Changing the Stem
Depending on your body type and riding style, you may ride better with a replacement stem. The stem is the metal part that connects the steering mechanism to the handlebars. There are two variables: the length of the stem, and the angle that it rises away from the steering tube.
How the stem affects your riding:
The photos in this section assume that you're riding a real mountain bike with a newer threadless headset. But the same principles apply to stems in threaded headsets.
Summary: Long and flat = advantage uphill, "body" steering. Short and high = advantage downhill, "twisting" steering. For more information on selecting a stem (and how it affects the "fit" of your bike), see our page on fitting the bike to your body.
Placing a new stem:
Now loosen the bolts that connect the stem to the steering tube. You don't need to take them completely out.
Carefully lower the handlebar, being careful not to kink any cables or break any wires!
Now slide the stem up off the steerer tube.
This bike is being geared up as a downhiller -- short stem, high handlebars.
Once the headset cap is snug, you can tighten up the bolts that hold the stem on the steerer tube.
Now put the handlebars in the receiver at the end of the stem, and loosely attach the end plate.
The new stem will affect the position of your body. Take a test ride. Do you want to skooch the seat back a bit? Or forward? If you bought the new stem to be a more-rad downhiller, you probably want to slide the seat back a bit. If you bought it to keep the front wheel on the ground while climbing, you'll want to move the seat slightly forward.
The new stem also changes how your hand meets the shifters and brakes. Take a test ride and see if you need to rotate the handlebar a bit up or down. Take this opportunity to review how your thumb and fingers meet the controls and make adjustments.