· By Erik Svedin
Which rear derailleur is on the bike? How to find the right replacement
How to tell which rear derailleur is on a bike, what has to match when you replace it, and how BikeOffice finds the right model from a photo.
The rear derailleur that comes in without a box
A customer puts the bike on the counter. The rear derailleur took a hit in a crash, or it has simply done thousands of miles and has play in every pivot. The question is always the same: which rear derailleur is on the bike, and what should replace it?
The answer is rarely as simple as it sounds. The model name is often worn off, the bike is ten years old and the manufacturer has changed its range three times since. Meanwhile the suppliers' catalogues hold hundreds of rear derailleurs, and several of them look almost identical. Here is how to identify the derailleur, what has to match when you pick a replacement – and how to let a photo do the heavy lifting.
Where is the model name on a rear derailleur?
Always start by looking for the model code. On a Shimano rear derailleur it usually sits on the back of the parallelogram or under the outer plate, in the format RD-M3100-SGS or RD-TY300. The code tells you more than you might think:
- RD stands for rear derailleur
- The letters and digits that follow (M3100, R7000, TY300) give the series and model – Tourney, Altus, Acera, Alivio, Deore, 105 and so on
- The suffix (SS, GS, SGS) gives the cage length, the length of the cage that holds the two jockey wheels
SRAM often prints the series name (X5, NX, GX Eagle) clearly on the side, but the exact variant may require you to look at the number of speeds and the cage length too. If the text is worn away, the detective work begins: colour, cage shape, jockey wheel size, whether it has Shimano's lower-profile Shadow design, and whether it bolts straight to the derailleur hanger or through a separate link.
The model code sits right where the grime collects. A photo is faster than a rag and a magnifying glass.
What has to match when you replace a rear derailleur?
Even when you know exactly which derailleur was on the bike, the same model may no longer be available. The replacement then has to match on the points that actually affect how it works:
- Number of speeds – a 7 or 8-speed derailleur is built for a different sprocket pitch than an 11 or 12-speed one
- Cable pull – Shimano and SRAM pull different amounts of cable per click, and some Shimano road and MTB series differ from each other as well. The derailleur has to match the shifter, not just the cassette
- Cage length and capacity – the largest rear sprocket and the difference between the chainrings decide whether you need a short, medium or long cage
- Mounting – a standard mount to the derailleur hanger or direct mount, and whether the bike uses Shimano's mounting link
Miss one of these and the bike shifts badly or not at all. The most common mistake at the counter is swapping to a nicer derailleur with more speeds and then finding that the shifter can't drive it.
Check the derailleur hanger at the same time
A rear derailleur that has taken a knock has almost always bent the derailleur hanger too. Never replace or straighten the derailleur without checking the hanger – otherwise the new derailleur will shift as badly as the old one. We have written more about that in our article on derailleur hangers.
Find the right rear derailleur from a photo with BikeOffice
This is where image search comes in. In the BikeOffice app and in Product search on the web you pick Rear derailleur (Bakväxel) – or Auto, and the system recognises the part by itself – photograph it, and get the most similar rear derailleurs from the catalogue back. The photo is compared with the product photos of hundreds of rear derailleurs from the suppliers, weighing cage length and shape, body shape, colour, logo and model text.
Every match comes with its article number and supplier, so the next step is to check price and stock in Product search and place the order. If the derailleur in the photo is probably not in the catalogue, the system says so and shows the closest alternatives rather than pretending it found an exact match.
Photograph the derailleur from the side, ideally fitted to the bike and in good light. Dirt does not matter much, but a photo where the frame hides the cage makes it harder.
Why it matters for the shop
A wrongly ordered rear derailleur costs more than the shipping. The bike stays in the workshop, the customer waits and someone has to pack a return. When the workshop can give an answer right at the counter – this is an Alivio with a long cage, two suppliers have it in stock – the whole repair gets shorter.
It is the same idea as the derailleur hanger search that has been in the app since the summer. Image search now works for more parts: rear derailleurs, shifters, e-bike displays and derailleur hangers, in the same flow.
Get started
Image search is in the BikeOffice app for iOS and Android and in Product search in BikeOffice on the web. You sign in with the same account in both. Take a photo the next time a rear derailleur without a box lands on the counter – and see whether you find the right replacement before the customer has taken their helmet off.
Frequently asked questions
- How do I know which rear derailleur I have?
- Look for the model code on the derailleur, on Shimano usually in the format RD-M3100-SGS on the back of the parallelogram. The code gives the series, model and cage length. If the text is worn off you can photograph the derailleur in BikeOffice, which compares it with hundreds of rear derailleurs in the suppliers' catalogues and shows the most similar ones.
- What do SS, GS and SGS mean on a Shimano rear derailleur?
- They are the cage length, the length of the cage holding the two jockey wheels. SS is short, GS medium and SGS long. A longer cage handles a bigger largest sprocket and a bigger difference between the chainrings.
- Can I switch to a rear derailleur with more speeds?
- Only if the cassette and the shifter are replaced at the same time. The number of speeds and the cable pull have to match between derailleur, cassette and shifter, or the bike will not shift cleanly.
- Where can I find spare parts for the gears?
- Bike shops order rear derailleurs and shifters from wholesalers such as Shimano, Hartje, Unicykel and CSN. In BikeOffice Product search you search every supplier at once and see price and stock in the same list.
Which rear derailleur is on the bike? How to find the right replacement
