We all want the chain to be on the right gear, moving precisely at the right moment without it bouncing off and above all without ghost shifting. This is the rear derailleurs duty: mechanical or Di2 electronic, it has to ensure smooth precise shifting. Companies such as Shimano, Campagnolo and Sram invest important resources in research and development to reach the best performances and reducing the weight at the same time. They even managed to use carbon technology in the rear derailleur structure so as to obtain an ultralight and efficient product. In MTB, Sram has revolutionized the market by introducing the single chainring transmission, whereas Shimano has applied the most advanced road technology (Di2) on the off-road combining it with the popular Shadow RD+ rear derailleur.