Rear derailleur total capacity versus maximum sprocket: keep every limit separate
Read total tooth difference, largest/smallest sprocket bounds, front difference and system identity without mistaking one passed calculation for assembly permission.
Published 11 October 2026 · OpenBikeFit
Capacity and rear endpoint are different tests
- Fictional 48/32 and 11–37: (48−32)+(37−11) = 42 teeth.
- Invented limits: total ≤39; largest rear ≤36. Both conflicts remain.
- One ring and 11–40: total difference 29, but largest rear 40.
- Passing 29≤39 does not pass 40≤36; other limits remain independent.
Original schematic, not to scale. These limits belong to no real product. Tooth difference is not chain length, clearance, physical inspection or assembly permission.
On this page
- 01Find the exact derailleur and configuration
- 02Calculate total tooth difference without losing either end
- 03Maximum sprocket answers a different question
- 04Smallest sprocket and front difference stay independent
- 05Read the source matrix, not just one convenient cell
- 06Capacity is not a chain-length recipe
- 07A complete system still needs interface permission
- 08Keep a whole constraint record for discussion
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Retain every numerical tooth pairing, optional wheel quantities, source-specific constraints and unknowns. No ranking, chain-length instruction or compatibility verdict.
Open toolFind the exact derailleur and configuration
Start with the full rear-derailleur model, suffix, generation and source revision, plus the specified front arrangement and compatible control/chain system. A long-looking cage or the letters GS and SGS without a complete model are not universal capacity values. Preserve whether the source concerns one ring, two rings, a particular cassette variant or a frame-specific system. Do not transplant a familiar limit from another model.
Write current and proposed front tooth counts and the complete rear inventory separately. Keep the largest and smallest counts, but retain all original identities too. If a source is missing, the declaration remains unknown. This guide explains reading the constraints; it does not diagnose shifting faults, inspect a hanger or approve a replacement derailleur through a screen.
Calculate total tooth difference without losing either end
For the ordinary multi-ring derailleur accounting convention, required total tooth difference K = (F_max − F_min) + (R_max − R_min). Fictional rings 48/32 and rear endpoints 11–37 produce K = 16 + 26 = 42 teeth. It is a difference sum, not the largest rear sprocket, the total teeth on all sprockets or the number of gears. With one ring, the front difference is zero because both front endpoints are that same known ring.
An unknown front arrangement is not a one-ring arrangement. Unknown rear endpoints likewise cannot be replaced with a customary 11–something range. Keep the separate terms beside the sum so a future change can be traced. This accounting expression does not set actual chain length, pulley position, frame clearance or how much chain is retained in a physical condition.
Maximum sprocket answers a different question
A source-defined largest-sprocket limit restricts the rear endpoint in that configuration independently of K. To see why, suppose a fictional, unnamed source permits K at most 39 teeth and largest rear sprocket at most 36 teeth. A one-ring 11–40 declaration needs K = 29, below the fictional capacity limit, but its 40-tooth endpoint exceeds the separate 36-tooth limit. One passing inequality does not cancel another failure.
Conversely, the earlier 48/32 with 11–37 needs 42 teeth and also exceeds that fictional largest-sprocket limit. Keep both conflicts visible rather than displaying only the first. These numbers are deliberately invented teaching constraints, not specifications for any real derailleur. The diagram does not imply a qualified assembly just because a numerical inequality can be evaluated.
Smallest sprocket and front difference stay independent
An exact source may give both minimum and maximum values for largest and smallest sprocket positions, not merely a maximum. It may also limit front tooth difference. Record which endpoint each field governs; a minimum acceptable largest sprocket is not the same as a minimum smallest sprocket. Wording such as top and low sprocket must be resolved from that document’s definitions before entering numbers.
A fictional total-capacity allowance of 42 with a front-difference limit of 14 does not approve rings 48/32: their front difference is 16. A different rear cassette that lowers K cannot repair that independent conflict. Likewise, satisfying front difference does not establish front-derailleur, chainring or crank compatibility. Do not average limits from two sources or turn a missing maximum into infinity.
Read the source matrix, not just one convenient cell
Shimano’s historical RD-6700/RD-5700 instructions separately label total capacity, largest sprocket, smallest sprocket and front chainwheel difference. That document shows why these are different questions; its numbers and installation procedures are not generalized to modern parts. A current decision needs the actual current manual for the exact model and arrangement, including all footnotes and exceptions.
Keep a small evidence ledger: source title/revision, model and suffix, relevant front arrangement, each limit name and whether it was actually found. If the product page is unavailable or only a search snippet was seen, mark that limitation. A snippet is not a fully checked specification table. Conflicting exact sources should remain a question for the manufacturer or a qualified mechanic, not an invitation to select the more permissive cell.
Capacity is not a chain-length recipe
K is a tooth-difference accounting quantity. It cannot be converted directly into a chain-link count, a cutting instruction or a tension setting. Actual chain sizing and connection depend on the exact system, frame and manufacturer procedure. A chain already present is not proven suitable for a proposed change merely because the new K is smaller. This guide gives no chain-cutting, joining or adjustment sequence.
Frame motion can also change the physical chain path on a suspension bicycle. Eye-to-eye length, stroke, wheel travel and the relevant frame kinematics are separate references; total tooth arithmetic supplies none of them. Do not model an unknown path as fixed. Mechanical condition, mounting, clearance and chain retention need appropriate documentation and physical checks, independently of this numerical ledger.
A complete system still needs interface permission
Before calling anything compatible, resolve the cassette/driver and body/hub identities from the previous guide, chain family, control actuation or electronic system, mounting interface and the source-defined front system. A same-speed label or a passed capacity calculation is not sufficient evidence for those relationships. Numerical gears can be listed even when their physical use remains unresolved.
The existing crankset-interface article remains the owner of shell, spindle, chainring, chainline and related clearance questions. Do not replace it with this narrower capacity sum. A supposed solution based on an unverified adapter, extender or altered limit setting needs its own explicit manufacturer permission; none is generated here. There is no choose-the-best-derailleur score, automatic winner or recommendation to exceed documented limits.
Keep a whole constraint record for discussion
Record raw front counts, rear sequence, exact derailleur identity, control/chain system and sources. List total difference and every independent endpoint/front constraint beside each other. Distinguish known conflicts, missing documentation and physical checks not performed. If only K is known, keep the other questions rather than marking the row compatible. A useful record makes uncertainty legible, not smaller.
Use supplementary paper notes alongside the free blank worksheet; its existing slots are not dedicated chain-capacity or chain-length fields. Reading or opening it creates no saved bike and changes no assembly. Return to the ratio and step guides when a proposed tooth inventory changes, but do not let a desirable numerical result override an unresolved constraint. This journey supplies reference literacy, not installation, riding, fit, medical or performance certification.
Practical questions
Frequently asked questions
Is total capacity the maximum sprocket size?
No. Capacity accounts for combined front and rear tooth differences; the largest rear endpoint has a separate source-defined limit.
How do I calculate capacity for one chainring?
For the ordinary declared convention the front difference is zero and the rear difference remains. This is not permission for a cassette, chain length or whole system.
Does passing capacity prove that my existing chain is long enough?
No. Tooth-difference accounting is not a chain-link count or sizing procedure. The exact system and frame instructions govern chain sizing and connection.
Can a cage-length label supply missing limits?
Not reliably across models. Keep full model/suffix/configuration and the actual current source. Missing limits remain unknown rather than becoming universal GS/SGS values.