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A spoke wrench that fits three common nipple sizes

A spoke wrench or spoke key is a small wrench or tool used to increase or decrease the tension in the spokes of a bicycle wheel, sulky wheel, wheelchair wheel, or similar. A spoke wrench is sometimes called a nipple wrench, as it is the spoke nipple - not the spoke - that is turned in the process of changing the tension of a spoke.

Spoke nipples are typically T-shaped in cross-section, with an internal thread running part of the way through the hole that runs along the spoke nipple's principal axis. A spoke nipple rests in a hole drilled radially through the wheel's rim, and the nipple is threaded onto the external thread of a spoke. The spoke itself is fixed, at its other extreme, through a hole in the wheel's hub. The spoke and nipple are functionally equivalent to a bolt and a nut, respectively. However, unlike a typical nut-and-bolt pair, a spoke and nipple do not join two parts (the rim and the hub) so much as bridge them, under tension.

The area of the nipple around which the spoke wrench, or key, is fitted in use is square in cross-section.

Common sizes of spokes (flat-to-flat) include:

• 0.127" (3.23 mm)
• 0.130" (3.30 mm)
• 0.136" (3.45 mm)

A spoke wrench is used in the creation, maintenance, or repair of hand-built or machine-built spoked wheels. The goal in such activity is to bring the wheel (nearer) to a state of true - namely, a wheel in which the rim is: a perfect circle concentric with the axis of the axle in the hub; and, planar, such that the plane of the rim is centered laterally between the two extremes of the axle where it is supported in a frame.

By increasing or decreasing the tension of individual spokes, or groups of spokes, around the circumference, a wheelsmith can correct for local deviations from true - e.g. wobbles, or hops.

Wikibooks Bicycles has a page on the topic of

Truing a bicycle wheel