This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Cornering requires estimating the required lean angle before reaching
the apex of the turn where the angle with the road surface is the
critical parameter rather the angle with the vertical, as is evident
from banked curves. Lean angle is limited by the available traction
that must be assessed from velocity and appearance of the surface.
For good pavement, this angle is about 45 degrees, in the absence of
oil, water, or smooth and slick spots. Therefore, a curve banked
inward 10 degrees, allows a lean of up to at least 55 degrees from the
vertical, while a crowned road with no banking, where the surface
falls off about 10 degrees, would allow only up to 35 degrees.
Banked curves have a greater effect than just adding to the maximum
lean angle, because with a steeper banking, more of the centripetal
cornering force goes into increasing traction directly into the
banking up to the point of a vertical wall where only the maximum
G-forces limit what speed a bicyclists can attain. In contrast, an
off banked curve makes cornering progressively more difficult until
the bicycle will slip even at zero speed. This effect is more
naturally apparent to riders who exceeded these limits early in life
and have added the experience to expected natural phenomena.
The skill of visualizing effects of speed, traction, braking, and
curvature are complex, but is something humans and other creatures do
regularly in self propulsion. The difficulty arises in adapting this
to higher speeds. When running, we anticipate how fast and sharply to
turn on a sidewalk, dirt track, or lawn, to avoid sliding. The method
is the same on a bicycle although the consequences of error are more
Cornering requires reflexes to dynamics that are easily developed in
youth, while people who have not exercised this in a long time find
they can no longer summon these skills. A single fall strongly
reinforces doubt, so cautious practice is advisable if returning to
bicycling after a long time.