Stem (bicycle part)
Encyclopedia
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
that connects the handlebars
Bicycle handlebar
Bicycle handlebar or often bicycle handlebars refers to the steering mechanism for bicycles; the equivalent of a steering wheel. Besides steering, handlebars also often support a portion of the rider's weight, depending on their riding position, and provide a convenient mounting place for brake...
to the steerer tube of the bicycle fork
Bicycle fork
A bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork ends which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...
. 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
Headset (bicycle part)
The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed...
and fork designs:
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-
- Quill: the stem inserts into the steerer tubeBicycle forkA bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork ends which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...
which is threaded and which does not extend above the headset. - Threadless: the stem clamps around the steerer tubeBicycle forkA bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork ends which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...
which is not threaded and which extends above the headset.
- Quill: the stem inserts into the steerer tube
-
Quill vs. Threadless
Somewhat counter-intuitively, the term threadless derives not from whether the stem itself is threaded— but rather from whether a headsetHeadset (bicycle part)
The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed...
lock nut threads on to the fork
Bicycle fork
A bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork ends which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...
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.
Quill stems
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 less expensive sport bikes and higher-end retroRetro
Retro is a culturally outdated or aged style, trend, mode, or fashion, from the overall postmodern past, that has since that time become functionally or superficially the norm once again. The use of "retro" style iconography and imagery interjected into post-modern art, advertising, mass media, etc...
bikes. The quill stem fits down into the inside of the top of the fork steerer tube
Bicycle fork
A bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork ends which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...
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
Threadless stems, the newer of the two styles, are widely popular, and have displaced quill stems as the industry standard on sport bikes. 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 headsetHeadset (bicycle part)
The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed...
. Threadless stems are commonly available in lengths up to about 130 millimeters. (5 inches).
With threadless stems, a "star-nut" is driven down into 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 type
Advantage Threadless:- Threadless stems offer a 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.
Advantage Quill:
- 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.
- Quill stems can be removed without disrupting the headset.
Materials
Stems are often constructed of aluminum, but are also available in steelSteel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, carbon fiber
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...
, and carbon fiber over aluminum.
Handlebar attachment
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 BMXBMX
Bicycle motocross or BMX refers to the sport in which the main goal is extreme racing on bicycles in motocross style on tracks with inline start and expressive obstacles, and it is also the term that refers to the bicycle itself that is designed for dirt and motocross cycling.- History :BMX started...
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.
Dimensions
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.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 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.
Length
The length of the stem determines how far forward of the steerer tube the handlebars are.Minimum insertion (quill stem)
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
Stems come mainly in two common nominal sizes: 1" and 1". Less commonly 1" is an obsolete size, and 1" is found on very high end Downhill bikeDownhill bike
A downhill bike is a full suspension bicycle designed for downhill cycling on particularly steep, rocky trails. Unlike a typical mountain bike, durability and stability are the most important design features, compared to lighter, more versatile cross-country bikes...
s.
While the 1" 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 26.0 mm 25.4 mm, others exist as nonstandard variants, often found on low end bikes in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. The variations of the 1" standards, when they differ, regarding the other press-fit
Interference fit
An interference fit, also known as a press fit or friction fit, is a fastening between two parts which is achieved by friction after the parts are pushed together, rather than by any other means of fastening...
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
Both quill and threadless stems come in a variety of bicycle handlebarBicycle handlebar
Bicycle handlebar or often bicycle handlebars refers to the steering mechanism for bicycles; the equivalent of a steering wheel. Besides steering, handlebars also often support a portion of the rider's weight, depending on their riding position, and provide a convenient mounting place for brake...
clamp diameters. The ISO standard for the clamping area of a handlebar is 25.4 mm (1 inch), 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 to 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.
Attachments
Some stems have a hole through the horizontal part to support the front brake cable on bikes with cantilever brakes such as cyclo-cross bicycleCyclo-cross bicycle
A cyclo-cross bicycle is a bicycle specifically designed for the rigors of a cyclo-cross race. Cyclo-cross bicycles roughly resemble the racing bicycles used in road racing...
s and older mountain bike
Mountain bike
A mountain bike or mountain bicycle is a bicycle created for off-road cycling. This activity includes traversing of rocks and washouts, and steep declines,...
s.
Adjustable
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 are sometimes called Major Taylor Outriggers.Suspension
At one time, some manufacturers (Softride) marketed suspension stems. Softride's stem allowed for up to 3 inches of travel, used a parallelogramParallelogram
In Euclidean geometry, a parallelogram is a convex quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of equal measure...
linkage
Linkage (mechanical)
A mechanical linkage is an assembly of bodies connected together to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for...
, and used a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
bushing and a steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
coil spring
Coil spring
A Coil spring, also known as a helical spring, is a mechanical device, which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces...
for shock absorption.
Tandem stoker stem
The stem for the stoker (rear rider) on a tandemTandem bicycle
The tandem bicycle or twin is a form of bicycle designed to be ridden by more than one person. The term tandem refers to the seating arrangement , not the number of riders. A bike with two riders side-by-side is called a sociable.-History:Patents related to tandem bicycles date from the late 19th...
is similar to a stem for a threadless fork and headset, but clamps on the captain's (front rider's) seatpost
Seatpost
A bicycle seatpost, seatpin, saddlepole, saddle pillar, or saddle pin is a tube that extends upwards from the bicycle frame to the saddle. The amount that it extends out of the frame can usually be adjusted, and there is usually a mark that indicates the minimum insertion...
. This type of stem may be adjustable in length with one section of tubing telescoping into another.
External links
- Hands Up! Adjusting Handlebar Height by Sheldon BrownSheldon Brown (bicycle mechanic)Sheldon Brown was an American bicycle mechanic and technical authority on bicycles. He contributed to numerous print and online sources related to bicycling, bicycle mechanics and maintenance, including his own website — and received numerous awards for his contributions.-Biography:Brown...