Front and Rear Derailleurs

A quick guide to bicycle drivetrain components begins with the derailleurs.  The front derailleur controls the front chainrings, and the rear derailleur controls the cogs (cassette).  To illustrate it more visually, the front derailleur paints the big picture (hills, rolling terrain or flats) and the rear derailleur paints the details (fine-tuning the gearing).

Most modern mountain bikes are 9 speed.  The chainring (front rings) is ignored in this scenario - a bike is not considered a 27-speed because there are certain gear combinations that are not recommended; only the cogs on the cassette are counted.  Very entry-level mountain bikes can still be found with 8 speeds, but 9 is the norm.  Most 9-speed mountain cassettes are either 11-32t or 11-34t, meaning a range of 11 teeth (high) to 34 teeth (low).

Chainrings

Most modern road bikes are 10 speed.  Road bikes can accommodate an extra gear because the chain is narrower (less "beefy") than a mountain bike chain, allowing for closer spacing between cogs in the cassette - and less of a noticeable jump between gears in terms of pedaling difficulty or ease.  Most 10-speed road cassettes are 12-25t or 12-27t.

Chainrings are essentially the same in road bikes and mountain bikes except for the number of teeth.  A typical mountain chainring tooth count is 44 teeth on the big ring, 32 teeth on the middle, and 22 teeth on the inner.  A triple road chainring has 52/42/30, and a double road chainring has a 53/39.  Another option for road bikes is the compact double, a typical example being 50/36.  Strong riders often favor a traditional double, while those who prefer a little easier gearing on hills usually prefer a compact over a triple chainring because of a cleaner chain line (no awkward angles), lighter weight and smoother shifting.

Crankset

A crankset is the chainring/crankarm combination; chainrings can be replaced individually.  The crankarm connects the pedal to the chainrings.  Cranks (as the crankarm is commonly known) come in different lengths, with 172.5 and 175mm being the most common for road and mountain use.