The front hub service is a good place to start – it’s less tricky than a rear service because you have no gear clutter on the hub. If you haven’t serviced a hub before, start on the front for practice.
If you have disc brakes, take care not to get grease on the rotors during this procedure. Some people stick a plastic bag over the rotor, then cut a hole in the bag through which to work on the hub. However, this does seem excessive – simply keep sticky fingers and bearings off the rotor. You need to check what size spanners you need to adjust the cones. The cone itself needs a special flat cone spanner because its spanner flats are very narrow. The size you need is normally 13mm or 15mm. The locknut is usually turned with an ordinary spanner, often a 17mm, but sometimes you also need a cone spanner. Enough variations exist to merit taking your bike to the shop to buy the size you need. While you’ve got it there, ask them for the correct-size ball bearings for your hub. They’re nearly always 5mm.
Step 1: Take the front wheel off and remove the skewer completely. Inspect each side of the hub. Seals may cover the cones, as in the picture. Peel them off by hand if you can to avoid damaging them, otherwise use a thin, flat screwdriver to peel back the edges. In order, you can see the threaded end of the axle, the locknut with wrench flats, spacers and the cone with wrench flats. Most of the cone is invisible, as it’s inside the hub.
Step 2: Slide your narrow cone wrench onto the flats of one cone. Use the cone wrench to hold the axle still, then undo the locknut with the other wrench. Take it off completely. Remove any spacers. Lay them out in order on a clean rag so you can remember which order to put them back in. It’s not difficult to work out the order for the front hub if they get jumbled, but it’s good practice for when you come to the back one, which is harder.
Step 3: Switch the locknut spanner onto the other side of the wheel and use it to hold the locknut, and thus the axle, on that side still. Keeping your cone spanner on its original cone, undo that cone completely and take it off the axle. You should now be able to draw out the axle from inside the hub. Some bearings may drop out as you pull out the axle; be ready to catch them.
Step 4: Use a small flat-bladed screwdriver to extract all the bearings. Count them to be sure you replace the same number. Clean the axle and everything still attached to it. Wrap rag around the end of a screwdriver and use it to get grease off the bearing surface. For stubborn grime, you may need bikewash or degreaser. Rinse the bearing surface out and dry with a clean rag. Once all the old parts are clean, wash your hands to start fitting everything back.
Step 5: Inspect both the bearing surfaces on each side of the hub. A track will be worn in both cup and cone. Look for pits in the bearing surfaces. Any pitting prevents you adjusting the bearings nicely. Cones can sometimes be replaced individually, although you may have to buy a whole new axle. Pitted bearing surfaces in your hubs are more serious – you have to replace the hub, which means either buying a new wheel or rebuilding your rim around a new hub.
Step 6: Check the locknut is tightened against the cone. If the cone is pitted, note how much axle sticks out beyond the locknut, then hold the cone still with the cone spanner, and release the locknut. Wind all parts off axle. Fit the new cone and replace any spacers and the locknut. Wind the locknut onto the axle so the same amount of axle thread pokes out the end, then wind the cone back up to the locknut. Hold the locknut steady and tighten the cone against it firmly.
Step 7: Grease the cups on both sides of the hub. There should be enough grease for each bearing to sit in grease up to its middle. With clean hands, pop as many bearings back into each side as you took out. If you push a bearing into place and another pops out, that’s one too many bearings. The grease should hold them in place.
Step 8: Slide the axle assembly back through the hub. If you have a rotor, slide through from the rotor side, otherwise it doesn’t matter. Wind the new or cleaned loose cone back onto the axle all the way, so that it touches the bearings and traps everything into place. Now replace any spacers and wind the locknut onto the axle so that it butts up against the spacer or cone.
Step 9: All the adjustment must be done from the side of the axle to which you refitted the cone; we’ll call this the ’adjustable’ side. You tightened the locknut and cone onto the axle on the other side, the stationary side, before you refitted it, so you can use them to hold the axle still to adjust the cone on the other side. You don’t adjust the cone on this side at all; it stays where it is.
Refitting the axle
It’s always worth making sure the cones and bearing surfaces are really clean and dry before you refit the bearings. Any dirt left inside the hub will shorten the hub lifespan and make the cones more difficult to adjust. A light oil, like GT85 or WD40, will help to dissolve the old grease and dirt that becomes compacted onto the bearing surface – squirt a little into each end of the hub, then roll the wheel around so that the oil has a chance to soak into the whole bearing surface. A toothbrush is just the right size and shape to shift stubborn dirt stains. Clean the central part of the hub between the two bearing surfaces – it’s easiest if you use a screwdriver to poke a thin strip of cloth right through the hub, then twist and pull to clean. Dry the bearing surfaces, so that the grease sticks to it properly.
Once everything is clean, check again for pits in both bearing surfaces and both cones. Smooth worn tracks are fine – these just show where the bearings have been running. But any pitting at all – dents or rough patches on the bearing surface – means that the part must be replaced. Cones are slightly softer than the hub-bearing surface. This is so that they wear first, since they’re easier and cheaper to replace. Take the old ones with you to the bike shop for replacement, since there are a number of variations in depth and diameter. Replace both cones at the same time – if one side is worn out, the other hasn’t got much life left in it either. You may find that your hub cones are not available separately, so you may have to buy a whole new axle. If your hub surfaces are worn, you have no option but to replace the hub. If your rim is in good condition, you may wish to consider taking the wheel apart and rebuilding the rim onto a new hub.
“Any dirt left inside the hub will shorten the hub lifespan and make the cones more difficult to adjust”
Now the tricky bit. Adjust the space between cones and cups so the wheel can turn smoothly, but keep it tight enough to eliminate side-to-side play. Hold the cone still, and tighten the locknut against the cone so the cone can’t rattle out of place. The problem is that tightening down the locknut usually shifts the adjustment. Test by rocking the axle across the hub, and then rotating. If it rocks from side to side, the cones are too loose. If it won’t turn freely, the cones are too tight. Tighten up the adjusting cone gently against the bearings, so it can still turn freely, but without side-to-side wobble. Hold the axle by sliding your cone spanner onto the stationary cone and holding it still, then use your locknut spanner to wind the adjustable locknut gently up to the adjusting cone. When they touch, transfer the cone spanner from the stationary side of the hub to the adjusting side and wedge the adjustable locknut firmly against the adjustable cone. Test the bearing adjustment – twirl the axle between your fingers. It should turn smoothly with little resistance. Turn the wheel to face you and wiggle the axle from side to side – there should be no play. Occasionally you adjust it perfectly first time but this is rare – normally you have to go back and try again. If the axle doesn’t turn smoothly, move the adjustable cone out a little. Using your two spanners, hold the adjustable cone still and undo the locknut one complete turn. Transfer the locknut over to the stationary side and use it on that locknut to hold the axle still. Undo the adjustable cone a little. Swap sides with both spanners, hold the axle still with the cone spanner on the stationary cone, and tighten the adjustable locknut so it touches the adjustable cone. Swap both spanners back onto the adjustable side and tighten them against each other so they lock together. Test again and repeat until satisfied. If there is play between the axle and hub, you need to undo the locknut; tighten the cone instead of loosening it and lock the locknut back down onto the cone. Replace any seals you took off and refit the wheel.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Checking and adjusting cones”