How Mountain Bike Gears Work

The gears in mountain bikes just keep getting more
and more intricate.  The bikes of today have as many
as 27 gear ratios.  A mountain bike will use a
combination of three different sized sprockets in
front and nine in the back to produce gear ratios.

The idea behind all these gears is to allow the
rider to crank the pedals at a constant pace no
matter what kind of slope the bike is on.  You can
understand this better by picturing a bike with
just a single gear.  Each time you rotate the pedals
one turn, the rear wheel would rotate one turn
as well (1:1 gear ratio).

If the rear wheel is 26 inches in diameter, then
with 1:1 gearing, one full twist on the pedals
would result in the wheel covering 81.6 inches of
ground.  If you are pedaling at a speed of 50 RPM,
this means that the bike can cover over 340 feet of
ground per minute.  This is only 3.8 MPH, which
is the equivalence of walking speed.  This is ideal
for climbing a steep hill, although bad for ground
or going downhill.

To go faster you'll need a different ratio.  To
ride downhill at 25 MPH with a 50 RPM cadence at the
pedals, you'll need a 5.6:1 gear ratio.  A bike
with a lot of gears will give you a large number
of increments between a 1:1 gear ratio and a 6.5:1
gear ratio so that you can always pedal at 50 RPM,
no matter how fast you are actually going.

On a normal 27 speed mountain bike, six of the gear
ratios are so close to each other that you can't
notice any difference between them. 

With actual use, bike riders tend to choose a front
sprocket suitable for the slope they are riding on
and stick with it, although the front sprocket can
be difficult to shift under heavy load.  It's much
easier to shit between the gears on the rear.

If you are cranking up a hill, it's best to choose
the smallest sprocket on the front then shift
between the nine gears available on the rear.  The
more speeds you have on the back sprocket, the
bigger advantage you'll have.

All in all, gears are very important to mountain
bikes as they dictate your overall speed.  Without
gears you wouldn't be able to build speed nor would
you be able to pound pedals.  The gears will move
the pedals and help you build up speed. 

There are all types of gears available in mountain
bikes, all of which will help you build up a lot
of momentum if you use them the right way.