Hub
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[edit] Hubs
Hubs are situated at the center of a fully built wheel. They are what allows the wheel to rotate while being attached to the frame or forks. They are made of an axle, bearings and a hub shell. The axle is like any other axle and allows the rotation of of the hub, the hub shell will usually have 2 flanges so that the spokes can be attached to the hub and the rim.
[edit] Axle
The axle is attached to dropouts on the fork or the frame. There are a few different methods of attachment:
- Quick Release - a lever and skewer that pass through a hollow axle designed to allow for installation and removal of the wheel without any tools (found on most modern road and mountain bikes, not recommended for trials however).
- Nut - the axle is threaded and protrudes past the edges of the fork/frame. (often found on track, fixed gear, single speed, BMX and inexpensive bikes)
- Screw/Bolt - the axle has a hole with threads cut into it and a bolt can be screwed into those threads. (found on some single speed hubs.)
- Thru Axle - a long axle, typically 20mm (110mm width), 9mm (100.33mm width) in diameter for durability, onto which the fork/frame clamps. (found on MOST free ride and downhill mountain bikes)
- Female Axle - hollow center axle, typically 14, 17, or 20mm in diameter made of chromoly and aluminum, which two bolts thread into on either side. This design can be much stronger than traditional axles. (found on higher end BMX hubs and some mountain bike hubs)
Trials bikes generally have a 116mm or 135mm rear spacing between the dropouts so when buying a new hub getting the right size is crucial. Front hubs generally are made to fit inbetween the 100mm seperated dropouts.
Click for a list of Hub manufacturers.
[edit] Bearings
The Bearing allows the hub shell (and the rest of the wheel parts) to rotate freely about the axle. Most bicycle hubs use steel or ceramic ball bearings. Older designs used "cup and cone", whereas some modern wheels utilize pre-assembled "cartridge" bearings.
A "cup and cone" hub contains loose balls that contact an adjustable 'cone' that is screwed onto the axle and a 'race' that is pressed permanently into the hub shell. Both surfaces are smooth to allow the bearings to roll with little friction. This type of hub can be easily disassembled for lubrication, but it must be adjusted correctly; incorrect adjustment can lead to premature wear or failure.
In a "cartridge bearing" hub, the bearings are contained in a cartridge that is shaped like a hollow cylinder where the inner surface rotates with respect to the outer surface by the use of ball bearings. The manufacturing tolerances, as well as seal quality, can be significantly superior to loose ball bearings. The cartridge is pressed into the hub shell and the axle rests against the inner race of the cartridge. The cartridge bearing itself is generally not serviceable or adjustable; instead the entire cartridge bearing is replaced in case of wear or failure. Hope hubs are one brand of hub that has this type of bearing.
[edit] Hub shell
The hub shell is the part of the hub to which the spokes (or disc structure) attach. The hub shell of spoked wheels generally have two flanges extending radially outward from the axle. Each flange has holes or slots to which spokes are affixed. Some wheels (like the Full Speed Ahead RD-800) have an additional flange in the center of the hub. Others do not have a noticeable flange. The spokes still attach to the edge of the hub but not through visible holes. Other wheels have a threaded hub shell that the spokes thread into.
[edit] Hub brakes
Some hubs have attachments for disc brakes or form an integral part of drum brakes.
- Disc brakes - a disc brake comprises circular plate or disc attached to the hub which is squeezed between brake pads mounted within a caliper that is fixed to one side of the wheel forks. The brake disc can be attached in a variety of ways using bolts or a central locking ring.
- Drum brakes - a drum brake has two brake shoes that expand out into the inside of the hub shell. Rear mounted drum brakes are often used on tandems to supplement the rear rim brake and give additional stopping power.
- Coaster brake - coaster brakes are a particular type of drum brake which is actuated by a backward pressure applied to the pedals. The mechanism is contained inside the bicycle wheel hub shell.
Neither of the last two are used for trials as neither are really suitable. For information on other types of bicycle brakes see the full article on brakes.
[edit] Gears
The rear hubs have one or more methods for attaching a gear to it.
- Freehub - The mechanism that allows the rider to coast is built into the hub. Splines on the freehub body allow a single sprocket or, more commonly, a Cassette containing several sprockets to be slid on. A lock ring then holds the cog(s) in place. This is the case for most modern bicycles.
- Freewheel - The mechanism that allows the rider to coast is not part of the hub, it is contained in a separate freewheel body. The hub has threads that allow the freewheel body to be screwed on, and the freewheel body has threads and/or splines for fitting sprockets, or in the case of most single speed freewheels an integral sprocket. This style of hub was used before the freehub became practical.
- Track sprocket - There is no mechanism that allows the rider to coast. There are two sets of threads on the hub shell. The threads are in opposite directions. The inner (clockwise) set of threads is for a track sprocket and the outer (counter-clockwise) set is for a reverse threaded lock ring. The reverse threads on the lock ring keep the sprocket from unscrewing from the hub, which is otherwise possible when slowing down.
- Flip-flop hub - Both sides of the hub are threaded, allowing the wheel to be removed and reversed in order to change which gear is used. Depending on the style of threads, may be used with either a single speed freewheel or a track sprocket.
- Internal geared hub - the mechanism to provide multiple speeds is contained inside the shell of the hub. Many three-speed internal geared hub bicycles were built in the last century. This is an extremely robust design, although much heavier than more modern designs of multi-speed arrangements. Modern hubs are available from three-speed to 14 speeds
