With each new model of mountain bikes there seems to be a more intricate gear system. Today’s bikes can have up to 27 gear ratios. A typical mountain bike uses a combination of 3 different sized sprockets in the front and 9 different ones in back, which produce the various gear ratios.
The reasoning for all these gears is so the rider can keep cranking the pedals at a consistent pace regardless of the slope. To follow this reasoning, picture a bike with only one gear. Each time you would rotate the pedals one turn, the wheels in the rear would rotate once as well. This is a 1:1 gear ratio.
If the wheels on the rear are 26 in. in diameter, with a 1:1 gearing, one complete twist on the pedals means that the wheels would cover 81.6” of ground. If your speed were at 50 RPM, you would cover more than 340 ft. of ground a minute. This only comes to 3.8 MPH, which is no more than walking speed. This might be perfect if you were trying to climb a steep hill, but not very good on flat ground or if you were going downhill.
To increase your speed, you’d need a different gear ratio. To go downhill at a speed of 25 MPH, at a cadence of 50 RPM at the pedals, you’d need a gear ratio of 5.6:1. If your bike has a lot of gears you’ll have many different increments between a gear ratio of 1:1 and 6.5:1. This means you can consistently pedal at 50 RPM, whether you’re going fast or slow.
On a typical 27-speed mountain bike, 6 of the gear ratios on the bike are so close that you wouldn’t really notice any difference from one gear to the next. Normally, bike riders will select a front sprocket that works for the slope they’re on and just stick with it, even though the front sprocket might be hard to shift under a heavy load. It really is much easier to shift gears on the rear.
When cranking up a slope, you should select the tiniest sprocket on the front and then shift gears on the rear since there are 9 gears to choose from. The more speeds you have available on the back sprocket, the greater your advantage. The gears on mountain bikes are vital as this is what determines your overall speed. Without all these gears there would be no way you could build up speed or pound the pedals. The gears are what help you move those pedals so you can build up your speed. There are many types of gears offered on mountain bikes, all designed to help you go fast.