The hub is a piece that has many machines where it rests and rotates the shaft. There are bushings that hold simple metal cylinders and others that are a set of different components that together form a point of union.
In the cycling, the bushings can be classified by the type of turn they use, whether it is by balls or by sealed bearings. Each one of them has advantages and disadvantages, but for the world of this sport sealed bearings are being used much more, since they do not get as dirty as those of the ball system although they are somewhat heavier. There are hubs that can only be used for the front wheels and others for the rear, the main difference they have is that the hubs of the front wheel are not attached to the cassettes of the bicycle and if there are the hubs of the rear wheel .
Another very important thing for cyclists, is that the bushings loosen very easily in case you have to do a repair such as changing a tire or changing the brake pads. That's why they have to have a small crank that can be tightened easily but with great force to avoid any accident.
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