Understanding Bike Stems
The stem connects the top of the fork to the handlebars.
In addition to providing a necessary connection between the handlebars and steerer (the top part of the fork), the length and rise of the stem allow for greater variation in bike fit, offers some mechanical advantage when turning the fork, and helps position the handlebars closer to the point above which the front tire makes contact with the ground, stabilizing the bike's steering.
By selecting (or adjusting) the height of the stem, its length, and the angle it makes relative to the head tube, the stem allows the rider to make minor adjustments to bike fit with relative ease.
See Brightspoke's Bike Stem Calculator for an easy tool for calculating the impact of these adjustments.
There are two basic types of stem: "quill" or "conventional" stems and "threadless" stems. A third type, "direct mount" stems, are sometimes used for downhill racing bikes and other specialized applications. The stem must be matched to both the size of the steerer (the upper part of the fork) and the diameter of the handlebars, as well as the type of headset.
Quill stems are shaped like an inverted 'L', with a long tube that is inserted into the steerer of the fork and is held tight by an expander or wedge. This wedge or expander is typically adjusted by a long hex bolt that is exposed at the top of the bend in the "L".
Quill stems are the older of the two styles but are still common on both on inexpensive bicycles and on higher-end retro-styled bikes, in part due to the clean, classic look they provide. Unlike threadless stems, the height of a quill stem can be adjusted by inserting more or less of the tube into the steerer of the fork, much like the height of a seat or saddle can be adjusted by inserting more or less of the seat post into the frame's seat tube.
A quill stem requires a "threaded" headset, which is held in place by a threaded "lock nut" which tightens around the top of the fork's steerer.
Threadless stems are a straight tube with clamps on either end to attach to the fork's steerer and the handlebars, respectively. Instead of being inserted into the steerer, a threadless stem is clamped around the outside of the steerer by a set of bolts.
Threadless stems are the newer of the two basic styles and are the most common type. Their modular design makes it easier to find compatible stems for a given handlebar and fork combination. Threadless stems can also often be "flipped" (essentially, installed upside down so that they angle down instead of up) to offer more choices in ride geometry. The height of a threadless stem can sometimes be adjusted by adding or removing "spacers" from the steerer's stack, but this is neither as easy nor as flexible as adjusting the height of a conventional stem and is limited by the height of the steerer tube that extends above the headset.
A threadless stem requires a "threadless" headset, which is held in place by a "star nut" or expander plug that helps anchor the stem and headset to the fork.
(Note that counter-intuitively there are no threads on either threadless or quill stems. The term "threaded" or "threadless" applies to the type of headset and fork to which the stem attaches.)
A direct mount stem is similar to a very short threadless stem (typically 30 mm to 50 mm long), but rather than clamping around the steerer tube, a direct mount stem is bolted directly onto of the crown of the fork. Direct mount stems are most commonly found on downhill racing mountain bikes where the extremely solid connection between the handlebars and the fork offer a clear advantage.
Direct mount stems must be matched to the specific type of fork (such as the Rockshox BoXXer or Marzocchi 888) as manufacturers haven't yet settled on a universal standard.
Bicycle stems are largely defined by eight basic parameters.
1. Stem Type
The stem type (quill or threadless) must be matched to the type of headset on the bike. Quill stems are used with threaded headsets (although adapters are sometimes available to connect a threadless stem to a threaded headset). Threadless stems are used with threadless headsets. Direct mount stems must be matched to a specific compatible fork
2. Steerer Tube Diameter
In order to fit properly on the fork and headset, a stem must be sized to match the diameter of steerer tube.
Quill stems are inserted into the steerer tube itself and held in place by compression of an expander or wedge against the inside of the steerer tube. The outer dimension of the stem must be matched to the inner dimension of the steerer. One inch (25.4 mm) is the most common size of quill stem, although 1 1/8 inch (28.6 mm) and 1 1/4 inch (31.8 mm) diameters are also popular.
Threadless stems clamp around the outer portion of the steerer tube. The outer dimension of the steerer must be matched to the inner dimension of the clamp. One and one eighth inch (28.6 mm) is the most common size of threadless stem, although 1 inch (25.4 mm), 1 1/4 inch (31.8 mm) and 1 1/2 inch (38.1 mm) diameters are also popular.
The larger diameter steerer tubes and stems will be stronger but also heavier than a smaller stem made of the same material.
3. Handlebar Clamp Size
In order to attach to the handlebars properly, the handlebar clamp must be sized to match the outside diameter of the handlebars at the center of the bar. Handlebars (and therefore handlebar clamps) are often 25.4 mm (1 inch), 26.0 mm (approximately 1.02 inches) or 31.8 mm (1 1/4 inch) in diameter. Fatter bars are stronger but also heavier than a thinner handlebar of the same length and material.
4. Stem Length
A given stem model is often available in a number of lengths, generally around 90 mm to 140 mm. By replacing a stem with a shorter or longer one, riders can adjust the distance from the seat to the handlebars to find the most comfortable riding position.
Stem length is typically measured from the center of the steerer tube to the center of the handlebar clamp along the length of the stem. (That is, on the same angle as the stem rise or drop, not parallel to the ground.)
In addition to providing a more stretched out riding position, longer stems will "relax" the steering of the bicycle, so that the handlebars will need to move a bit more (and with a bit less force) to achieve a given rotation of the front tire relative to a smaller stem. Short stems will create a more "twitchy" or responsive handling. The width of the handlebars and to a lesser extent the size of the fork rake are other significant factors in this handling. Think of the extreme case of very short handlebars installed directly on top of the fork. You'd need to apply more force to turn the wheel but a small movement of the handlebars would produce a comparatively large rotation of the fork and wheel--the handlebars are moving through a very small circle. For very wide handlebars installed on a very long stem, the grips on the handlebars would need to move a greater distance (along a much wider arc) to achieve the same turn at the fork and wheel.
5. Stem Rise (or Drop) Angle
Most stems have a slight rise that lifts the handlebar clamp above the steerer clamp (or the bend in a quill stem). Road bikes typically have smaller stem rise angles, often 6 or 7 degrees. A more upright riding position is created by larger stem rise angles and it is not uncommon to find comfort bikes with stems with a 25 to 30 degree rise. Extremely aerodynamic bikes, like those used for triathlons and time trials, sometimes have no rise at all.
Most threadless stems can be flipped over so that a 6 degree stem can raise or lower the handlebars by 6 degrees. Some quill stems have a negative rise (in other words, a drop) as well, so that the top of the handlebars are below the top of the stem.
Some stems are composed of two independent parts joined together by an adjustable joint. On these "adjustable" stems the effective rise angle can be changed by loosening a hex bolt that is accessible on the side of the stem.
Note that the stem rise angle does not, by itself, determine the angle of the stem relative to the ground. The stem rise angle is measured from a line perpendicular to steerer clamp (or the steerer upon which it would be mounted). Hence a stem with 6 degree rise will form a 96 degree angle relative to the head tube (or 84 degrees when flipped). The angle of the stem relative to the ground can be determined by subtracting stem tube angle from this formulation of the stem rise angle, so that threadless stem with 6 degree rise installed on a bike with a 73 degree head tube angle will create an angle of 23 degrees relative to the ground (96 - 73 = 23), or 11 degrees when flipped (84 - 73 = 11).
You can use Brightspoke's stem calculator to experiment with different stem rise and head tube angle combinations.
6. Handlebar Clamp Type
There are two basic types of clamp that are used to connect the handlebars to the stem.
A "pinch clamp" is a "C" shaped clamp typically held together by a bolt or two. To install or remove a set of handlebars on pinch clamp stem, the bars must be threaded through the clamp starting at one end. This is an awkward maneuver for bars with any significant bend or curve and requires that any grips, tape or brake levers be removed from the handlebars prior to installation or removal.
To work around this problem many stems have a "removable face plate" clamp, which can be completely removed from the stem by loosening two or four bolts. With a removable face plate clamp, the handlebars can be installed or removed without stripping components or grips attached to the handlebars.
7. Number of Bolts
The number and type of bolts used to attach the handlebars to the stem (and for threadless stems, to attach the stem to the steerer tube) can impact the strength of the assembly and to a lesser extent, the weight of the bike. Most stems with removable face plates use four bolts to attach the handlebars to the stem, although two bolts are sometimes used to reduce cost or weight at a loss of some strength.
8. Stem Material
Most modern threadless stems are made of aluminium or carbon, although quill stems and some inexpensive stems are sometimes made from a steel alloy. As a general rule, steel stems are the heaviest of the three, but are stronger for a given diameter. Aluminium stems are light weight, sufficiently strong and relatively inexpensive, and are the most common type. Carbon stems are lighter still but relatively expensive. Carbon stems are often found on high end racing bikes where weight is at a premium. Carbon stems, much like carbon frames, can also help dampen road vibrations leading to a more comfortable ride. Rarely, stems are made from more exotic bike materials such as titanium.
While these guidelines are generally accurate, be aware that the stem material isn't the only factor in determining a stem's quality, weight, comfort and price. Different engineering approaches and manufacturing techniques can create significant variation within the selection of stems made of a given material. For example, some high quality aluminum stems can be lighter, stronger and better at reducing road vibration than some inexpensive carbon stems.
(Be careful to always use an expander plug, never a star nut, with a carbon stem. The star nut is designed to dig slightly into the inner wall of a stem. While these minor gouges do not weaken steel or aluminum, they can significantly compromise the integrity of carbon fiber, making it prone to failure.)
What to Look For
Generally speaking any stem from a reputable manufacturer will work safely and well under the riding conditions you're likely to encounter. More expensive stems may be easier to work with, weigh less, be better at absorbing road vibrations, look better, or be more durable in the face of a crash or extreme riding conditions, but even a low priced stem will connect your handlebars to your steerer tube securely and effectively.
Unless you are replacing the entire front end assembly of your bicycle (fork, headtube, handlebars and stem), your first order of business should be to ensure that the stem is compatible with your headset (threaded or threadless) and with the diameter of your steerer tube and handlebars.
For a given type of stem, you'll usually have some options about the stem length and rise angle. These are primarily a matter of fit--longer stems provide a more stretched out riding position, steeper rise angles offer a more upright (or when flipped, aerodynamic) riding position.
The handlebar clamp type and the number of bolts used to attach the stem the are often indicators of the overall quality of the stem. Pinch clamps and fewer bolts are often used on less expensive stems although they offer some minor weight advantages.
The stem material is another indicator of the overall quality and price of the stem. Aluminum stems are most common, and with good reason. Where used, threadless steel stems are often an indication of efforts to reduce costs, although quill stems can take advantage of their increased strength. Carbon stems are generally lighter and stronger than either aluminum or steel and can help dampen road vibration, but are generally priced accordingly. Given the relatively small size and weight of all stems, carbon stems are probably overkill for all but the most weight conscious rider.