The headset is the component on a bicycle which provides a rotatable interface between the bicycle fork and the bicycle frame itself. The short tube through which the steerer of the fork runs is called the headtube. A typical headset consists of two cups which are pressed into the top and bottom of the headtube. Inside the two cups are bearings which provide a low friction contact between the bearing cup and the steerer.
Traditional bicycle head tubes and headsets are sized for a 1 inch diameter steerer tube. Many frame and fork manufacturers are now building their parts around a steerer tube with a diameter of 1 1/8 inch. The larger diameter of the head tube and headset gives added stiffness to the steering portion of the bicycle.
Threaded headsets
Threaded headsets are for use with threaded steerers. After the steerer is cut to the appropriate size, the top inch of the steerer is threaded using a die. The upper cap of a threaded headsets have the appropriate threads to interface with the threads that are cut into the steerer. The cap is threaded onto the steerer during installation and is forced down onto the bearings in the upper cup, which also pulls the crown race on the fork into the bearings in the lower cup. This compression is necessary to insure that the fork does not have any perceivable "play", and will not shift other than to rotate inside the headtube. It is necessary to adjust the tension on the top cap so there is no play, yet the steerer can still move freely. This is an adjustment that at times requires patience.
Threadless headsets
The threadless headset is a more modern design than the traditional threaded headset. It features two cups like a threaded headset, but the upper cap of the headset is not threaded. The compression in a threadless headset is provided via a bolt that is connected to a star-fangled nut that is driven into the steerer. The "star-nut" resists upward movement and allows the bolt to create an upward force as it is tightened into the nut. The bolt is resting in a cap that pulls the stem down onto the top cap of the headset. The bolt is tightened in a manner that is similar to the threaded top cap on a threaded headset; the bolt is tightened so that there is no play, yet the steering is fluid.
Integrated headsets
A relatively new and emergent headset design, integrated headsets do away with the upper and lower caps on threadless headsets and instead seat the bearings directly against the head tube of the frame. Favored sometimes for its aesthetic appeal, integrated headsets reduce the number of parts involved in the headset assembly. Chris King®, a leading manufacturer of bicycle headsets, offers a vehement argument against the implementation of integrated headsets.
Internal headsets
Sometimes referred to as semi-integrated headsets, internal headsets include all of the parts of conventional headsets, but situate the bearings inside the head tube, instead of outside. Unlike integrated headsets, however, internal headsets still employ cups to between the bearings and the frame itself.
