Bicycle Stems
The stem connects the handlebar to the bike's fork steering tube.
Types
Bike stems can be:
for threaded forks, often called quill stems
for threadless forks
Most bike stems are rigid--made from one piece so that the angle cannot be adjusted. Other stems are adjustable--the angle with the fork steering tube can be adjusted. Adjustable stems are common on comfort bikes and hybrids, but not-often used on nicer bikes.
Quill Stem on Left, Threadless Stem on Right
Sizes
Stems are measured several ways:
Diameter of fork steering tube they fit--1", 1 1/8" or 1 1/4". BMX and real cheap or old bikes may have odd stem sizes.
Diameter of handlebar they fit.
Length, measured in millimeters from the center of the fork steering tube position to the center of the handlebar position.
Angles
Stems come in different angles, usually measured from the stem centerline and a horizontal line perpendicular to the fork steering tube. A 0 degree stem sticks straight out. A 10 degree fork angles up at 10 degrees. Many threadless stems can be flipped over to achieve a different riding position. For example, many road stems are 10 degrees. A young athlete may want the stem 10 degrees down for better aerodynamics, and an old coot may want it 10 degrees up for more comfort.
High Rise Stems
Many riders want a more upright riding position, which can be accomplished by installing a high rise stem--a stem with a large angle. A high rise stem can be combined with a riser handlebar--a bar that is curved so the grips are above the stem--to achieve a more upright riding position.
Adjusting Stems
Quill stems--on threaded forks--can be adjusted up or down in the fork steering tube, to make your riding position more or less upright. Simply loosen the chock bolt that is on top of the stem, move the stem up or down, and tighten the chock bolt. It has to be very tight. There is a safety line on the stem--don't extend it past the safety line. Usually you will not have to readjust the headset, but you should test the adjustment.
Threadless stems can be adjusted up or down only if there are spacers between the bike's head tube and the bottom of the stem. The stem can be moved downward by removing spacers and either cutting the fork steering tube or putting the spacers between the top of the stem and the top cap of the headset. You will have to readjust the headset.
The angle of a threadless stem can be changed by flipping the stem over as suggested above. Don't do this if the stem angle is greater than 10 degrees, unless you want a real weird setup.
Adjustable stems allow the angle to be changed. Usually, there's a bolt under the stem that controls a little chock. Loosen the bolt counterclockwise, change the angle, and retighten. Be careful not to strip the threads--it's easy to do.
Replacing a Stem
Replacing a stem is an easy way to customize your bike's feel. A longer or shorter stem changes your cockpit length. A lower or higher angle changes the uprightness of your riding position.
Make sure you buy the right size stem--for fork steering tube and handlebar.
If you install a longer or more high rise stem, you may need to replace your brake cable or derailleur cable and housing to accommodate the greater lengths.
The tools are few--usually 5 or 6 mm hex wrenches.
The procedure is simple--take the old one off and put the new one on. Threadless stem bolts are notorious to strip, so be careful. All the bolts loosen counterclockwise and tighten clockwise and should be lightly greased. Test the headset. Readjust a threadless headset.