My currrent cx (and occassional gravel) bike has the following setup: Shimano Dura Ace/Ultegra mix, crankset is 165mm with 46/39, Ultegra medium cage RD and 11/28 cassette. I feel like I need a few more (easier) gears with really steep power climbs. I have a spare Ultegra crankset that is 170mm, 50/34. I get that 34/28 is easier than 39/28, but will the 5mm crank arm difference negate that? Also, with a medium cage RD, could I run 12/30? Thanks!
I’m not sure crank length makes much difference to how any particular gear “feels”.
As for capacity, you’ve currently got 7T difference up front and 17T out back so 24T. You are thinking of moving to 16T up front and 18T out back so 34T. According to CRC the medium cage Ultegra has a 39T capacity and can use an 11-34T cassette so you should be fine.
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Just note that going to a 5mm longer crank will impact your bike fit. So if do make this swap, be sure to adjust your saddle down and adjust the seat fore-aft if you want to keep the exact same saddle position.
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Good call. Actually all my other bikes are 170 cranks, and I feel a tiny bit “off” on the bike w/ 165’s. hadn’t considered the crank length in making adjustments. thanks!
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that kind of hurts my brain, but thanks! I’ll have to study the numbers and see if I can understand them
Sheldon browns website is old and clunky but a great tool once you figure it out. I used it to calculate out converting my tri bike to a 1x and make sure I had the gears I needed - worked great.
That other site listed earlier looks similar and maybe more user friendly
170/165mm = 3% more leverage with the 170 crank, so a 50t on the 170 will feel like a 51.5t on the 165mm crank. Your 46t will feel like a 44.5T on the 170.
Have you thought about running 46x 11-34t 1x? It’d be easier to deal with for CS.
That or changing the inside from 39t to 36t (did you mean 36t?) If you’ve got a 130mm bcd crank, you can run a 38t inside ring, which is about the same as the 170 to 165 change
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Crank arms does Not effect the gear ratio.
But wheel diameter does.
I run a 50/34 up front with a 32T in back. I would typically look for a 1:1 ratio for gravel set up with any type of climbing, but this seems to be just fine - I really should lose 4 kilos and it wouldn’t even be a consideration 
And FYI, I have an Ultegra mid cage derailleur that will handle the 34T in back.
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Neither cranck length nor wheel diameter affect the gear ratio @Ede71 , if you want to be precise.
However both affect the force you need to put in the pedals to push the same watts at a given rpm…
For cranck arms it is as @jfranci3 says: larger crank arms => bigger leverage => „feels“ like you are riding a bigger chain ring
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Fehlst like…
But with a 48 chainring, you will turn a rearwheel with a 16 tooth sprocket 3 Times.
You will make more way with a 28 mm tyre than a 23mm tyre.
In fact IT is easy… divide the big Chainring with the small ONE and there is the ratio. With big I mean the Front Chainring. In Case you ride a Bike with 44x 52. Today the is passable.
I think you should be fine with the medium cage. At least the RD-R8000-GS has a 39t wrap, so (30-12)+(50-34)=34t. Not sure how much that changed over generations, but it should probably be ok.