Stem (bicycle part)

Sometimes called a goose neck,[1] a stem's design belongs to either a quill or threadless system, and each system is compatible with respective headset and fork designs: Although stems are commonly referred to as being of either the quill or threaded type, the thread in question is the one on the fork steerer tube.

Threadless systems use an unthreaded steerer tube, which extends into the stem and may be cut to length as desired in order to accommodate the height of the headset, head tube, stem and any spacers used to adjust the handlebar height.

The older of the two handlebar stem styles, quill types have been largely displaced as the industry standard on sport bikes.

The quill stem requires the threaded steerer tube of the fork to extend up through the headset but not protrude beyond it.

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.

The top cap bolts into, and pulls against, the star nut, thereby preloading the headset bearings.

[5] Stems normally have two dimensions that affect bicycle fit: an angle and a forward length or extension.

On the other hand, a quill stem must be sized to match the inner diameter of the steerer tube.

However, the Italian unofficial standard is 26.0 mm, which is the most common clamp size for road bars.

This is rapidly taking over from the previous mix of sizes, although other accessories such as some light or computer brackets may also need to be oversized to fit the thicker bars.

Certain uncommon adjustable stems have the handlebar clamp unit mounted on a moveable slide, permitting variable fore and aft settings.

Softride's stem allowed for up to 76 mm (3 in) of travel, used a parallelogram linkage, and used a polymer bushing and a steel coil spring for shock absorption.

[8] Urbancyclo Quicktwist is a start-up company that has designed a stem that can be rotated so that the handlebar points upwards parallel to the steerer tube.