Bearings have been around since Roman times and appear in working drawings by Leonardo Da Vinci. His design for an early tank featured a device for enabling the gun turret to turn in different directions. He rested the upper part of the structure on a circle of wooden balls that allowed it to turn freely and support the weight (wood isn’t the best bearing material, but is still used in cycle track racing rims). The modern ball bearing pioneer was Sven Wingquist, a visionary Swedish inventor who founded the SKF bearing company in 1907. The company still produces quality stainless steel bearings.
There are bearings in the centre of your wheels. They take different forms, ranging from handfuls of cheap steel balls to fancy sealed units, but they all do the same job – keep the wheel securely fixed onto your bike with no side-to-side movement, while allowing it to spin as freely as possible. Well-adjusted bearings run for years without complaint. Bearings that are too loose or too tight slow you down either way and wear out in no time, so it pays to check them regularly.
Pick up each wheel and spin it gently. It should continue rolling a couple of times on its own, even after a really gentle spin. If it slows down quickly, first check that the brake blocks or pads are not rubbing on the rim or rotor, which can have the same effect as overtight bearings. If that’s the problem, go to the brakes chapter and sort them out first, then come back to bearings. If the brakes aren’t the problem, then your bearing is too tight and it’s slowing you down. Put the wheel down again and crouch beside the bike. Hold the rim of the back wheel where it passes between the stays (seatstays or chainstays on a hardtail, otherwise whatever lies between the main frame and the back wheel). Just pinch the rim between your thumb and finger. Hold onto the nearest bit of frame with your other hand and rock your hands toward and away from each other, pulling the rim toward the frame then pushing it away.
The rim may flex slightly, but that’s not what you’re looking for. You need to check if there’s a knocking feeling, or even a clicking noise, as you pull the rim back and forth. This indicates movement between the bearings and the surfaces supporting them, and means the bearings need adjustment.
Repeat with the front wheel, holding the rim where it passes through the fork and rocking gently across the bike. Again, the rim may flex slightly, but it shouldn’t knock at all. The wheel should spin freely, gradually slowing down over a couple of revolutions.
Wheel bearings can be divided into two types:
- Cup-and-cone: Traditionally, bicycle bearings were of the cup-andcone type – a cone-shaped nut on the axle traps a ring of bearings into a cup-shaped dip in the hub. The cone can be adjusted along the axle, making enough space for the bearings to spin but not enough for the wheel to move sideways. The cones are locked into place by wedging a locknut against each one, then tightening the cone against the locknut. The advantage here is that the parts can be serviced and adjusted with a minimum of tools.
- Sealed bearing hub: The modern type is called a sealed bearing hub, although the name is a bit misleading because the cupand-cone type usually has seals too – anyway, you’re liable to open up either kind and find your bearings in a mess. Instead of the cup shape, this type has a flat-bottomed round hole on each side of the hub. The bearings and the races they run on come as a unit, which is then pressed into the hole. They are trickier to fit because the bearings on each side of the wheel have to be prefectly parallel to run smoothly. The advantage here is that both the bearings and the bearing surface can be replaced when they wear. With the cup-and-cone type, the bearings and cones can be replaced, but the cup is integral to the hub and cannot be replaced cheaply.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “How to remove and refit wheels”