Installing a new chain - length and direction
When it comes to installing a chain there isn’t really any difference between Di2 and mechanical groupsets. I get asked this question often enough to publish a quick guide on this though, so here we go.
Chain length
While there are numerous methods and guidelines on how to size a chain, I tend to stick to the Shimano manuals. These show how a chain should be installed, depending on the type of rear derailleur and cassette size.
Note that these manuals are still relevant even if you’re not using a Shimano chain - they’re derailleur specific, not tied to any chain brand or type.
You see, Shimano has two types of derailleur: Shadow RD and "other", or non-shadow RD.
For GRX and 12-speed MTBs, see the off-road chain sizing section below.
Shadow RD
All current and previous generation derailleurs are of the Shadow RD type. This includes the 12-speed derailleurs, 11-speed MTB derailleurs, and the last generation 11-speed road rear derailleurs.
- DURA-ACE RD-R9250 (12-speed), RD-R9150
- Ultegra RD-R8150 (12-speed), RD-R8050-GS (medium cage), RD-R8050-SS (short cage), RD-RX805 (clutch)
- XTR RD-M9050-SS (MTB, long cage), RD-M9050-GS (MTB, medium cage)
- XT RD-M8050-GS (MTB, medium cage)
For all of these rear derailleurs, sizing the chain is surprisingly simple. This is what you do:
Without routing the chain through the rear derailleur, set the chain on the largest chainring and the largest sprocket.
Have the two chain ends meet at the large chainring, and then add two or three links, depending on the zero point shown in Shimano’s image below:
Basically, if the point where the two chain ends meet is the ‘end’ of an outer link, add three links (left image).
If the point where the two chain ends meet is the beginning of an outer link, add two links (right image).
Then add the quick-link to connect both ends, and you’re done!
Road: Non-shadow RD
What is a "non-shadow RD", you ask? Well, all 10-speed Di2 derailleurs and the first generation 11-speed Di2 road derailleurs:
- DURA-ACE RD-9070, RD-7970 (10-speed)
- Ultegra RD-6870-GS (medium cage), RD-6870-SS (small cage), RD-6770 (10-speed)
If you’re using one of the older Di2 derailleurs, the chain sizing method depends on the cassette installed on the bike.
Does your cassette have a largest sprocket of 28T or more? Then use the Shadow RD method described above.
Do you have a smaller cassette? With a largest sprocket of 27T or less? You then put the chain on the largest chainring and smallest sprocket, and route the chain through the rear derailleur
Size the chain so that the derailleur’s jockey wheels are at 90 degrees to the ground. This may be a bit cryptic, but the image below should clarify things:
You don’t need to add any extra links, just connect the chain and you’re good to go. Note that this last method only really applies to non-shadow rd derailleurs and cassettes smaller than 28T.
Off-road (GRX and 12-speed MTB) chain sizing
Do you have a GRX (RD-RX815 (2x), RD-RX817 (1x)) bike, or a 12-speed MTB? Shimano's recommendations for these bikes are slightly different.
This is how you size a chain for off-road use. I'll assume you're using a quick-link for all of these. If you're using a connecting pin, add one extra link.
Hardtail / Gravel
Without routing the chain through the rear derailleur, set the chain on the largest chainring and the largest sprocket.
Have the two chain ends meet at the large chainring, and then add four or five links, depending on the zero point shown in Shimano’s image below:
If you're using a connecting pin, add one extra link.
Full-suspension bikes
12-speed full-suspension mountain bikes require slightly longer chains. The difference is just one link. Here is Shimano's recommended sizing method for full-suspension bikes.
Again, if you're using a connecting pin, add one extra link.
Chain directionality
You’re almost there, but there’s one more thing to keep in mind. Some chains are directional. They’ll either have an arrow ‘printed’ on them, or something that indicates the inside or outside of the chain.
For Shimano chains in particular, pay attention to the text and logos on the chain. These will be on one side of the chain only, and that side should be facing out (away from the frame).
Are you installing a different brand of chain? Then be sure to check the chain manual, or check with the chain manufacturer.
Done!
There you go! You should now be able to size the chain. If you need some chain tools, these are the ones I use:
