I ride primarily in an area with rolling hills but nothing too steep. I am getting 52/36 chainrings and debating either an 11-30 or 11-34 12 speed cassette. I don’t need any more than the 36/30 for my local riding. Was thinking about possibly going up to the 11-34 cassette to add a good climbing gear for the odd time I am in the mountains. There would obviously be a big gap between the 30 and 34 tooth cog, so even when in mountains, wondering if this will be a good setup or if it would be better to swap out chainrings and cassette for those occasions? That approach obviously means buying all the extra components and the hassle of the switch.
Go with the 11-34 and you have a better chain line in the 30.
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12 Speed
11, 12, 13, 14, 15, 16, 17, 19, 21, 24, 27, 30
11, 12, 13, 14, 15, 17, 19, 21, 24, 27, 30, 34
The jumps are all 2-3 except to the 34.
I don’t need anything over 28 locally but I’d like to go to a 30 or 34 so I can stay in the big ring more efficiently over some lumps, especially with a headwind.
I’d still be crossing my chain over from big gear to gear 2 or 3 for a short while (30 seconds) but I’d rather do that than be changing front gears more often on these little hills that always seem to spark accelerations.
I got the lbs to put a 34t instead of a 36t inside ring for the mountains most of my riding is done in the 52t ring (especially round here) with an 11-28 cassette, and that set up suits me. I think however, I’m going to dig out a spare 36/52 (or even a 34/50) for the currently 38/52 commuter (it means changing the BB from Sram to shimano though) with an 11-28t cassette, laptop, mudguards etc that can be a grind on somedays as I have a moderately hilly commute (circa 500-600ft in 11miles). I could get a bigger cassette but I’d have to look into a Wolf Tooth.
11-34
I did it and while not using the 34 too much it is nice to have the option.
I find, if you have a low gear on your bike, you’ll end up using it. Our low gear used to be 39-23, and that was fine, now i ride the same mountains with a 36-30, do the same hills, only slower, some of thats age, some of thats going hey, i can just shift down instead of standing and powering up something. Whatever is fine, but my opinion is it may slow you down unless you stay conscious of how you ride
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If you’re going 12 speed, then for all but the strongest riders, I don’t see why you wouldn’t ride the 11-34 unless you’re a committed flatlander or it’s a crit or TT bike. The 11-34, iirc, has identical ratios to the old 11sp 11-30 with the addition of the 34t.
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Why not go 50/34 with 11/30 in the back? There’s been discussion about terrain and climbing, but how strong of a rider are you? I think a 50t up front for most people allows for jamming in the middle of the cassette while in the big ring a lot more than a 52 does.
For reference, I’m a 4.7-4.8 W/kg rider (FTP 360ish) and I ride a 52/36 with 11/30 12sp. Most of my riding is in rolling hills outside Boston, though I occasionally ride in the mountains of northern New England. The 52t is fine, but I still find myself wanting the 50 for every day riding–better chain line. My previous bike was 50/34 with 11/28 11sp (and I had an 11/30 for climbing).
I went up Lincoln Gap in VT (has a continuous mile that is 15.5%) recently in the 36/30 and having 34/30 would’ve made my life a lot nicer 
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I’m not sure I’ve ever heard someone complain about having too much gear range. The opposite, however…
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Came to say this.
I like having something near 1:1 gearing for longer rides in the hills. I don’t need it for local group rides or training, but 3-4 hours in the hills and it’s useful.
You could do both? If the mountain rides are infrequent then it’s not that bad to change out cassettes. I race on a 11-28 and do most of my regular riding on a 11-32. It takes < 5min to swap back and forth.
Same! I started off the same and didn’t know any better. Somehow I’m stronger now but using 34-34.
Don’t you also need to change out the chain for the different cassette? So need two sets of chains I suppose?
I can out spin a 50/30, which is why I want to go with a 52. The 34/30 is also not quite as low as 36/34 would be. My main concern is that the gap between the 30 and 34 on the cassette will be quite a lot.
I ride a 52-34 with the 11-34 cassette on my Aethos. No issues with this setup. Having the 1:1 is a lifesaver on some of the steep roads in my area. Heck, my house is up on a hill with 12-20 degree pitches.
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Is it a 12 speed shimano? Do you think this setup would work in sustained mountains? or would you switch to something with smaller gaps on the easier gear if doing a trip to the alps or something like that?
Nah, if you size the chain for the bigger cassette then you should be fine. My chain gets a touch loose in the 36-11 combo but I never really ever get in that gear so it’s fine.
My bike is 11spd. No issues with gaps in gearing. Note that going from the inner 36t to 34t chainring isn’t technically “supported”, but I have had no issues, though the shift from 34t to 52t is ever so slightly slower than the stock 52-36, but I tend not to shift under load going from little to big chainring since that usually means I’m starting to go downhill.
Per @KevinKlaes post quoted, the step between 30->34 isn’t the big gap you imagine it to be: proportionally, it’s less than the step between 21->24 that’s present in both cassettes and which you’re not expressing concern about…
And with 30->34 being at the end of the range it’s the one place where you might actually welcome a larger step - even though it isn’t particularly a large step.
=> The 30->34 step size isn’t a reason to avoid that cassette, if you think the 36/34 bottom ratio could be useful to you on occasion.