Why Is There A Bigger Front Sprocket On High End Downhill Bikes?

Downhill mountain biking races are increasing in popularity in the United States and United Kingdom. Manufacturers are keeping pace by developing bikes specific to the sport. Many of the high-end downhill bikes feature a larger front sprocket as well as special suspension systems to absorb extra shock and large brakes.

Power and Control

A larger front sprocket gives you more speed for any given rpm. That's because it creates shorter gearing, which gives you more power. If you want to continue pedaling to gain speed instead of freewheeling, or when you need to maintain control as you go downhill, you need a system that accommodates your downhill power needs.

Ratio

The larger front sprocket creates a larger gear ratio, which you want for downhill cycling. You get the gear ratio by dividing the size, or number of teeth, on your front chain ring with the size, or number of teeth, in your rear sprocket. The larger the number, the harder -- and faster -- the gear is, note Susanna Mills and Herman Mills in the book "Mountain Biking." Thus, using a larger sprocket on the front and a smaller sprocket on the back puts you into a high gear ratio, which translates to more power. If you use a front sprocket with fewer teeth, this will work your chain harder, which will decrease your power.

Gearing Limits

You get your upper limit for your gearing ratio with the following equation: pi, which is 3.14, times the diameter of your wheel in meters times the number of teeth on your largest chain wheel divided by the number of teeth on the smallest sprocket. You get your lower limit with a similar equation, except you use the number of teeth on the smallest chain wheel divided by the teeth on the largest sprocket. For example, the equation might look like this: 3.14 x 0.7m x 28/34 = 1.8m on the lower limit and 3.14 x 0.7m x 48/14 = 7.5m. Your upper limit is what you use on downhills.

Number of Gears

A bike that has more gears gives you more choices for riding uphill versus downhill. A bike with fewer gears is more sport-specific for downhill. In fact, some downhill cyclists stay in the same gear for the entire ride. It's important to avoid gears that cause your chain to cross from the front sprocket to the back at an angle, such as using the smallest chainrings on both front and back. This is bad for your sprockets and puts strain on your chain, which will cause it to wear out.