Glossary of Bicycle Wheel Terms

Wheel: The sum of all the components, including hub, spokes, spoke nipples, rim, and sometimes the tire and tube. “Wheelset” is a paired front and rear wheel.

Hub: The central part of a wheel that houses the bearings and axle and anchors the spokes.

Freehub: A rear hub that incorporates a freewheeling clutch.
Single-speed hub: Refers to a special rear hub with wider-spaced spoke flanges and a thin threaded flange on the right side that accepts a single-cog freewheel.

Disc brake hub: Any hub with a drilled and threaded flange that accepts a disc brake rotor.

Through-axle hub (Also thru-axle): Generally, an oversized, sealed-bearing hub that accepts a large-diameter axle which can be pulled free from the hub in order to remove the wheel. Most through-axle designs are specific to downhill bikes.

Hub width: Refers to the hub’s measurement between the faces that engage the inside of the dropouts. Most mountain bikes use 135mm-wide rear hubs and 100mm-wide front hubs. Downhill-specific hubs are up to 25mm wider.

Cup-and-cone bearings: Old fashioned, adjustable bearings that use a threaded-on inner “cone” race that traps ball bearings inside a bowl-shaped “cup” race that is pressed into the hub. Until the late ’80s, when sealed cartridge bearings became the mountain bike standard, almost every bicycle used this type of hub bearing.
Cartridge bearings: One-piece ball bearing assemblies that are pressed into the hub shell.

Quick-release axle: A hollow axle that does not protrude past the outside of the dropouts to allow the use of a quick-release mechanism.

Quick release: A type of over-center clamp that uses a rotating cam inside a housing to draw a shaft through a hollow hub axle in order to fix the hub to a frame or fork.

Spokes: Generally, threaded high-strength wire that, when tensioned and laced properly, transfers the weight of the rider to the rim. Aluminum (Mavic CrossMax) and composite spokes (Spinergy) are available, but are not widely used.

Butted spokes: Not necessary, but very common. Butting refers to a thicker section of the spoke shaft, generally near the threads and head, that adds strength and durability to these high stress areas. The proper way to describe a butted spoke is by its wire gauges (14/15 gauge spoke) or its diameter in millimeters (2mm x 1.5mm spoke). These are the most commonly used for mountain bikes.

Rim: A circular shape, generally extruded from aluminum alloy, that a conventional tire can be mounted to. The parts of a mountain bike rim are as follows:

Flanges: A pair of hooked ridges designed to catch the reinforced “bead” of a tire.

Braking tracks: Flat sides with smooth surfaces that interface with conventional rim brake pads.

Spoke holes: Two rows of equally spaced holes that interface with the spoke nipples. Spoke holes may or may not have metal reinforcement “ferrules.”

Well: A depressed section between the rim’s flanges that facilitates the removal of the tire bead.

Valve hole: There are two diameters: the smaller “presta valve” size, and the larger, automotive “Schrader” valve sized hole.

Box-section rim (Also, double-walled): A hollow cross section in the rim extrusion used to add stiffness.

Single-wall rim: A thick extrusion with no hollow cross-section that is rarely used for mountain bikes.

Disc brake rim: An extrusion designed without a brake track on its sides. (Any conventional rim may be used with disc brakes).

Tubeless rim: A rim designed to hold air. Tubeless rims usually have a box-section profile without the usual spoke hole perforations in the outer wall in order to create an air seal. Special O-ring-sealed valve stems are a requirement (Mavic UST).

Welded rim: A construction technique that melts the rim extrusion together with pressure and an electrical arc.

Pinned rim: Any type of internal pressed-in fitting that is used to complete the rim extrusion into a circle.

Ceramic coated rim: A type of ceramic, dark gray in color, that is applied to the rim’s braking track, usually with an electric plasma arc. The rough, abrasion-resistant surface improves the life of an aluminum rim and stops better in dry conditions.

One-piece wheels: Various types of carbon fiber and cast metal mountain bike rims have been introduced over the past two decades with poor performance results. The advantage of a cast or molded wheel is that it should never need truing (straightening). Avoid this area of technology.

Spoke lacing: Both an art and a science, lacing refers to the number of spokes used to build a wheel and the method in which the spokes are woven together. Here are the high points:

Spoke count: Most mountain bikes have 32 spokes per hub—a number that has proven to produce the lightest and most reliable bicycle wheel. Hubs and matching rims are made for 48-, 36-, 28-, and 24-spoke wheels for special purposes.

Cross: Crossing the spokes as they leave the hub flanges reduces the stress on the hub flange and increases the strength of the wheel when using disc brakes. The crossed spokes are counted from the hub flange out. Most wheels use a “three-cross” pattern. One-cross, two cross and four-cross patterns are used for special applications.

Radial lacing: A fancy-looking pattern with each spoke radiating directly from the hub to the rim without crossing any others. Radial lacing creates a laterally rigid wheel, but tends to overstress the hub flanges. It is also undesirable for disc brake wheels.

Weaving: Crossed spokes are woven together at the outermost intersection to cause some of the wheel’s stresses to be transferred to adjoining spokes. Weaving the spokes has also proven to extend the lifespan of a wheelset. Fancy twisted spoke lacing serves no purpose except to impress non-cycling beach goers.

Tension: Spokes must be stretched slightly so they won’t go slack when the wheel is deflected. The perfect wheel is fine tuned so that every spoke is equally tensioned. When the rim of a properly tensioned wheel hits a bump, it deforms slightly, and momentarily increases the tension of all spokes. Uneven tension forces fewer spokes to do more work. Over-tightening the spokes can bring the wheel’s components to the breaking point, leading to premature failure. (Top wheel builders use a gauge as a final check.)

Paired spokes: A minimally-spoked wheel that uses paired crosses of spokes on opposing sides of the rim to maintain a higher degree of lateral strength than would be possible using a standard lacing with an equal number of spokes. A rigid rim is an essential component of this exotic-looking configuration (popularized by Trek, and Shimano).

TIRES AND TUBES WHAT WE REALLY ROLL ON
Dumb luck is as important as cutting-edge science when it comes to designing a successful mountain bike tire. The following tire lingo should get you up to speed:

Beads: The pair of reinforced lips that interface with the inside edges of the rim. The tire’s beads do not expand when the tire is inflated and thus keep the tire on the rim.

Kevlar tire (Also folding tire): A tire with the beads reinforced with flexible aramid fibers instead of the typical steel wires that line most commercial tire beads. Kevlar is Dupont’s trade name for the material. Kevlar fabric is rarely used elsewhere in the tire.

Carcass: Refers to several bonded layers of nylon cloth and rubber that support the tread and give the tire its strength.

Tread: A thick layer of rubber that is molded onto the carcass. Tread rubber is formulated into hard and soft compound depending on the aggressiveness of the tread design.

Hot patch: The fancy lettering and logos that adorn most tires, which are hot-pressed on in the final manufacturing operation.

Bead-filler rubber: A thick layer of rubber that is applied between the cloth layers of the carcass near the beads to stiffen the sidewalls and reduce impact damage. Usually found only on downhill and freeriding tires because of the added weight.

Plies: Refers to the number of cloth layers in the carcass under the tread.

Size: A confusing measurement molded onto the tire’s sidewall. The standard rim size that any mountain bike tire will fit is 26 by 1.75 inches. The width of the tire is considered its size. Most manufacturers measure the width of the carcass; some use the widest part of the tread. The average width of a cross-country mountain bike tire is 2.0 inches. Downhill and freeriding tires range from 2.2 to 3 inches wide. A number of mountain bikes use smaller-diameter, 24-inch rims, and the matching tires share conventional widths and tread patterns.

Knobby tread: Any aggressive tread design with tall, individual blocks.

Racing slick: A narrow, lightweight tire with a smooth tread pattern at the top of the tread and small edging tread block near the sides. Used for cross-country racing because they roll faster.

Semi-slick: A more aggressive racing tread pattern that retains the minimal center-tread design.

Sticky tread: An ultra-soft rubber compound used primarily on downhill racing knobbies. Small rocks and sand actually adhere to the rubber, hence its name (Intense and Tioga tires).

Butyl tube: A type of synthetic rubber that retains air well and is used to produce most of the world’s inner tubes.
Natural rubber tube (Also latex tube): Natural rubber is stretchier than synthetics and can be made into a lighter weight innertube—and is believed to better resist punctures. In practice, latex tubes are unreliable and expensive.

Schrader tube: Refers to any tube with an automotive-type filler valve.

Presta tube: Any tube with a small-diameter “French-type” filler valve. The presta valve is lighter weight and better suited to high-pressure applications because its valve seat can be threaded tightly closed.

Rim strip: A liner of special tape or a band of rubber that protects the inner tube from the sharp edges of the rim’s spoke holes.

Tubeless tire: A conventional tire that has been sealed to retain air and uses a modified bead designed to seal against a special sealed rim. (Mavic, Continental, Kenda, Michelin, Hutchinson and Specialized.)

Slime tubes: A proprietary brand of sealant injected into a ready-to-use innertube. Jargon for any inner tube that has been injected with puncture sealant. (Slime, Stan’s No-Tubes).
Pinch flat (Also, snake-bite puncture): The most common off-highway flat is caused by a harsh impact that compresses the tire against the rim flanges. The tube shears in two places as it becomes caught between the folds in the tire carcass.