Bicycle stem

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 requires the threaded steerer tube of the fork to extend up through the headset but not protrude beyond it. The quill stem fits down into the inside of the top of steerer tube and is held in place by either a wedge-shaped nut and bolt or a cone-shaped expander nut and bolt. In the case of a wedge-shaped nut, the bottom of the stem is cut diagonally to match the wedge and the bolt pulls the wedge against the stem to expand against the inside of the steerer tube and hold the stem in place. In the case of an expander nut, the bottom of the stem is cut perpendicular to its length and also has two slits cut parallel to its length. The cone-shaped expander nut is pulled upwards by the bolt causing the sides of the stem to spread and press against the inside of the steerer tube to hold it in place.

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.

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Wikipedia