Rear Derailleur Standards Explained: Hangers, Cage Length, Actuation & Compatibility | BikeQuiver Cyclopedia

The rear derailleur is the spring-loaded arm that moves your chain across the rear cassette when you shift. To work correctly it has to match your shifter's cable pull, your cassette's speed count and biggest cog, your chainring setup, and your frame's mounting standard. This guide explains every rear derailleur standard in plain English so you can identify what you have and know what fits.

Why these standards exist

A rear derailleur sits at the intersection of four other parts: the shifter that commands it, the cassette it shifts across, the chain it guides, and the frame it bolts to. Each connection has its own standard. The shifter link is governed by 'actuation ratio' — how far the derailleur moves per millimeter of cable pulled. Shimano, SRAM, and Campagnolo each use different ratios, which even differ between road and mountain lines from the same brand — the reason you generally cannot mix a Shimano shifter with a SRAM derailleur (or road with mountain) and get crisp shifting.

The frame connection has its own messy history. For decades nearly every frame model used its own unique 'derailleur hanger' — a small sacrificial aluminum plate designed to bend or break in a crash instead of your frame or derailleur. With well over 400 distinct hanger shapes in circulation, finding a replacement often meant detective work. SRAM's Universal Derailleur Hanger (UDH, 2019) finally standardized this for new bikes, and its follow-up, the hangerless Full Mount 'Transmission' design (2023), bolts the derailleur straight through the frame's dropout.

Finally, the derailleur's own geometry limits what gearing it can handle: its rated maximum cog size caps how big a cassette you can run, and its cage length sets its 'total capacity' — how much chain slack it can take up across your full gear range. Knowing these few numbers is usually all it takes to tell whether a derailleur, cassette, shifter, and frame will play nicely together.

Modern standards

What you’ll find on bikes from roughly 2018 onward. Everything here is current: clutch cages and wide-range 12-speed are the 2018+ norm, with SRAM UDH (2020+) and hangerless T-Type (2023+) becoming the frame-interface standards to know.

Traditional (frame-specific) derailleur hanger

~1990–present (being displaced by UDH on new bikes)

Also called: Replaceable dropout hanger, Mech hanger

Derailleur attaches via a 10 mm diameter, M10x1 threaded B-knuckle bolt; the hanger plate itself is unique to each frame model

On most aluminum and carbon bikes from the 1990s to today, the derailleur screws into a small replaceable aluminum plate bolted to the frame's rear dropout. The plate is deliberately soft so that in a crash it bends or snaps instead of your frame or derailleur. The catch: hangers were never standardized. Well over 400 shapes exist, differing in bolt pattern, thickness, and offset, and you must find the exact one for your frame model and year.

Why it exists: Aluminum and carbon frames can't be bent back like old steel dropouts, so a cheap sacrificial part protects the expensive frame. Each manufacturer designed its own to suit its dropout shape, creating decades of replacement-part chaos that UDH was invented to end.

Typically found on: Nearly all geared aluminum/carbon road, gravel, commuter, and mountain bikes made before ~2020; steel frames often use an integrated (non-replaceable) hanger instead

SRAM UDH (Universal Derailleur Hanger)

2019–present

Also called: UDH

One standardized hanger; requires a thru-axle with M12x1.0 thread and 12.7 mm thread length; hanger part number AC-DRHG-UDH-A1

Launched by SRAM in 2019 as a free-to-license standard, UDH replaces frame-specific hangers with one universal part (about $15) that any shop stocks. It works with all normal derailleurs — Shimano, SRAM, microSHIFT, and others — and is designed to rotate rearward in an impact rather than bend, then be nudged back into place.

Why it exists: To end the hunt for obscure frame-specific hangers, and (as it turned out) to prepare frames for SRAM's hangerless Full Mount derailleurs, which bolt into the same standardized dropout.

Typically found on: Most new mountain bikes from ~2020 on, and a rapidly growing share of gravel and road frames from ~2023 on (Trek, Specialized, Canyon, Santa Cruz, and most major brands)

SRAM Full Mount / T-Type Transmission (hangerless)

2023–present

Also called: T-Type, Eagle Transmission, Hangerless direct mount

No hanger: the derailleur clamps directly around the frame's UDH-spec dropout on the thru-axle, indexed off the rear axle centerline

Introduced in 2023 with SRAM's Eagle Transmission mountain groups and later RED/Force/Rival XPLR AXS for gravel, Full Mount removes the hanger entirely. The derailleur sandwiches the dropout and locates directly off the axle, so there is nothing to bend out of alignment, no B-screw or limit screws to adjust, and the unit is robust enough to step on. It only fits frames built to the UDH standard and only works as part of a T-Type ecosystem (matching flattop chain and cassette).

Why it exists: Hanger flex and misalignment were the biggest causes of poor shifting; mounting rigidly to the axle centerline gives perfect, permanent alignment and lets the derailleur shift under full power.

Typically found on: Higher-end mountain bikes (XX/X0/GX/S1000 Eagle Transmission), e-MTBs, and 2024+ gravel bikes with RED/Force/Rival XPLR AXS 13-speed

Shimano Direct Mount hanger

2012–present (declining)

Also called: Shimano DRM

A longer two-bolt hanger that replaces the derailleur's B-link; the derailleur's upper knuckle bolts straight to it

A Shimano-specific option from around 2012: compatible Shadow derailleurs shed their small B-link and bolt directly to a special longer hanger, stiffening the mount and easing wheel removal. The frame must accept the direct-mount style hanger. Not to be confused with SRAM's hangerless Full Mount.

Why it exists: Shimano wanted a stiffer, simpler interface and faster wheel changes; adoption was modest and it remains an optional variant.

Typically found on: Some mountain and e-bikes from ~2013–2022 (certain Trek, Kona, Norco, and others)

Shimano road actuation

1984–present

Also called: Shimano 2:1 road pull

Roughly 1.7–2:1 ratio (derailleur moves ~1.7 mm per 1 mm of cable) for 8/9/10-speed; 11-speed road uses a new, slightly different pull

The cable-pull system used by all Shimano mechanical road shifters (STI levers). All Shimano 8-, 9-, and 10-speed road parts share one ratio, so those generations mix fairly freely. With 11-speed (Dura-Ace 9000 onward), Shimano changed the pull, so 11-speed road shifters need 11-speed road derailleurs.

Why it exists: A brand-proprietary ratio dating to Shimano's original SIS indexing (1984); the 11-speed revision tightened tolerances as cog spacing shrank. Old Shimano MTB derailleurs (9-speed and earlier) share the same 2:1-family pull, which is why classic road/MTB mixing tricks worked.

Typically found on: All mechanical Shimano road and gravel drivetrains: Claris, Sora, Tiagra, 105, Ultegra, Dura-Ace, mechanical GRX

Shimano Dynasys (MTB wide-pull)

2010–present

Also called: Dynasys, Dynasys 11, Shimano MTB 10/11/12-speed pull

~1.2:1 actuation ratio, distinct from Shimano road pull

Starting with 10-speed mountain groups in 2010, Shimano switched its MTB line to a new lower ratio called Dynasys. From that point on, Shimano mountain shifters and derailleurs stopped being compatible with Shimano road parts of the same speed count. 12-speed MTB (M6100/M7100/M8100/M9100) and the newer CUES/Linkglide families continue with their own matched pulls.

Why it exists: A lower ratio makes shifting less sensitive to cable friction and hanger flex — valuable on mud-exposed mountain bikes with long housing runs.

Typically found on: Shimano Deore, SLX, XT, XTR 10/11/12-speed; CUES on commuters and e-bikes

SRAM Exact Actuation

1990s–present (mechanical road through 2019 Rival/Force 11-speed; Apex 1)

Also called: SRAM 1:1 (family), Exact Actuation

1:1 actuation ratio — 1 mm of cable pull moves the derailleur ~1 mm

SRAM's original hallmark, first on 1990s MTB groups as '1:1' and carried into road groups (2006 onward) as Exact Actuation. All SRAM 9/10-speed MTB and all SRAM 10/11-speed mechanical road parts share this family, which allows some handy cross-matching — e.g. a SRAM 11-speed road shifter can run certain SRAM MTB derailleurs of the same family.

Why it exists: SRAM chose 1:1 partly to differentiate from Shimano's patents and partly because equal pull-to-movement is more tolerant of cable friction and contamination.

Typically found on: SRAM X5/X7/X9 9/10-speed MTB; SRAM Apex, Rival, Force, RED 10/11-speed mechanical road and many gravel builds

SRAM X-Actuation

2012–present

Also called: X-Actuation, Eagle actuation

~1.12:1 ratio, unique to SRAM 11- and 12-speed mechanical MTB

When SRAM launched 1x11 (XX1, 2012) and later 12-speed Eagle (2016), it introduced a new pull ratio called X-Actuation. These derailleurs only work with SRAM 11/12-speed MTB trigger or grip shifters — not with SRAM road shifters or any 10-speed parts.

Why it exists: The wider cassettes (10–42t, then 10–50/52t) and tighter cog spacing of 1x drivetrains needed a purpose-tuned ratio for accurate indexing across a huge range.

Typically found on: SRAM NX, GX, X01, XX1 Eagle 12-speed and SX; earlier X1/X01/XX1 11-speed mountain and gravel bikes

Electronic shifting (Di2, AXS, EPS)

2009–present (Di2), 2015–present (eTap/AXS)

Also called: Shimano Di2, SRAM eTap / AXS, Campagnolo EPS

No cable ratio; motorized derailleur commanded by wire (Di2/EPS) or 2.4 GHz wireless (AXS, newer Di2 levers); AXS uses a removable rechargeable battery on the derailleur, Di2 a central battery (BT-DN110/DN300 family)

Electronic derailleurs replace the cable with a motor. Shimano Di2 (2009 onward) runs wires to a shared internal battery; current 12-speed Di2 is 'semi-wireless' (wireless levers, wired derailleurs). SRAM eTap/AXS (2015 onward) is fully wireless with a swappable battery on each derailleur. Campagnolo EPS is the wired equivalent, now joined by wireless Super Record. Electronic systems self-trim, never suffer cable stretch, and are configured by app.

Why it exists: Removing the cable removes friction, stretch, and hanger-flex compensation from the rider's hands, giving identical shifts for the life of the system and enabling features like automatic front trim and cross-brand 'mullet' pairings within one maker's wireless protocol.

Typically found on: Mid- to high-end road, gravel, and mountain bikes: Shimano 105/Ultegra/Dura-Ace Di2, XT/XTR Di2; SRAM Rival/Force/RED AXS, GX–XX Eagle AXS; Campagnolo EPS/Super Record Wireless

Clutch derailleurs

2011–present

Also called: Shimano Shadow RD+, SRAM Type 2 / Type 3 Roller Bearing Clutch

No fit dimension — a damping mechanism on the cage pivot; Shimano's has an on/off lever, SRAM's is always on with a Cage Lock button for wheel removal

A clutch adds one-way friction to the derailleur cage so the chain can't whip up and down over bumps. It dramatically reduces chain slap and dropped chains, which is why virtually every modern mountain and gravel derailleur has one (Shimano brands it Shadow RD+, SRAM calls it Type 2/2.1/3). Road derailleurs usually omit it to save weight and drag, though Shimano GRX and some Ultegra RX models include it.

Why it exists: The rise of 1x drivetrains removed the front derailleur that used to help retain the chain; a clutch plus a narrow-wide chainring keeps the chain on over rough terrain.

Typically found on: All modern MTB derailleurs, Shimano GRX and SRAM gravel/XPLR lines, many e-bike drivetrains

Cage length designations (SS / GS / SGS)

1980s–present

Also called: Short / medium / long cage, Shimano SS-GS-SGS

Shimano suffixes: SS = short, GS = medium, SGS = long; typical total capacity roughly 29–33t (SS), 33–41t (GS), 43–47t (SGS)

The cage is the plate assembly holding the two small pulley wheels. Shimano appends SS/GS/SGS to model numbers (e.g. RD-R7000-GS is a medium-cage 105). SRAM historically used short/medium/long wording; modern 1x-specific derailleurs come in a single cage sized for their intended cassette. Longer cages wrap more chain — needed for wide-range cassettes or 2x/3x chainring setups — while short cages shift snappier and hang less close to the ground.

Why it exists: One derailleur geometry can't cover everything from an 11–28t road cassette to a 10–52t MTB monster; offering cage options lets one series span many gearing setups.

Typically found on: Every derailleur line; SGS/long dominates modern MTB, SS/short dominates road, GS/medium is typical for gravel and wide-range road

What to record about yours

These are the values worth writing down when you add this part to your quiver — the critical ones drive compatibility matching.

Spec	Matters for fit	Common values
Speeds(speeds) critical	The number of cogs on the rear cassette the derailleur is designed to index across. A derailleur must match the speed count of the shifter, cassette, and chain — an 11-speed derailleur will not index correctly with a 12-speed shifter.	12 · 11 · 10 · 9 · 8 · 13 · 7
Actuation / Cable Pull System critical	The ratio between cable pulled by the shifter and sideways derailleur movement. Shifter and derailleur must use the same system — this is the main reason brands and road/MTB lines cannot be freely mixed.	Shimano road · Shimano MTB Dynasys (10/11/12-speed) · SRAM X-Actuation (MTB 11/12-speed) · SRAM Exact Actuation (1:1) · Shimano MTB 2:1 (9-speed and older) · Campagnolo · Electronic (no cable)
Mount Standard critical	How the derailleur attaches to the frame: a traditional frame-specific hanger, a standardized SRAM UDH hanger, or SRAM's hangerless Full Mount which bolts directly through the dropout and only fits UDH-ready frames.	Standard hanger (10 mm B-knuckle bolt) · SRAM UDH · SRAM Full Mount (T-Type, hangerless) · Shimano Direct Mount hanger
Cage Length critical	The length of the arm holding the two pulley wheels. Longer cages handle wider gear ranges (bigger total capacity and bigger cogs); shorter cages shift faster and clear the ground better. Shimano marks these SS (short), GS (medium), and SGS (long).	Long (SGS) · Medium (GS) · Short (SS)
Max Cog Size(teeth) critical	The largest cassette cog the derailleur can reach without the upper pulley jamming into it. Running a cassette bigger than this rating causes poor shifting or damage.	51 · 52 · 50 · 46 · 42 · 36 · 34 · 33 · 32 · 30 · 28
Total Capacity(teeth) helpful	How much chain slack the derailleur can take up, calculated as (largest cog − smallest cog) + (largest chainring − smallest chainring). Exceed it and the chain hangs loose in small gears or binds in big ones.	47 · 45 · 43 · 41 · 39 · 37 · 35 · 33 · 29
Mechanical or Electronic critical	Mechanical derailleurs are moved by a steel cable; electronic ones use a motor commanded by wires (Shimano Di2, Campagnolo EPS) or wirelessly (SRAM AXS, newer Di2). Electronic and mechanical parts never mix, and electronic parts only pair within their own brand ecosystem.	Mechanical (cable) · Electronic wireless (SRAM AXS) · Electronic (Shimano Di2) · Electronic (Campagnolo EPS / Super Record Wireless)
Clutch helpful	A friction or spring damper on the cage pivot that stops the chain from bouncing and slapping on rough ground — near-universal on modern mountain and gravel derailleurs, and important for chain retention on 1x drivetrains.	Yes — Shimano Shadow RD+ (on/off lever) · Yes — SRAM Type 2/2.1/3 Roller Bearing Clutch · Yes — SRAM T-Type overload clutch · No clutch
Brand & Series helpful	The brand and groupset (e.g. Shimano Deore XT, SRAM GX Eagle) pins down speed count, actuation system, and typical capacity, since these are consistent within a series generation.	Shimano Deore / SLX / XT / XTR · Shimano Claris / Sora / Tiagra / 105 / Ultegra / Dura-Ace · Shimano GRX / CUES · SRAM SX / NX / GX / X01 / XX1 Eagle · SRAM Apex / Rival / Force / RED · Campagnolo Centaur / Chorus / Record / Super Record · microSHIFT Advent / Advent X
Model Number optional	The exact code stamped or printed on the derailleur (e.g. RD-M8100-SGS) identifies everything at once — series, generation, and cage length — making it the fastest way to look up official specs.	RD-M8100-SGS (XT 12s) · RD-R7000-GS (105 11s) · RD-M5120-SGS (Deore 10/11s) · RD GX Eagle AXS · RD-RX812 (GRX 11s 1x)

How to identify which one you have

By markings

Look for the model code printed or stamped on the back of the outer cage plate or on the parallelogram body — e.g. 'RD-M8100-SGS' means Shimano XT 12-speed, long cage; SRAM derailleurs say the series (GX Eagle, Rival) on the cage or knuckle, plus 'Type 2.1/3' if clutched and 'AXS' if electronic. Search that exact code online for the official max cog, capacity, and speed spec.

By measuring / counting

Count the cogs on your rear cassette — that is your speed count and the derailleur must match it. For cage length, measure center-to-center between the two pulley bolts: roughly 55 mm or less is short, 70–80 mm medium, 85 mm+ long. Read the tooth count stamped on your largest cog and smallest chainring to sanity-check against the derailleur's rated max cog and capacity.

By the frame mount

Look at where the derailleur meets the frame. A small separate aluminum plate between derailleur and dropout is a traditional hanger (photograph both sides before ordering a spare — shape matters). A chunky one-piece hanger that wraps the dropout and tightens with the thru-axle is a SRAM UDH. If there is no hanger at all and the derailleur clamps directly around the dropout, it is SRAM Full Mount (T-Type / Transmission).

By cables and batteries

A steel cable entering the derailleur means mechanical. A thin wire and no cable means wired electronic (Shimano Di2 or Campagnolo EPS). No cable or wire at all, with a rectangular battery clipped to the back, means SRAM AXS. A small on/off lever on the cage pivot marks a Shimano clutch (Shadow RD+); a round 'Cage Lock' button marks a SRAM clutch derailleur.

Compatibility gotchas

Match the speed count everywhere: derailleur, shifter, cassette, and chain must all be the same speed (e.g. all 11-speed). The chain's width and the shifter's click spacing are both tied to it.
Shifter and derailleur must share an actuation system. Safe pairings: any Shimano road 8/9/10-speed together; Shimano 11-speed road with 11-speed road; Shimano Dynasys MTB (10/11/12) only with matching MTB shifters; SRAM Exact Actuation parts together; SRAM X-Actuation (11/12 MTB) only with SRAM 11/12 MTB shifters. Shimano shifters never index SRAM derailleurs, and vice versa.
Shimano road and Shimano MTB stopped being cross-compatible at 10-speed: a 10-speed road STI lever will not correctly drive a 10-speed Dynasys MTB derailleur. At 9-speed and below, Shimano road and MTB mix freely.
Check max cog size before upgrading a cassette. A short-cage road derailleur rated for a 28t cog will not climb onto a 34t; hanger extenders (e.g. Wolf Tooth RoadLink) can reposition a derailleur for a bigger cog but do not increase its chain-wrap capacity.
Total capacity formula: (largest cog − smallest cog) + (largest chainring − smallest chainring). If your gearing exceeds the derailleur's rated capacity, the chain will droop in small-small or bind dangerously in big-big.
SRAM T-Type Transmission is a closed ecosystem: it requires a UDH-spec frame, a T-Type flattop chain, and a T-Type cassette on an XD driver. You cannot put a T-Type derailleur on a frame with a traditional hanger, and older Eagle 12-speed shifter pods will not control it mechanically (it is AXS-electronic only).
A UDH-equipped frame accepts any normal derailleur (Shimano, SRAM, microSHIFT) via the UDH hanger — UDH standardizes the hanger, not the drivetrain.
Within SRAM AXS, wireless parts cross-talk: a road AXS shifter can run an Eagle AXS rear derailleur (the popular 'mullet' gravel build), but AXS parts never mix with mechanical or with Shimano Di2.
Shimano Di2 generations matter: 11-speed Di2 derailleurs, wires, and batteries do not mix with the current 12-speed semi-wireless generation. Stay within one Di2 generation and speed count.
Electronic derailleurs need charging: SRAM AXS batteries pop off and recharge in a cradle (swap between derailleurs in a pinch); Shimano Di2 charges a central battery in the frame via the lever or junction port.
Campagnolo is its own world: Campagnolo shifters, derailleurs, and (mostly) cassettes pair only with Campagnolo. Cage and cog limits still work the same way.
If shifting is mysteriously bad in the biggest cogs, suspect a bent hanger before blaming the derailleur — traditional hangers bend easily and a shop can check alignment with a gauge in minutes. This is exactly the failure UDH and Full Mount were designed to reduce.

References

Related components

Cassette Chain Shifters Front Derailleur