Will Your Rear Derailleur Handle a Bigger Cassette?

Shimano and SRAM each publish two different limits for every rear derailleur they sell: how much chain slack the cage can take up, and the largest cog the cage can physically clear. This page shows you where to read both numbers for your own model, so you know before you buy whether the cassette you want needs a new derailleur.

The short answer

Two different numbers decide this, and the manufacturer publishes both for every rear derailleur it sells: total capacity and max low sprocket (SRAM calls its version of the second number "max cog"). Check both against the exact cassette and crank you plan to run, because a derailleur can pass one and still fail the other. If it clears both for your combination, keep it. If it fails either one, you need a longer-cage or wider-range model — no adjustment gets around a hard sprocket limit.

Two separate limits, not one

Shimano's own specification handbook prints two separate fields for every rear derailleur it sells: Total capacity and Max. Low sprocket. They look like the same kind of number. They aren't.

Total capacity is how much chain slack the derailleur's spring-loaded cage can take up as the chain moves between your smallest and largest chainring and your smallest and largest cog. It's arithmetic: (largest chainring − smallest chainring) plus (largest cog − smallest cog). On a 1x drivetrain the chainring term is zero, so total capacity is just the spread of the cassette.

Max low sprocket is a different kind of limit. It's set by the physical length of the derailleur's cage and the geometry of its B-knuckle, the pivot that sets how far the upper guide pulley sits from the cassette. Go past that number and the cage or the upper pulley runs into the cassette's largest cog. No adjustment fixes that — it's a clearance problem, not a slack problem.

Work out your own total capacity

Use the actual numbers stamped on your crank and printed on your cassette, not a rough guess: (largest chainring − smallest chainring) + (largest cog − smallest cog) = required total capacity. A 50/34 crank with an 11-34 cassette needs (50 − 34) + (34 − 11) = 16 + 23 = 39 teeth of capacity — arithmetic on those two example parts, not a published spec. Compare that number against your derailleur's own published total capacity. Separately, compare your cassette's largest cog against the derailleur's max low sprocket. Both have to clear.

Real numbers, read off the spec sheet

The table below comes from Shimano's current Specifications handbook and from SRAM's own per-model service pages. It isn't every derailleur either brand sells — look up your exact model number for the definitive figures — but the pattern holds across the catalog.

That pattern is the cage-length system. Within one derailleur family, the model with no suffix or -SS (short cage) carries the smallest total capacity and lowest max low sprocket. -GS (mid-length cage) steps both numbers up. -SGS (long cage) goes highest. Shimano's own product page for the 11-speed Ultegra short-cage derailleur names it exactly that in the page title — "Ultegra Rear Derailleur Short Cage 11s," for the RD-R8000-SS — and the spec handbook backs it up: 35T total capacity and a 30T max low sprocket, against 39T and 34T for the -GS version of the same family.

Look closely at the current 12-speed MTB and GRX rows and you'll notice the Max and Min low-sprocket figures are the same number — 45T and 45T, or 51T and 51T, not a range. Shimano publishes these current derailleurs as matched to one specific largest-cog size, the one used by its own same-generation cassette, rather than "up to" a ceiling. If your cassette's largest cog doesn't match that number exactly, check your own model's row rather than assume a nearby-looking spec applies.

One previous-generation Deore XT model is worth calling out on its own: RD-M8000-SGS publishes a max low sprocket that changes with how many chainrings you're running on that same physical derailleur — 40T for 3x11-speed, 42T for 2x11-speed, 46T for 1x11-speed, all under one 47T total-capacity figure. The handbook is telling you that your own drivetrain layout, not just the derailleur model, changes the ceiling.

SRAM: a matched range, not a total-capacity number

SRAM's current derailleurs don't publish a total-capacity figure the way Shimano does. Look up a specific model on SRAM's own service pages and you'll find a maximum cog size and, separately, a minimum cassette size the derailleur is built for — not a chain-capacity formula you calculate yourself. The GX Eagle rear derailleur (RD-GX-1-B2) is listed with a long cage, a 1x-only configuration, a 52-tooth maximum cog and a 50-tooth minimum — built around 10-50T and 10-52T cassettes, not an open-ended range. SRAM's 2x road derailleurs work the same way but narrower: the Force AXS rear derailleur (RD-FRC-E-D2) lists a 36-tooth maximum and a 28-tooth minimum, with no cage-length choice listed. The Rival XPLR AXS derailleur (RD-RIV1-E-D1), built for 1x gravel and road, lists a 44-tooth maximum and a 36-tooth minimum. If you're on SRAM, check your exact model's service page for its stated maximum and minimum, and stay inside both — there's no capacity arithmetic to fall back on the way there is with Shimano.

Why the two limits fail differently

The two Shimano limits fail differently because they're different kinds of problems. Exceeding max low sprocket is a collision: the cage or upper pulley runs into the largest cog because there isn't room for it, and the derailleur can't complete the shift. Exceeding total capacity is a slack problem, and it shows up at the opposite end of the range, in the small-chainring/small-cog combination — a gear you might rarely use, but the derailleur still has to manage the chain there. The cage runs out of spring travel to take up the extra chain, and the chain goes slack and can slap or jump. Neither failure is "close enough": a derailleur one tooth over its max low sprocket doesn't settle in with a bit more chain tension, because the geometry either clears the cog or it doesn't.

What actually shifts the number on your own bike

Two things on your own bike shift where you land inside the published range. The B-tension adjustment sets how far the upper pulley sits from the largest cog, within whatever room the cage geometry allows — it doesn't create clearance that isn't there, but a poorly set B-tension can make an in-spec derailleur behave like it isn't. Hanger geometry and length also vary by frame, and a non-standard, damaged or bent hanger changes the derailleur's position relative to the cassette. That's why a forum comment that someone "ran a 51-tooth cog on a 45T-rated GS cage" isn't a specification — it describes one bike, one hanger and one B-tension setting, not something you can rely on for yours.

Before you buy: two adjacent checks

While you're checking your cassette's largest cog against your derailleur's rating, it's worth checking two adjacent things before you buy: whether the new cassette actually fits your wheel's freehub body — see which cassette fits your freehub — and whether your chain has enough life left to survive a cassette swap, or needs replacing at the same time — see how worn is too worn.

What to check on your own equipment

1. Find your derailleur's exact model number — stamped on the parallelogram or printed on the cage — and look it up by that number, not by groupset name alone. One groupset name, such as Deore XT, Ultegra or GX Eagle, ships in more than one cage length with different ratings.
2. For Shimano, cross-reference that model number against the current Specifications handbook.
3. For SRAM, open that exact model's page on SRAM's own service site.
4. Check your cassette's largest cog against the derailleur's max low sprocket. This is a hard limit — no exceptions.
5. Work out your total-capacity arithmetic and check it against the derailleur's published total capacity (Shimano) or its stated cassette range (SRAM).
6. If either check fails, the fix is a longer-cage or higher-rated derailleur — not a different B-tension setting, and not a new hanger.

Check it on your own equipment

These take you to the manufacturers and standards bodies the page draws on, so you can look up your exact model.

• Manufacturer specification
  Open Shimano's current Specifications handbook and find your derailleur's exact model number (opens in a new tab)The 'Rear Derailleur' tables (Ver. 2.7, Sep. 17, 2026), listing every current model by its exact code with its own Total capacity, Max./Min. Low sprocket and Max. front difference rows. The document runs to 302 pages, so use your PDF viewer's find function (Ctrl+F on Windows, Cmd+F on Mac) and search your derailleur's exact model code to jump straight to its table. — productinfo.shimano.com
• Official support page
  Shimano's compatibility chart index (opens in a new tab)A directory of numbered compatibility charts, including 'Compatibility between rear derailleur and cassette gear teeth' for road, gravel and METREA. — productinfo.shimano.com
• Official support page
  SRAM Service — look up your exact rear derailleur model (opens in a new tab)A service hub that opens each rear derailleur's own model page, showing its maximum cog and minimum-cassette figures. — www.sram.com
• Manufacturer specification
  Shimano Compatibility handbook (PDF) (opens in a new tab)Cross-reference tables pairing specific rear derailleur model numbers with compatible shifters, cassettes and chains. — productinfo.shimano.com