Engineering:Stem (bicycle part)
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,Cite error: Closing </ref> missing for <ref> tag
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 BMX bicycles,[1] 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.[2]
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. So, for example, a 17° stem angled downward would mimic the angle of the typical road quill stem.
There are also models of quill and threadless stems with adjustable angles, although this adjustability often comes at the expense of some lost rigidity.
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
A stem must be compatible with the dimensions of the fork steerer tube. Steerer tubes come mainly in two common nominal sizes: 25.4 and 28.6 mm (1 and 1 1⁄8 in). Less commonly 31.8 mm (1 1⁄4 in) was used on some mountain bikes and tandems and has now returned on some carbon fiber forks, and 38.1 mm (1 1⁄2 in) is found on high end Downhill bikes.
A threadless stem should match the outside diameter of the steerer tube; a reducing shim may be employed to match a 28.6 mm (1 1⁄8 in) stem to a 25.4 mm (1 in) steerer tube. On the other hand, a quill stem must be sized to match the inner diameter of the steerer tube. Thus a quill stem made to fit a 28.6 mm (1 1⁄8 in) steerer tube has an outer diameter of 25.4 mm (1 in). For 25.4 mm (1 in) steerer tubes the quill diameter is most often 22.2 mm (7⁄8 in) but some older American bicycles used 21.15 mm. Some older French bicycles used 25 mm steerer tubes with 22 mm diameter quill stems.[3]
Handle bar diameter
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 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 "number 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 bicycles and older mountain bikes.
Variations
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 a 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 76 mm (3 in) of travel, used a parallelogram linkage, and used a polymer bushing and a steel coil spring for shock absorption.[4]
Tandem stoker stem
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.
Rotatable
There are several examples of stems that allow the handlebars to be rotated when the bike is not in use without the use of tools. Such stems may provide benefits such as the bike taking up less storage space in bike racks or apartments, being easier to transport through doors and small corridors, or as a form of bicycle theft protection. They have also been used on folding bicycles. Most are rotated to the side so that the handlebar becomes approximately parallel to the frame's top tube, with examples such as Revelo THINstem, N-Lock Rotating Stem, Speedlifter Stem Twist SDS and Satori-ET2.[5] 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.[6]
See also
References
- ↑ Brown, Sheldon. "Sheldon Brown Glossary: Pillow block". http://www.sheldonbrown.com/gloss_p.html#pillowblock.
- ↑ Brown, Sheldon. "Sheldon Brown Glossary: Pop-top". http://www.sheldonbrown.com/gloss_p.html#poptop.
- ↑ Brown, Sheldon. "Bicycle Glossary (Stem)". http://sheldonbrown.com/gloss_st-z.html#stem.
- ↑ "SoftRide Stems: SOFTRIDE ALUMINUM SUSPENSION STEM". http://www.bikepro.com/products/stems/soft.html.
- ↑ "Rotating Handlebar Stems for Folding Bikes" (in en-US). www.foldingwheels.com. https://www.foldingwheels.com/rotating-handlebar-stems-for-folding-bikes/.
- ↑ "This stem rotates in the middle to make your bike more compact" (in en). BikeRadar. 2018-05-23. https://www.bikeradar.com/news/this-stem-rotates-in-the-middle-to-make-your-bike-more-compact/.
External links
- Hands Up! Adjusting Handlebar Height by Sheldon Brown
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