Stem (bike)
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The stem is the component on a bicycle that connects the handlebars to the steer tube of a bicycle fork. Depending on the type of headset the stem either clamps around the steer tube or inserts into the steer tube and is held into place with a wedge. This latter style is referred to as a quill stem.
[edit] Varieties
Stems come in several varieties to suit different uses.
[edit] Quill
An older style, still in use on less-expensive new bikes, has a part called the quill, that fits into the inside of the top of the fork steer tube and clamps it internally in place with a wedge and bolt. The steer tube does not protrude above the headset.
[edit] Threadless
A newer style, used almost exclusively on all but the least-expensive new bikes, clamps around the outside of the top of the fork steer tube (or quill to threadless adapter) that protrudes above the headset.
[edit] Suspension
At one time, some manufacturers (Softride) played with including an elastomer or spring suspension in the stem, but that seems to have been a passing fad.
[edit] Tandem Stoker
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.
[edit] Dimensions
Stems normally have two dimensions that affect bicycle fit: an angle and a length (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 steer tube diameter.
[edit] Angle
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 17° 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.
[edit] Length
The length of the stem determines how far forward of the steer tube the handlebars are.
[edit] Minimum insertion height (quill stem)
Quill stems also have a maximum length that they can extend upward out of the steer tube.
[edit] Steer tube diameter
Quill stems come in at least two sizes for 1" and 1 1/8" steer tubes.
Threadless stems come in sizes for 1", 1 1/8", and even 1 1/4" steer tubes.
[edit] Handle bar diameter
Both quill and threadless stems come in a variety of handlebar clamp diameters. The ISO standard for the clamping area of a handlebar is 25.4 mm (1"), which is used on the majority of 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 new emerging standard is an oversize 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 computers and brake levers also need to be designed with the thicker bars in mind. Shims are available to fit a 31.8mm stem to either a 25.4mm or 26.0mm bar, so it is likely that most new stems will be made oversize-only.[original research?]
On older quill stems, the handlebars are inserted through the clamp hole and secured with a single bolt. On newer stems, two or four bolts and a separate plate hold the handlebars in place, allowing the handlebars to be removed without having to remove the brake levers.
[edit] Construction
[edit] Materials
Stems are often constructed of aluminum, but are also available in steel, carbon fiber, and carbon fiber over aluminum.
[edit] Attachments
Some stems have a hole through the horizontal part to support the front brake cable.
