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What's confusing is that what's happening at the front
derailleur is the opposite of what's happening at the rear
derailleur (for details, read the instructions on Shifting the
Rear Derailleur and Shifting the Front Derailleur below).
For example, you can select a gear which will make
pedaling easier on a hill (make a downshift) in one of two
ways: shift the chain down the gear "steps" to a smaller
gear at the front, or up the gear "steps" to a larger gear at
the rear. So, at the rear gear cluster, what is called a
downshift looks like an upshift. The way to keep things
straight is to remember that shifting the chain in towards
the centerline of the bike is for accelerating and climbing
and is called a downshift. Moving the chain out or away
from the centerline of the bike is for speed and is called
an upshift.
Whether upshifting or downshifting, the bicycle derailleur
system design requires that the drive chain be moving
forward and be under at least some tension. A derailleur
will shift only if you are pedaling forward.
CAUTION!
Never move the shifter while pedaling backward, nor pedal
backwards immediately after having moved the shifter. This
could jam the chain and cause serious damage to the bicycle.
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B. Shifting the Rear Derailleur
The rear derailleur is controlled by the right shifter.
The function of the rear derailleur is to move the drive
chain from one gear sprocket to another. The smaller
sprockets on the gear cluster produce higher gear ratios.
Pedaling in the higher gears requires greater pedaling
effort, but takes you a greater distance with each
revolution of the pedal cranks. The larger sprockets
produce lower gear ratios. Using them requires less
pedaling effort, but takes you a shorter distance with
each pedal crank revolution. Moving the chain from a
smaller sprocket of the gear cluster to a larger sprocket
results in a downshift. Moving the chain from a larger
sprocket to a smaller sprocket results in an upshift. In
order for the derailleur to move the chain from one
sprocket to another, the rider must be pedaling forward.
C. Shifting the Front Derailleur:
The front derailleur, which is controlled by the left shifter,
shifts the chain between the larger and smaller chainrings.
Shifting the chain onto a smaller chainring makes pedaling
easier (a downshift). Shifting to a larger chainring makes
pedaling harder (an upshift).