and sorting out hub seals”