Mountain bike gears are becoming more and more sophisticated. Nowadays it's not unheard of for a bike to have 27 gear ratios. Mountain bikes use a combination of nine different sized sprockets in the back and three in the front to create gear ratios.

The idea behind it is to allow the cyclist to crank the pedals at a constant pace regardless of the terrain the bike is on. It's easier to understand this by imagining the bike with only one gear. On your rotating the pedals once, the rear wheel undergoes one rotation too.

If the back wheel has a 26 inch diameter and the gearing ratio is 1:1, twisting the pedals one full time would result in the wheel covering 81.6 inches of ground. If you are pedaling at 50 RPM, this means the bicycle can cover over 340 feet of ground every minute. This is only 3.8 MPH, which is the same as a person walking. Cycling at this speed is perfect when trying to ascend a steep hill, although it's far from appropriate for going downhill.

If you want to go faster then a different ratio is required. If you wanted to cycle downhill at 25 MPH with a 50 RPM cadence at the pedals, you would need to have a 5.6:1 gear ratio. A bicycle with many gears also means you have lots of increments between a 1:1 gear ratio and a 6.5:1 gear ratio so that you will always be able to pedal at 50 RPM no matter how fast you are riding.

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

Most cyclists tend to choose a front sprocket appropriate for the slope they are cycling on and stick with it even though its tough moving the front sprocket under heavy weight. It is much easier to shift between the gears on the rear of the bike.

If you are trying to cycle up somewhere steep, you should choose the smallest front sprocket and after that shifting between the nine available back gears. The more speeds available on the back sprocket, the greater the advantage you will have.

Generally speaking, its the gears that decide your final speed. If your bike had no gears it would not have been possible for you to even pound your pedals, let alone build up some speed. The job of the gears is to move your pedals and help you build speed.

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