Will an 11-51T cassette work with my 11-speed rear derailleur?
By Crankworthy · Published · Updated
Only if the derailleur is rated for a 51-tooth cog. Plenty of 11-speed mountain derailleurs top out well below that — our catalog records several at 42T — and a derailleur rated for less cannot wrap a 51T cog however the B-tension is set.
Why
Every rear derailleur has a maximum sprocket size it is designed to clear. Beyond it, the upper jockey wheel runs into the biggest cog, and the cage cannot take up the extra chain across the full range of gears.
Wide-range 11-speed cassettes arrived after many 11-speed derailleurs were designed, so a derailleur being "11-speed" says nothing about whether it will handle 51 teeth.
Getting it wrong is the single most common reason a cassette upgrade fails: the cassette fits the freehub, matches the speed count — and still will not work.
How to check your derailleur
- Find the model number
- It is printed on the derailleur body or cage, for example "RD-M5100". Look up its maximum sprocket.
- Use the catalog field
- We record each derailleur's rating as "Largest Cog". It has to be 51 or more for an 11-51T cassette.
11-speed rear derailleurs rated for 51T
Recorded in our catalog as 11-speed with a largest cog of 51 or more.
- Shimano Deore RD-M5100-SGS11-speed Shadow RD+ clutch, long cage, 51T max cog$52.49Amazon
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