Bicycles have gears because people are lazy. To make the bicycle move, you have to push on the pedals. If you only had one gear, you could set the bicycle up so you had to push very hard, but each pedal stroke would make the bicycle go a long way. This is called a high gear. Alternatively, you could set it up so you didn’t have to push the pedals hard, but one pedal stroke wouldn’t take you far. You would have to do a lot of strokes to get anywhere. This is called a low gear.
Both extremes work in their own way, but your body is most efficient pedalling at a medium rate – pushing moderately hard and pedalling moderately fast – between 80 and 100 revolutions per minute. Gears were invented so that you can maintain a steady pedalling rate (’cadence’ – roughly speaking, how fast your legs are going round) while the bicycle travels at different speeds.
Mountain bikes are designed to have a very wide range of gears so you can maintain an efficient cadence both when moving very slowly – for example, up a steep, rough hill at 2mph – as well as when moving very fast – for example, plummeting downhill at 40mph.
Small steps between the gears allow you to make subtle changes from one gear to the next, matching your pedalling speed precisely to the terrain you’re cycling over. In recent years, manufacturers have steadily increased the number of gears on your cassette, giving you smaller, subtler gaps between gears, and making modern bikes more responsive than their oldfashioned counterparts.
Shifting to a higher gear
Less haste, more speed
New cyclists – along with many who’ve been around long enough to know better – are seduced by the idea that in order to go faster, it’s imperative to force the pedals around using as much strength as possible with every stroke. With experience, it becomes plain that this only gives an illusion of speed and, in fact, serves mainly to exhaust you in the short term and wear your knees out in the long term. Generally, you’ll get where you want to go faster, feeling less exhausted, by using a lower gear: your legs spin around faster, but you don’t have to press down so hard on each pedal stroke. It’s worth watching the next person who overtakes you; chances are, their legs will be spinning faster than yours in a lower gear, and that’s the main reason they’re flying past you.
Turning on the power
There are some circumstances in which it makes sense to use a relatively high gear. For short bursts of speed, nothing beats standing up on the pedals in a high gear and hauling the bike forwards. It means you can use your shoulder muscles, pulling up on the bars as well as stamping down on the pedals – perfect for keeping up momentum while you power up short inclines. Sometimes ground surfaces dictate gear choice as well. If you’re approaching a sand trough or trying to cross a swathe of deep, sticky mud, the best approach can often be to shift into a high gear, keeping your cadence low so that your tyres have a chance to find the last scraps of traction without you getting bogged down.
By comparison, road-racing bikes have fewer gears, but they are very closely spaced. This reflects the fact that road-racing bikes are designed to be used at a much narrower range of speeds – fast, very fast and faster. Close ratios are vital to allow the rider to maintain a comfortable, efficient cadence while being able to ride at the speed a tightly packed peloton.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES