Stem (bicycle part)
The stem is the component on a bicycle that connects the handlebars to the steerer tube of the bicycle fork. Sometimes called a goose neck, a stem's design belongs to either a quill or threadless system, and each system is compatible with respective headset and fork designs:
Quill: the stem inserts into the steerer tube, which is threaded and does not extend above the headset.
Threadless: the stem clamps around the steerer tube, which is not threaded and extends above the headset.
Quill versus threadless
Somewhat counterintuitively, the term threadless derives not from whether the stem itself is threaded, but from whether a headset lock nut threads onto the fork steer tube. Quill stems require a threaded headset of specific length for each bicycle model. Threadless stems require an unthreaded steerer tube, which may vary in length for each bicycle model.
Quill systems predate threadless systems. With the advent of threadless stems, manufacturers no longer need to provide a range of threaded forks for a given model; all threadless system forks are made with the same length steerer tube (long). The steerer tubes are then cut to length to fit upon installation. Unthreaded forks often require less labor to swap than threaded forks.
