Stem angle is one of the factors that determines handlebar
height. A listed angle of 0 degrees actually provides a rise of
16-20 degrees, depending on the bike's head-tube angle.
Flexible, athletic riders prefer lower-rise stems, while
recreational riders usually favor higher-rise stems. Stem length
is measured along the side of the stem from the center of the
steerer to the center of the handlebar clamp (see diagram). Keep
in mind that as a stem provides more rise, its effective length
decreases. In the example above, if the +17-degree stem measures
135 mm, it actually extends 108 mm parallel to the ground.