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 vs. ThreadlessEdit
Somewhat counter-intuitively, the term threadless derives from not from whether the stem itself is threaded— but rather from whether a headset lock nut threads on to the fork steer tube. Quill stems require a threaded headset — of specific length for each bicycle model. Threadless stems require an un-threaded 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. Un-threaded forks often require less labor to swap than threaded.
The older of the two handlebar stem styles, quill types have been largely displaced as the industry standard but remain in use on less expensive 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 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.
Threadless stems, the newer of the two styles, are widely popular, and have displaced quill stems as the industry standard. 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.
With threadless stems, a "star-nut" is driven down into the the threadless steerer tube and held in place by two barbed flanges. The top cap bolts into, and pulls against, the star-nut preloading the headset bearings. (See: photograph of Star-nut) Newer model forks, with carbon fiber steerer tubes, use an expander plug instead of a star nut, which once installed serves the same purpose as the star nut, but will not damage carbon fiber forks as a starnut will (by design a Star-nut digs into the surrounding tube, which, while safe with aluminum, can weaken carbon fiber to the point of failure)
Special adaptors may allow a threaded fork to receive a threadless stem.
Advantages of each typeEdit
- Threadless stems offer simple way to swap, flip, mix and match stems, which are readily available in various configurations and variations of construction, color, reach and angle.
- Threadless stems allow for the lighter carbon fiber or aluminum alloy steerer tubes, and hence a lighter overall bicycle.
- Threadless stems can be changed with a single allen wrench. (Some quill stems also use a single allen wrench.)
- The threadless stem's centering can be adjusted without disturbing handlebar height.
- Threadless stems avoid the internal binding or seizing possible with a quill stem's wedge or cone bolt.
- Quill stems offer the ability to make fine adjustments to handlebar height.
- Quill stems can simply be raised, especially examples with long shafts. To raise or lower a threadless headset beyond pre-determined (spacer) increments requires another stem.
- Quill stems may offer a slender, smoother appearance vs. the comparatively modular, jointed appearance of the threadless stem.
Stems tighten around and hold the handlebar either by pinch bolts, which require 'feeding' the handlebar through the stem — after removing controls, accessories and bar covering; or via detachable faceplates, also called pillow blocks, especially on BMX bicycles, which allow a handlebar with controls, accessories or bar covering to be removed intact. Stems with faceplates or pillow blocks are known as pop-top stems.
Stems normally have two dimensions that affect bicycle fit: an angle and a forward length or extension. Quill stems may also have a height (above minimum insertion mark). Stems must also be compatible with the dimensions of the components that they connect, namely the handlebar clamp diameter and steerer tube diameter.
For road quill stems, the angle is normally 73° which causes the extension of the stem to be nearly parallel with the ground. Some quill stems also have other angles, e.g. 90°, which results in the stem pointing forwards and upwards.
Newer style stems for threadless headsets come in a wide variety of angles from 0° to 40° and can be flip-flopped, or inverted so that the angle is up or down.
There are also models of quill and threadless stems with adjustable angles.
The length of the stem determines how far forward of the steerer tube the handlebars are.
Minimum insertion (quill stem) Edit
Quill stems each require a minimum length which must insert into the headset steerer tube, thereby determining a maximum length that may extend above steerer tube.
Steerer tube diameter Edit
Stems come in at two common nominal sizes: 1" and 1 1/8". Less commonly 1 1/4" is an obsolete size, and 1 1/2" is found on very high end Downhill bikes.
While the 1 1/8" size is standardized, the 1" size comes in a number of variants, depending on the diameter of the fork Crown Race (at the base of the steerer tube). The two common sizes are 27.0 mm 26.4 mm, others exist as nonstandard variants, often found on low end bikes in the United States. The variations of the 1" standards, when they differ, regarding the other press-fit parts of the head set is 0.2 mm, a small enough difference that it can be safely ignored. 
Tapered steerer tubes: As of 2008 a number of manufacturers of high end road and mountain bikes are coming out with tapered steerer tubes. While there are purported advantages, there are not any standards yet developed, with each manufacturer following its own conventions. This makes replacement parts difficult to come by, only available from the original manufacturer.
Handle bar diameter Edit
Both quill and threadless stems come in a variety of bicycle handlebar clamp diameters. The ISO standard for the clamping area of a handlebar is 25.4 mm (1"), which is used on mountain bikes and many Japanese-made road handlebars. However, the Italian unofficial standard is 26.0 mm, which is the most common clamp size for road bars. There are also intermediate sizes such as 25.8 mm to try and achieve compatibility with either an ISO or Italian stem, and the old Cinelli-specific size of 26.4 mm. In practice, many modern stems with removable faceplates allow for slight differences in handlebar clamp diameter, but the older type of stem with a single pinch bolt must be accurately matched. In the days of quill stems, a road stem was clearly identifiable from its "7" shape, but nowadays it can be hard to tell the difference between a "road" (26.0 mm) and "MTB" (25.4 mm) stem. Manufacturers frequently omit the clamp size from advertising or packaging.
BMX bikes usually have a 22.2 mm diameter clamp size.
A more recent standard is a 31.8 mm (1.25") clamp for both MTB and 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. Standard brake levers can be used as it is only the stem clamp central section that is oversized. Shims are available to fit a 31.8 mm stem to either a 25.4 mm or 26.0 mm bar, so many new models of stems are oversize-only.
Certain uncommon adjustable stems have the handlebar clamp unit mounted on a moveable slide, permitting variable fore and aft settings. This adjustable stem was developed by the famous cyclist, Major Taylor, hence they're sometimes called Major Taylor Outriggers.
At one time, some manufacturers (Softride) marketed suspension stems. Softride's stem allowed for up to 3 inches of travel, used a parallelogram linkage, and used a polymer bushing and a steel coil spring for shock absorption.
Tandem stoker stem Edit
The stem for the stoker (rear rider) on a tandem is similar to a stem for a threadless fork and headset, but clamps on the captain's (front rider's) seatpost. This type of stem may be adjustable in length with one section of tubing telescoping into another.