An integral part of the drive system - the rear derailleurs. When choosing this part, it is important to consider how many gears the bicycle has, and what kind of fixation it has - screwed directly to the frame/axle or through an adapter. If you are changing to a different type of derailleur than you had, the capacity of the derailleur is also relevant - if the capacity is too large, the gears will not work properly. You can calculate the capacity by adding the differences between the big and small chainrings. Shimano’s capacity is usually indicated in the specifications. As technology advances, rear derailleurs become stronger, lighter, and provide smoother shifting. More expensive Shimano models have Shadow+ technology, which makes the derailleur fit closer to the rear block - less chance of snagging and breaking the derailleur. There are also electronic derailleurs, the most widely known technology is Shimano Di2.
