How does a rear derailleur work? What does rapid rise mean? Chain wrap? What are high-normal and low-normal?
Rear derailleurs are pulled across the cassette cogs with the shift cable in one direction and returned by a stiff spring in the opposite way. The direction that the return spring moves the derailleur defines “high or low-normal” All road rear derailleurs are high-normal, because the spring returns the derailleur to the smallest cog. “Rapid Rise was Shimano’s marketing term for its low-normal mountain bike rear derailleur. Front derailleurs are all low-normal, because if you cut the cable, they would default to the small chainring or cog. “Chain Wrap” defines how much the rear derailleur will allow the chain to wrap around the cassette cogs. More wrap is considered to enhance shifting, especially in the smaller cogs. Most rear derailleurs have a set screw that butts against the spur on the frame’s dropout. Backing the screw out increases chain wrap by rocking the mechanism forward, which positions the upper pulley slightly in front of the cassette cogs. Overdo it, however, and the upper pulley will bounce on the larger cogs and buzz like a bumblebee.
Contact Richard Cunningham for questions or comments, or just to talk bikes at: askRC@roadbikeaction.com