A key feature of mountain bikes is their multiple gears. Using a cassette of seven, eight or nine sprockets and a triple chainset, mountain bikes have 21, 24 or even 27 gears (or 'speeds'). The system is exactly the same as on road bikes - using a derailleur. This literally 'derails' the chain from one cog to another to achieve a different gear.
Because of the varied trail surfaces it's useful to have a very wide range of mountain bike gears
Because of the varied trail surface - anything from tarmac or rock to grass, sand or gloopy mud - it's useful to have a very wide range of gears. This entails using a
triple chainset (see
Chainsets and bottom brackets) and a wide ratio freewheel or cassette of
sprockets on the rear wheel.
On better quality bikes, the gear mechanism will be screwed into a threaded '
hanger' on the right hand rear dropout of the bike's frame. Lower quality bikes have thinner dropouts that are not strong enough to take a gear, so a separate hanger is required.
A typical mountain bike cassette will have a range of
teeth from 11 or 12 up to 28 or maybe even 34 teeth. This compares with a road cassette, which might only range from 12-19 teeth for racing, or 12-23 for training or hilly roads. Due to the wide range of teeth on both the rear cassette sprockets and on the chainwheels, a longer length of
chain is required and, consequently, the derailleur mechanism must have a longer arm to take up the excess length.
Similarly, the front derailleur (or 'front mech') is slightly different to those used on road bikes, having a slightly deeper inner plate to deal with the very small, inner chainring and travel across three chainwheels instead of just two. The front derailleur uses a clamp secured around the seat tube of the bike. Note that seat-tubes are made in different diameters - so if replacing a front derailleur, do check that your new one will fit your bike!
Modern gear systems are designed to
index or click into place, for precise gear changes. This means that the
gear lever or 'shifter' has to match the gear mechanism (and it involves using a non-compressible outer casing for the gear cable). See also MTB controls.
Not strictly 'gears' but, without a chain, the gears are useless. The best quality chains are very strong and have lateral flex to cope with snaking across the multiple sprockets.