Short answer: a replacement rear derailleur has to match four things: the brand and shifting system, the number of speeds, the size of cassette it must wrap around, which decides the cage length, and the way it mounts to your frame. Mounting now comes in three forms: a standard hanger, Shimano direct mount, and SRAM's Universal Derailleur Hanger, or UDH. And if the derailleur stopped working after a fall, check the hanger before anything else, because it is designed to bend first and costs a fraction of the derailleur.
Contents
- The four things that must match
- Brand, system and speed
- Cage length: SS, GS and SGS
- How the derailleur mounts: hanger, direct mount, UDH and Full Mount
- Derailleur hangers
- Is it the derailleur or the hanger?
- Choosing a replacement
- Frequently asked questions
- Checking what fits
The four things that must match
| Match | Why it matters |
|---|---|
| Brand and system | The shifter pulls a set amount of cable, or sends a signal, for each gear. The derailleur must be built to respond to exactly that |
| Speed count | The derailleur must move the chain the right distance between cogs |
| Cassette size and capacity | The derailleur must reach the largest cog and take up the slack chain, which is what cage length is about |
| Mounting | It must physically bolt to your frame, through a hanger or directly |
Get all four right and the derailleur will shift properly. Get one wrong and no amount of adjustment will fix it.
Brand, system and speed
This is the match people most often get wrong, because derailleurs from different systems look alike.
- Shimano and SRAM do not mix. A Shimano shifter with a SRAM derailleur, or the reverse, will not index.
- Shimano road and mountain derailleurs are separate systems at most speed counts, even at 12-speed, so a road shifter needs a road derailleur.
- Shimano GRX 11-speed shares its shifting with Shimano road 11-speed, which is why a GRX derailleur can work with a road shifter. Our GRX reference covers this in detail.
- Shimano CUES uses its own LinkGlide system and does not mix with older Hyperglide groups, as our CUES reference explains.
- Mechanical and electronic do not mix. A Di2 or AXS derailleur needs a matching electronic shifter.
Within a single system, the speed count must match your shifter and cassette. An 11-speed derailleur belongs on an 11-speed drivetrain. Our groupset tiers reference shows which tiers share a system.
Cage length: SS, GS and SGS
The cage is the arm that holds the two jockey wheels below the derailleur. Its length decides how much chain slack the derailleur can take up, and so how wide a cassette and chainring spread it can handle.
| Shimano code | Cage | Typical use |
|---|---|---|
| SS | Short | Road doubles with close-ratio cassettes |
| GS | Medium | Wider road and gravel ranges, many hybrids |
| SGS | Long | Mountain bikes, single-chainring wide cassettes, triples |
SRAM simply describes its cages as short, medium and long.
Two numbers on every derailleur decide whether a cage is long enough: the largest cog it can reach and its total capacity. As an example, Shimano's 105 RD-R7100 is rated for a largest cog of 36 teeth and a total capacity of 41. Our cassette and gear ratios reference shows how to work out the capacity your drivetrain needs.
The practical rule: if you are fitting a wider cassette, check the derailleur first. Moving from an 11-28 to an 11-34 often needs a medium or long cage, and a short cage that cannot reach the big cog will either refuse to shift there or, worse, be pulled into the spokes.
Going longer than you need is rarely a problem within the same system. Going shorter than you need always is.
How the derailleur mounts: hanger, direct mount, UDH and Full Mount
| Mount | How it works | Since |
|---|---|---|
| Standard hanger | The derailleur bolts to a small replaceable hanger on the frame, through a short linking piece called the B-link | Long-standing |
| Shimano direct mount | The B-link is removed and the derailleur bolts straight onto a longer direct-mount hanger, for a stiffer connection | 2012 |
| SRAM UDH | A single standard hanger shape shared by many frame brands, which accepts standard derailleurs from SRAM, Shimano and others | 2019 |
| SRAM Full Mount | No hanger at all: the derailleur bolts directly to the frame's UDH interface | Recent |
UDH is the one to understand, because it is spreading fast. Before it, almost every frame had its own hanger shape, which made finding a replacement difficult, especially away from home. SRAM introduced the Universal Derailleur Hanger to fix that, and most modern mountain bikes and a growing number of gravel and road frames now use it. The Polygon Siskiu T7 UDH and the Pardus Super Sport Gen2 are two bikes in our range built around it. The UDH also rotates backwards on impact, which helps protect the derailleur in a crash.
Full Mount is SRAM's newest step. SRAM's Eagle Transmission mountain bike derailleurs, and its XPLR AXS gravel derailleurs, bolt straight to a UDH-equipped frame with no hanger in between. That only works on a frame built for UDH, so if your frame has an older proprietary hanger, Full Mount derailleurs will not fit.
Derailleur hangers
The hanger is deliberately the weakest link. In a fall, or when the bike tips over on its drive side, the hanger bends so that the frame does not. That is good design, but a bent hanger is also the single most common cause of bad shifting that nobody can explain.
Unless your frame uses UDH, the hanger is specific to the frame, often down to the model and year. That is why it is worth knowing which one your bike takes before you need it.
| Hanger | Fits | Price |
|---|---|---|
| SRAM Universal Derailleur Hanger | Any UDH frame | ₹1,120 |
| Merida dropout hanger | Merida Scultura and Reacto, SRAM type | ₹1,990 |
| Pilo D273 | Cannondale | ₹4,840 |
| Pilo D592 | Merida road, 2015 | ₹3,923 |
| Pilo D17 | Specialized | ₹3,500 |
| Pilo D437 | Trek road | ₹3,995 |
| Pilo D310 | Cervelo | ₹3,995 |
Each Pilo hanger fits specific models, so check the listing against your frame. A hanger that is only slightly bent can often be straightened with an alignment gauge such as the IceToolz E355, though a hanger bent badly or more than once should be replaced, because aluminium weakens each time it is bent back.
Is it the derailleur or the hanger?
| Symptom | Most likely cause |
|---|---|
| Shifting went wrong after a fall or the bike tipped over | Bent hanger |
| Derailleur visibly angled inwards, looking from behind | Bent hanger |
| Chain will not reach the largest cog after fitting a new cassette | Cage too short, or largest cog beyond the derailleur's limit |
| Chain slack and slapping in the smallest gears | Not enough capacity for the drivetrain |
| Shifting gradually worsening, no fall | Usually cable, housing or a worn chain, not the derailleur |
| Derailleur loose or wobbling side to side | Worn pivots, a sign the derailleur itself needs replacing |
Check the hanger and the cable before you buy a derailleur. Most derailleurs we see replaced in a hurry turn out to have been fine.
Choosing a replacement
- Read the code on your current derailleur, usually on the back of the body, such as RD-R7000 or RD-M7100. It tells you the system and speed.
- Check your cassette's largest cog and your chainrings, so you know the capacity you need.
- Check the mount: a standard hanger, a direct-mount hanger, or UDH.
- Match all three, then decide whether to stay at the same tier or move up within the same system.
Rear derailleurs currently in stock include:
| Derailleur | For | Price |
|---|---|---|
| Shimano Claris RD-R2000 | Road, 8-speed | ₹1,980 |
| Shimano Sora RD-R3000 | Road, 9-speed | ₹2,500 |
| Shimano 105 RD-R7000 | Road, 11-speed | ₹4,630 |
| Shimano Tiagra RD-R4000 | Road, 2x11, the 2026 Tiagra | ₹5,380 |
| Shimano Ultegra RD-R8000 | Road, 11-speed | ₹7,840 |
| Shimano GRX RD-RX810 | Gravel, 2x11 | ₹9,420 |
| Shimano GRX RD-RX812 | Gravel, 1x11 | ₹9,560 |
| Shimano SLX RD-M7100-SGS | Mountain bike, 12-speed | ₹6,440 |
| SRAM Rival 22 | Road, mechanical 11-speed | ₹7,900 |
| SRAM Force 22 | Road, mechanical 11-speed | ₹10,990 |
| SRAM Apex 1x11 | Single-chainring, 11-speed | ₹8,450 |
| SRAM X4 long cage | Mountain bike and hybrid, 9-speed | ₹2,990 |
The full range, including derailleurs currently out of stock, is in derailleurs. Standards outlive stock, and prices are indicative and move with exchange rates and import duty.
Frequently asked questions
Can I use a long-cage derailleur on a road bike?
Yes, within the same brand, system and speed count: a longer cage takes up more chain slack, so it handles a wider cassette or chainring spread, and it shifts perfectly well on a road bike. The trade-offs are small, mainly a cage that hangs a little lower and shifting that can feel a touch less crisp. If you are fitting a wider cassette for climbing, a longer cage is often exactly what you need, and it is always better than a cage that is too short.
How do I know if my frame has UDH?
Look at the bolt that holds the derailleur hanger to the frame: a UDH-equipped frame shows the UDH logo on that mounting bolt. Most modern mountain bikes use UDH, and an increasing number of gravel and road frames do too. If you cannot see a logo, tell us your bike's make, model and year, and we will confirm whether it uses UDH or a frame-specific hanger.
Can I fit a SRAM Transmission derailleur to my bike?
Only if your frame is built for UDH, because Transmission derailleurs bolt directly to the frame's UDH interface with no hanger, and they will not fit a frame with an older proprietary hanger. Transmission is also a complete drivetrain rather than a single part, with its own cassette, chain and shifter, so it is an upgrade of the whole system. Check your frame for UDH first, then plan the full drivetrain.
My derailleur went into the spokes. What do I need to replace?
Stop riding it, then check three things: the hanger, which has almost certainly bent or broken; the derailleur itself, whose cage and pivots may be damaged; and the rear wheel, whose spokes may be bent or broken. Often only the hanger needs replacing, which is the cheapest outcome, but a derailleur pulled into the spokes can also damage the wheel and occasionally the frame's dropout. Have it checked before riding again, because a damaged spoke can fail under load.
Should I carry a spare derailleur hanger?
Yes, on long rides, brevets and any trip far from a bike shop, because a hanger is light, cheap and frame-specific, and a bent one can end a ride or a holiday when no shop nearby stocks yours. If your frame uses UDH, a spare is especially easy to find and carry. Our guide to travelling with your bike covers what else is worth packing.
Checking what fits
Send us the code from your current derailleur, your cassette size and your bike's make and model. Ask TBA Guide and we will tell you which derailleur and which hanger fit, before you order anything.
