If you’ve read the ‘basic techniques’ page, you know that I’ve covered the basics of dealing with gears there. Here it is in more detail.
Shifters: The things on the handlebars that you use to change gear are called shifters. There are various different types of these, but they all work in generally the same way. They control the tension on cables that control the gears themselves. Generally they work either by twisting something that is wrapped around the handlebars, or by using the forefinger and thumb. The shifter on the left hand grip controls the chainrings, and the shifter on the right controls the sprockets.
Derailleurs: At the other end of the gear cable is the derailleur. Both front and back derailleurs are mechanisms that guide the chain onto a new ring. Once the cable is tensioned correctly and the indexing screws adjusted the chain should shift cleanly onto each ring and run on it easily. The derailleurs are balanced between springs and the tension in the gear cable, as controlled by the shifters.
Chainset: The set of chainrings – often three of them – mounted between the right pedal crank and the bottom bracket. These are controlled by the left shifter, and are really for broad strokes. The higher chainring is for high speed. The lower chainring will be for power, like when you’re going steeply uphill. The middle one is for everything else. See below.
Sprockets or cassette: These are the set of sprockets mounted on the rear hub. These are, broadly speaking, for fine tuning, although you’ll find you generally shift two or three at a time, as moving one gear is often too subtle to handle a change in terrain.
Chain line: Much as a bike will be described as having twenty-seven gears when it has nine sprockets and three chainrings, this is not strictly true in terms of usable gears. On the middle chain ring, you can use all the sprockets. On the others, however, the chain will only reach just over half of the sprockets without slipping out of gear and eventually damaging itself, the sprockets and the derailleur. So on the higher chainring, you should only use the higher half of the sprockets, and on the lower chainring you should only use the lower half of the sprockets. The way the gearing on a mountain bike is set up, however, the higher end of one chainring’s range will feel much like the lower end of the next, so the ‘lost’ gears are not missed. For example 1-5 and 2-1 will feel similar, as will 2-9 and 3-5.
For the chain to switch onto a new gear, you need the chain to be moving forwards, so you need to be pedaling. However, changing gear with the chain under heavy strain will cause problems. The chain will not shift cleanly but will grind on the gears, and sooner or later this will damage the chain, the gears, the derailleurs, or all of them.
So the solution is to anticipate changes in the terrain. Changing up for speed is less of an important difficulty, because changing late will still work smoothly, but you’ll lose a few moments of speed or control. It is still better to anticipate, just in order to get the most out of the ride. Changing down to climb is much more important. Failing to do so will mean a massive effort to ride, grinding the gears horribly, or possibly stalling completely. So read the trail ahead of you. If you see a climb approaching, build up some speed, change down in enough time that the chain has space to move before you start to climb, and then pick up the pedaling as your speed starts to drop on the hill.
If you should find yourself in a position where you’re in completely the wrong gear for the hill you’re trying to climb – if you grind to a stop and can’t see how to get going again – then lift the back wheel of the bike off the ground, switch down a few gears (normally three sprockets at a time will work ok) and turn the pedals with a hand or a foot until the chain moves.
A final point on gears: For all that mountain bikes have a huge number of gears, you might well not need all of them. Personally, I only use the middle chainring – at all times. The higher sprockets are plenty to give enough speed to propel the bike at speed down singletrack fun, and the lower have enough power to get me up the vast majority of climbs. Given the tension on the chain, it is easier to change the rear gears than the front, and so remaining on only one chainring makes a great deal of sense. However, don’t take my word for it. Ride more, experiment with gears, and find what suits you, in terms of which gears suit your riding and are most economical to change to.