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Oversized
Larger in diameter than the traditional sizes. This term is commonly used for headsets and frame tubing. Oversized parts are stiffer, and can be lighter for their weight if the walls are made thin enough. If the walls become too thin, however, they become too easy to damage by denting.
Traditional forks used 1" (25.4 mm) diameter steerers. Oversized steerers are 1 1/8" (28.6 mm) or, less commonly, 1 1/4" (31.8 mm). Details on sizes are listed under headsets.
Traditional frames used 1" (25.4 mm) top tubes, 1 1/8" (28.6 mm) seat tubes and down tubes, and 1 1/4" (31.8 mm) head tubes. When BMX racing became popular, frames built to these old standards couldn't always hold up to the rigors of BMX competition. Since the wide availability of T.I.G. welding freed designers from the need to use tubes that would fit standard-size lugs, oversized tubing became the norm for BMX frames. This design and technology were later adopted by builders of mountain bikes.
In the interest of improved chainwheel/tire clearance, some mountain bikes have "oversized" bottom brackets, with a shell width of 73 mm, rather than the standard 68 mm.
