How does cage length affect shifting?
Shorter cage rear derailleurs take up less chain slack than a long cage rear derailleur. The chain is held more snugly, which can result in smoother, more precise shifting. However, longer cage rear derailleurs are capable of taking up more chain slack than a short cage rear derailleur, allowing you to run a wider range drivetrain than would be possible with a short cage rear derailleur. On a mountain bike, your choice is slightly more difficult. Less experienced riders, or riders who ride frequently in steep terrain should probably stick to a long cage rear derailleur. This, combined with proper chain length, will allow the bike to be safely shifted into every gear on the bike. Racers might appreciate the lighter weight and slight shifting improvement a short cage rear derailleur offers. However, these benefits come with a tradeoff – because of the decreased capacity a short or medium cage rear derailleur offers, it may not be possible to safely use the small chainring in tandem with
