# Chain measurement on a 2021 Trek 9 Rail e-bike : Sram 12 speed Eagle GX



## CaspaPongo (Nov 6, 2021)

I am not sure I have correctly measured the chain I've fitted and can't kind much that can give an absolute assurance. When fitted I definately have issues, and the gear change is definately wrong .
Before I continue, can someone confirm if there is an exact number of links for this bike? 
Large, Trek 9 Rail 2021 e-bike. fitted the Sram directional 12 speed Eagle GX

I have taken what I thought was 1 1/2 links overlap and attached the power link after decompressing the rear sus. and tying it up to ensure the longest travel on the largest ring ,and ensured it was directional etc., pumped the rear sus. back up to 220psi and equalised the chambers.

But I wasn't sure if there was a 'dial in' number of links I could confirm? ( my old chain number can't be trusted as I may have lost links in the dark on a night ride)


----------



## [email protected] (Jan 7, 2019)

The tech information calls out 122 links for that chain. Do you have your old chain? I typically match my new chain to my old chain. Also, if a new drivetrain, be sure to check the B tension on the rear derailleur - the Eagle B adjustment is very finicky which is why they give you the tool to line it up perfectly.


----------



## CaspaPongo (Nov 6, 2021)

Thx Mitch, I could n't trust the old chain length as it was fitted by old owner as replacement with two power links (and meased 120 links or 60 dual links). I have a friend that counted 116 on his identicle bike, the original shops owner had the exact same bike and used 118, and I watched a video saying overlap of 1x outer and 1x inner link which I went with at 118 links (59 duals inclusive of power link). I completely agree that the b adjustemnt screw is impoertant but I don't have the original plastic guage, so went wit hadvice online of 15mm gap between the upper jockey wheel to the nearest tooth on the low (large) sprocket... but I must admit... it's something I am not confident is right.


----------

