Top tube of a bicycle.
The top tube, or cross-bar, connects the top of the head tube to the top of the seat tube.
In a traditional-geometry diamond frame, the tube is horizontal (parallel to the ground). In a compact-geometry frame, the tube is normally sloped downward toward the seat tube for additional standover clearance. In a mountain bike frame, the top tube is almost always sloped downward toward the seat tube. Radically sloped tubes that compromise the integrity of the traditional diamond frame may require additional gusseting tubes, alternative frame construction, or different materials for equivalent strength.
Step-through frames usually have a tube that slopes down steeply to allow the rider to mount and dismount the bicycle more easily. Alternative step-through designs may include leaving out the top tube out completely, as in monocoque mainframe designs using a separated or hinged seat tube, and twin top tubes that continue to the rear fork ends as with the Mixte frame. These alternatives to the diamond frame provide greater versatility, though at the expense of added weight to achieve equivalent strength and rigidity.
Control cables are routed along mounts on the tube, or sometimes inside the top tube. Most commonly, this includes the cable for the rear brake, but some mountain bikes and hybrid bicycles also route the front and rear derailleur cables along the tube.
The space between the top tube and the rider’s groin while straddling the bike and standing on the ground is called clearance. The total height from the ground to this point is called the height lever.