branched out for eleven-speed cassettes. With them, new freehub bodies emerged which were either wider (Shimano road eleven-speed) or entirely different (Sram XD, XDr, which differ in width and both provide the space for a ten-tooth cog). For twelve-speed, Shimano introduced a new freehub body as well (Microspline). This progression has provided more fine adjustment of gear ratio, however the use of thinner metal parts has had the effect of shortening the life-span of the chain and sprockets due to so called "stretching" of the chain. This is caused by frictional abrasion of the load-bearing surfaces of the chain causing elongation. As such, the chain and sprockets of a nine-speed system require more frequent replacement than an eight. The narrowing of the hub flanges has created more dish in the wheel, which weakens the wheel if all other factors are the same, but improvements in the strength and reliability of spokes and rims has more than balanced this out, and wheel strength is generally higher despite the increased dish. Improvements in shifting Shift ramps are complex tooth profiles, in the rear sprockets and front chainrings, designed to pick up and drop the chain during shifting. They allow for shifting under greater load than was previously possible, and for smoother and cleaner shifting. The different systems are branded Hyperglide by Shimano, UltraDrive by Campagnolo, and OpenGlide by SRAM. The chain itself is specifically manufactured for ease of shifting, and to interface with a particular manufacturer's shift ramps; using a different type of chain may result in sub-optimal shifting. See also Bicycle gearing Hub gear Sprocket Cycling Mountain biking Cycling terminology References External links Park Tools webpage containing basic freewheel & freehub repair info How to replace a cassette cluster (video) Bicycle gears