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:

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.

The older of the two handlebar stem styles, quill types have been largely displaced as the industry standard on sport bikes. However, they remain standard on the majority of utility bikes, regardless of price, as well as on less expensive sport bikes and higher-end retro bikes. The quill stem fits down into the inside of the top of the fork steerer tube to be held in place internally via either a wedge and bolt or a cone-shaped expander nut and bolt. With a quill stem, the steerer tube does not protrude above the headset. The quill stem requires a threaded fork that extends up through the headset but does not protrude beyond the headset.

Quill stems showing wedge (on left) and cone-shaped (on right) expander nuts.

Forged aluminum quill stem without a removable face plate.

Welded steel quill stem without a removable face plate.

Quill stem with a removable face plate.

Adjustable quill stem with a removable face plate.

A mid-1980s "slingshot" quill stem with removable face plates and an integrated brake cable hanger.

Adjustable quill stem.

Classic road quill stem.

Welded steel quill stem without a removable face plate.

One-piece quill stem and handlebar.

Threadless stems, the newer of the two styles, are widely popular and have displaced quill stems as the industry standard on sport bikes. Threadless stems feature a modular design where the stem clamps around the outside of the top of the fork steerer tube that protrudes above the headset. Threadless stems are commonly available in lengths up to about 130 millimeters (5 inches).

...
Wikipedia