Hello, currently have a set up with 12 speed Duraace Di2 front and rear derailleur.
I have 54/40 chainrings and an 11-34 cassette.
I am changing to a 50/34 chainring.
Would you shorten the chain? and if so by how many chain lengths?
The chain is relatively new (<500) miles.
Thanks,
KR
You could take a couple links out. I switch back and forth between an 11-34 and an 11-40 cassette without needing to worry about the chain, I’m probably slightly more susceptible to chain slap on the 34, but it hasn’t been as issue. You just want to make sure your derailleur is always under tension.
Put it in the 34/11 and see if there is slack in the system or generally looks super funny. If there’s tension I wouldnt worry about cutting the chain.
If I dont have an old chain to copy off or am running a different setup I tend to use the ‘Big, Big’ method.
I think instructions are in here:
This is a good answer too…only issue being if you ever want to put the old crankset back on…you’ll probably need a new chain too.
By which time you would probably need a new chain anyway 
Most likely yea 
You can also calculate the chain length, there is an equation for it, probably on the parktool website. You need to measure the length of your chainstays, plus know the crankset and cassette sizes. I did it once when changing from 50/34 to 52/36. It worked out as 1.5 links longer, but that is difficult to do, so I just left it and it works fine. But the change you want to make is bigger, so I think you might need to shorten it.
Put the chain in big ring and big cog with the quick link in place on the end of measuring, count out 4 links, break chain on the end of the inner plate to line up with the quick link.
ParkTool has a good video for such.
I’m aligned with the general consensus here. You could shorten (and if you’re running DA then might as well get it as dialed as possible!) but you almost certainly don’t need to. Bike shops are notorious for saving time on builds of new bikes by not even bothering to cut long chains, so a new bike often comes with a longer-than-spec chain anyway (though you’d hope they don’t cut corners like that on a DA build) - most customers never even notice.
Park Tool and other sizing methods are good and the videos/photos are great for understanding how to implement their methods, but I’m suprised nobody told you to check the Shimano dealer manual for your rear derailleur. I would always always always double check there before making a cut. In most cases you’ll end up with the same length as you’d get from the Park Tool instructions, but every once in a while Shimano change it up. For instance, I’m pretty sure they say big-big +4 for GRX rather than the usual big-big +2 (which is probably what you’ll find for 9200).
So a bit of a redundant comment for OP here, but if anyone comes across this thread in a search related to a different groupset they should make sure to double check the Shimano manuals just in case!
Just here to make the no doubt expected comment from the waxing crowd:
“Unless you’re waxing your chain, in which case it’ll probably not be showing any wear yet”
So what I ended up doing.
I used the Shimano guide to chain sizing.
My bike original came with a 54/40, 11/30 setup. I checked the chain length setup on this and it was exactly correct.
It turns out that when I switched it to a 50/34 and 11/34, the chain didn’t need to be adjusted at all. It was exactly the recommended length. I do use an 11/30 on the trainer, but I don’t think that will be an issue.
So much ado about nothing in the end.
Cheers,
KR
I would probably shorten the chain, depending upon how it fits with the change.
You’ll also likely have to lower the front derailleur.