slang for a tubular bicycle tire commonly used by professional road racers and track riding specialists.
A tubular tire is constructed by completely enclosing the inflatable inner tube within the rubberized tire casing, and then literally sewing the casing together around it with an industrial sewing machine (hence the term "sewup"). A protective strip is then bonded to the tire to protect the stitching from abrasion. The tire is physically attached to the wheel rim by gluing it in place with contact cement.
This design is somewhat archaic, and has been replaced for the most part by the more modern, beaded tire, or "clincher", which utilize a separate inner tube and open-style casing held in place by air pressure alone, much like an automobile tire.
Sewups, or tubular, tires are best suited to high-performance applications, because of their inherent lighter weight. However, due to the labor-intensive mounting procedure and higher manufacturing cost, the popularity of this design has diminished in recent years.
Lance Armstrong rides sewup tires in the Tour De France so his wheels are as light as possible for the mountain climbing stages.