(Say: dee - ray - lure) - Also called a "shifter" or in England, a "mech," a derailleur is a mechanism that literally derails the chain moving it to another cog or chainring. There are rear and front derailleurs. The rear shifts the chain across the cogs. The front moves the chain between the chainrings. You must be pedaling to shift and it's best to use light pedal pressure when shifting.
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