Main Types Of Bicycle Stems

Direct-Connect Stem: Clamps on the outside of the fork steerer tube, simple structure, easy to install, commonly seen on mountain bikes and road bikes.
Gooseneck Stem: Inserted into the fork steerer tube using an expandable wedge, appearance resembles a "gooseneck," often used on traditional or high-end road bikes.

Key Parameters
Length: Distance from the center of the fork steerer tube to the center of the handlebar clamp.
Mountain bike: Usually 50–80 mm (modern downhill bikes can be as short as 40 mm).
Road bike: Commonly 80–140 mm, medium frames mostly use 100–110 mm.

Angle (Rise/Drop Angle):
Positive angle (e.g., +7°): Raises handlebar height, suitable for commuting and long rides, enhances comfort.
Negative angle (e.g., -17°): Lowers handlebar, suitable for racing, downhill, or aggressive posture.
0°: Neutral horizontal, does not change height, balances control flexibility.

Clamp Diameter:
Handlebar clamp diameter is mainly 31.8 mm (common for both mountain/road bikes), some older or special models use 25.4 mm or 35 mm.
Fork steerer tube diameter commonly 1-1/8 inches (about 28.6 mm).