Downhill Mountain Bike Derailleur
As if downhill mountain biking wasn't intimidating enough, the names and varieties of different components act to further confuse innocent cycling enthusiasts and first-time consumers. Many bike patr names and functions are much less complex than they actually seem, though. Take the derailleur, for instance. It has a fancy French name, but it plays an easily understandable role in the functional mechanics of a downhill mountain bike.
A derailleur is the mechanism on the right-hand side of any bike - in particular, a downhill mountain bike - that shifts the chain from gear to gear. Before derailleurs were invented, when road and mountain bikers wanted to change gears to make it easier for them to climb, they had to physically dismount their bikes, take their chain ring in their hands, and move it manually from gear level to gear level. If your derailleur breaks, this is that you will be doing to shift gears - but it won't break, because hopefully you invested in a solid, reliable piece of machinery!
Everytime a downhill mountain biker is travelling uphill, and needs to enter into a lower gear, or is travelling downhill, and needs to enter into a higher gear, they will shift their hears, found on their handlebars. The shift in the gears corresponds to the derailleur via wires that run along the frame of the downhill mountain bike. The position of the wire will affect the position of the derailleur. The mountain bike's chain runs through the derailleur - as the position of the derailleur changes so does the position of the chain, as it will move up or down the gear cogs in the rear of the bike.
Downhill mountain bikes have two derailleurs - one for the front gears, and one for the rear. However, some mountain bikes, such as single speed mountain bikes, have no derailleur, as they have no gears - derailleurs are rendered useless by the asbesnce of a gear system.
Some people like to pay upwards of $300 for a derailleur, while others swear that all derailleurs are made of the same materials and perform similarly. Either way, replacing or selecting your mountain bike derailleur is something you may never have to deal with, as all downhill mountain bikes come already equipped with derailleurs. If it does become an issue, though, Shimano is an excellent company that develops derailleur, brake, and gear systems for all expense categories and performance demands.