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...