How do I choose a rear derailleur for a wide-range cassette?
By Crankworthy · Published · Updated
Check two numbers against the cassette: the derailleur's maximum cog must be at least the cassette's largest cog, and its total capacity must cover the spread of your gearing. If your current derailleur fails either, it has to change with the cassette.
The two limits
Maximum cog is the largest sprocket the derailleur can physically clear. Beyond it, the upper jockey wheel runs into the cog. A longer B-tension screw may squeeze in a tooth or two, but that is outside the manufacturer's spec and shifting suffers.
Total capacity is how much chain slack the cage can take up across your gears. Work it out as (largest cog − smallest cog) + (largest chainring − smallest chainring). On a 1x drivetrain the chainring term is zero, so capacity is simply the cassette's spread.
Cage length is how manufacturers deliver that capacity: short, medium and long cages (Shimano labels them SS, GS and SGS). A wide-range cassette, or a double or triple crank, generally needs the longer cage.
Where to find the numbers
- The derailleur
- Look up its model number, printed on the body or cage. Our catalog records each derailleur's rating as "Largest Cog".
- The cassette
- The range is in its name — an 11-51T cassette has 11 teeth on the smallest cog and 51 on the largest.
Your options
- Stay within the rating
- Pick the widest cassette your current derailleur is rated for. Often that is already a useful step up in climbing range.
- Change the derailleur with the cassette
- Buy them as a pair, with the derailleur rated for the new cassette's largest cog. Check the speed count matches your shifter.