Quite a bit of space is given to stems in this guide because they are a personality creating part on a bike, not unlike cranks. Many of the parts on your bike don't allow you the choice to make such a personal statement about what your bike is to you, like rear derailleurs, up to three makers, with maybe two or three choices. Stems are actually noticed by others, so material, style, and design permit you to quietly make a both a real choice and distinctive statement about your bike.
The parts of a standard stem are the quill, extension and binder, in a few instances the stem may have what we have chosen to call a "stem body" which is used on only a few stems. The quill is that part of the stem which is lowered into, and is tightened inside of, the fork steerer tube. The "stem body" as we call it, is an aluminum tube that surrounds, grips. and is fastened to the stem quill, but is not the extension itself. The Control Tech Control stem is an example of a stem that has a stem "body". Fastened to the quill usually, or the stem body occasionally, is the extension. The extension moves the handle bar forward of the quill, toward the front hub axles, and gives the stem it's commonly known size, dependent on it's length. Common extension lengths range from 70mm to 170mm. At the end of the extension is the binder tube, which holds the handlebar, and the binder fittings, which are the cinch fittings, (sometimes mistakenly called "pinch"). There are two types of bolts used on stems, the first is the expander bolt. This bolt passes through the quill from top to bottom. At the bottom of the expander bolt is the expander wedge. The wedge is frequently made of aluminum or steel, is threaded in the middle for the expander bolt. As the expander bolt is turned, it draws up the wedge, vertically, at an angle, tightening the stem into the fork, as one side of the wedge and one side of the stem, force themselves against the inside of the fork steerer tube. On the front of the binder tube are a pair of cinch fittings, that cinch, or bind the handlebar to the stem. In the binder fittings is a binder bolt., which is installed and seats in one of the fittings and is threaded and anchored in the other.
The extension is welded to the quill at an angle. This angle is called the "rise". The angle may be level, (0 degrees), drop below level where it's referred to in negative terms, "negative 17.5 degrees" or "minus 17.5 degrees" or "-17.5 degrees ", these are ways to indicate a negative rise, again, one where the handlebar is held at an angle less than 90 degrees to the quill. There are also a positive rises, which are referred to as "positive", "plus", or if it's left unexpressed it is agreed that it's a positive rise. Any extension that has an angle greater than 90 degrees from the quill is said to have a positive rise. As a rule, Road Racing bikes have rises that are 0 degrees or a negative rise. Hybrid bikes have a 0 degrees or a slightly positive or negative rise.
As headsets come in three sizes, so must the stem's quill diameter to fit them. The size is actually the outer dimension of the fork steerer tube, but for both fork, headset, and stem, it is referred to as 1", 1 1/8" or 1 1/4". If you actually measure the quill that fits into the 1" fork, and headset, you will find that it's really 7/8" (22.2mm), not 1" (25.4mm). The 1 1/8" quill is really just 1" (25.4mm) in diameter. The 1 1/4" quill measures out to be 1 1/8" or 28.6mm. Again the stem, headset, and fork are all referenced using the matching forks steerer tube outer diameter.
This may seem basic, and please forgive us, stems are sold generally with their extensions measured in centimeters. For consistency, we have given all stem extension lengths in millimeters, because more frequently stems are now described using millimeters. There are 10 millimeters to the centimeter, therefore if you divide the length by 10 you will have it's length in centimeters.
An important characteristic of a stem is the inner diameter of the binder tube which holds the handlebar. In each case we tell you what this diameter is because there are four handlebar center diameter sizes, 22.2mm (7/8"), 25.4mm (1"), 26.0mm, or 26.4mm. Handlebar grips for mountain bikes are made to fit 7/8" diameter tubing, so all mountain bars have a 22.2mm or 7/8" diameter at their ends. Mountain bike stems come with a 25.4mm inner binder diameter to fit the 25.4mm center bar diameter that all bulged mountain bike handles are made to. The 26.0mm inner diameter is the standard Road bike handlebar center diameter. Handlebars for all Road, Triathletic, and Hybrid purposes are made with a center bar diameter of 26.0mm, except those made by Cinelli of Torino, Italy. Cinelli is a Road bike stem and handlebar maker that makes their stems with a 26.4mm inner binder diameter to fit only their handlebars which have a 26.4mm handlebar center diameter. The Cinelli stem works comfortably only with their handlebar, any exceptions are noted in the handlebar section of this catalog.
The term "miter cut" is used in describing the manufacturing process of several stems. This is the term given when a piece of tubing is cut to contour the shape of another piece of tubing that it's joined to. The alternative is a square cut end, that doesn't leave as long a welding surface between the two pieces.
Many riders want to lighten the overall weight of their bike. One of the ways this is done is replace the steel bolts with either aluminum or Titanium bolts. A Titanium replacement bolt section exists toward the end of this section. It is only fair to tell however that a problem exists frequently when a Titanium bolt is threaded into another threaded piece of Titanium, specifically a Titanium binder bolt threaded into an all Titanium stem. Titanium is an unyielding metal, and the tendency is for the two surfaces to not slide against each other, but stop moving and "seize", preventing further installation or, even removal. There are coatings and processes that attempt to prevent this from happening, but this is an intrinsic quality of the metal, and the processes and coatings can't, at this time, be considered really perfect. Replacing with Titanium, the binder bolt in a Titanium stem is not recommended, but Finishline Ti Prep is made to solve this problem.
In Mountain bike stems, one of the considerations is how the front brake cable from the left lever is routed to avoid the cable engaging the brake if you make a hard left turn. On Road bikes the cable is routed in a tall curve vertically, but Mountain levers are mounted with the cable exiting horizontally, and there has to be some means to have the cable drop vertically to the brake if it's to work properly. For this reason several styles of brake cable routing have been developed and largely abandoned, to drop the cable ultimately in a straight line to the brake. One style that was popular is the "macaroni" style. This style has the brake cable exit the lever and loops under the extension tube where the is a welded 90 degrees bent tube that takes the cable from the horizontal and turns it to the vertical drop the brake requires. The name "macaroni" originated because this piece looks like a piece of elbow macaroni pasta. Top routing, is another method, in this case the cable stop is built into the stem extension itself. The extension is drilled top to bottom for a tube that is a brake cable housing stop to permit the brake wire to drop in a straight line to the brake itself. A less common, more expensive method, is roller routing. In this case the brake cable exits the lever and loops horizontally to a brake cable housing stop welded under the extension. The bare brake wire then runs over a vertically held roller that produces the required 90 degree drop. The roller style is desirable because it eliminates much of the friction and binding that can occur while engaging or releasing the brake. A rarely used method is "internal routing". This involves, at time of manufacture, the welding of a tube inside the extension tube, with two holes for the for the cable housing and wire to enter and exit through. The cable and housing loop horizontally returning to the forward end of the extension tube just behind the handlebar. An internal cable housing stop holds the housing while the bare cable moves through an interior tube and exits out the bottom of the extension, dropping vertically to the brake. The most popular style presently is headset cable hanger routing. Two types of cable hanger routing are in use, the headset style, and the binder style. With the headset type a piece of aluminum with a brake cable stop is positioned between the upper cup race and the headset head lock nut. The brake cable exits the lever, loops over the top of the bars, and descends vertically just in front of the quill to make the brake connection. The binder style clamps onto the fork steerer or the stem quill and routs the cable similarly. These are both discussed in the Brake section of this book.
A current trend in Mountain bike riding is "suspension". A part of the bike is permitted to move under the riders weight according to how much shock to bike is recieving. The moving piece has its travel restricted by compressed rubber, oil or air to dampen its movement and cushion the rider. Suspension products are made for the front or rear of the bike. "Front suspension" involves the use of either a shock absorbing/dampening fork, stem, while rear suspension involves the use of oil, air, or spring compression to separate mechanically the rear triangle, or in a few cases the seatpost is used to dampen it.
The Ahead Set concept eliminates some of the weaknesses of the headset and steerer tube, while eliminating the weight of the stem's quill, expander bolt and wedge. The steerer tube of an Ahead Set fork has no threads on it. Threads on a steerer tube are cut into the metal, which removes some of it's wall thickness and in essence creates a spiraling weakness down the tube. In eliminating the threads, there are several problems that go away with them. When replacing forks, rarely is the steerer tube the exact height required, once you figure out the head tube length and the stack height of the headset, you frequently have to cut the steerer down. It's very difficult to cut the threads squarely, so the headset upper cup and headlock nut turn on easily. More commonly, and even good mechanics cut through the threads at an angle, non-threaded steerers don't have this problem. In general use, headlock nuts sometimes loosen up, requiring periodic tightening, if the nut is over tightened, stripping the steerer threads is likely. You would have to buy another fork. The Ahead Set type stems use binder tube cinch fittings to cinch around the steerer tube rather than using a quill with expander/wedge system inside the steerer. With the Ahead Set type stem, you don't have the weight of these parts. To use the Dia Compe name "Aheadª" in relation to bicycle stems, headsets, or forks a license must be purchased from Dia Compe, in essence a royalty is paid to use the Aheadset name. Not having the license doesn't mean that you can't make non-threaded fork steerer items, you just can't use the word "Ahead" or "Aheadset".
Lastly, all the tubing wall thicknesses and lengths are dimensioned in millimeters, even though we know engineers measure metals in thousandths on an inch. Cyclists are familiar with what a millimeter is, rather than, say, ".082 304 SS". This presents measurements consistently, which we have personally measured with a micrometer to the tolerance of the thickness of the paint on the item. Also each of the weights have been acquired from weighing a production specimen of the item on a self calibrating digital scale. The weights are dead accurate, no matter what a manufacturers ad says.