compact double advice

My cx bike that I'm using as a commuter (Bianchi Axis) has a super-low triple on it (48/38/28). The shifting is a bit slow, I NEVER use the granny ring, and I've been finding lately that I always want a bigger gear than I have downhill, and now, frequently on flats too, even when I'm carrying a backpack.

It has a huge mtb rear cluster, I believe it's something like 11/34 (9sp).

So, I do enjoy having lower gears than on my roadbike. Commuting, I usually have an extra 20-30 lbs in my backpack, and there's one hill that's about a mile long with one somewhat steep section (not killer, but maybe 7-8%). There's also an outside chance I might someday want to try cx racing, so I'd want a setup that wouldn't rule that out. Plus, although I have road tires on it now, I'll be putting cross tires back on it when the weather gets worse, and that slows me down a little.

I'm considering switching to a compact double. Since the bike has 105, and I believe the shifters are double/triple compatible, I'd only have to change the cranks and maybe the front derailleur (but I think that would be more a weight saver than a functional necessity -- and I'm not super weight conscious with this bike). I'm hoping this would give me a little additional high-gear power, make the shifting a bit quicker, save a little weight, while preserving some lower gears for riding up hills with a load.

Do you have any advice? Does this sound like a good idea? I'm a little worried about the jump from 50t to 34t -- do those of you with compact cranks find there's a range where you have trouble finding the right gear, the big ring is too big but the 34 ring is too easy? Or not? Any advice would be appreciated.

Thanks for your help!

I don't have a compact double, so take this with a grain of salt...

My thoughts are that if you left the big mountain cluster on the back, that might help mitigate the loss of gears you're worried about. For instance, with a 34 on the back, that would make the 50 up front easier.

When I switched out my road bike's rear end to mountain derailleur and large cluster, I went online (WISH I could remember the site) and found a gearing table that showed me ALL the gear ratios in inches, so I could truly compare what I currently had to what I was considering going to.

Oh, and a thought about one day doing the other events...if you kept your low gear chain ring after you swapped it out, it wouldn't be that hard to have it re-installed for that what-if racing someday.

Just my .02 ...and probably only worth about that much! :p

Thanks Starfish, it is helpful :) .

It's actually not so much the low gears I'm concerned about losing, it's the in-between gears I get on my 38t ring & smaller cogs (11-13 or 14). It's a good suggestion to look at a gear ratio calculator, I actually hadn't thought of that. Time for Sheldon Brown I guess.

Yes, check the Sheldon Brown gear calculator! I use a compact, and the range of gears is excellent. Yes, there is less overlap than in your current setup, but plenty of overlap IMO to have smooth gear shifts. I always feel like I can find the right gear (until I'm looking for gear 0 on the steepest climbs!). :D

You are describing the set up I have on both my bikes, 11/34 in back and 50/34 in front . Nice wide range, and easier to keep tuned than a triple. No problem with the 50/34 shift. To me the only disadvantage is the larger gaps between gears in the rear, but you have that already so go for it!

If you get a good compact (such as DA) and FD, you should have no problems shifting between the 34 and 50. I find my FSA is finicky a lot of the time, and there's nothing I can do with my 105 FD to make it better. AND I have a 36 small ring. I just have to be mindful of where on the cassette I am when I shift.

With the MTB cassette, have you thought about a 36 small ring? You might be able to spend more time in the small ring that way and less time shifting back and forth. Plus, shifting with the 105s might be a tad easier. 36 rings are a little harder to find, though.

Also keep in mind if you want to use the same crankarms what your BCD is. Compacts are generally 110mm while 130mm is common for other cranksets.

Sounds like a good idea to me- if I had a cross bike that's exactly the gearing I'd want.

I currently have a compact on my tri-bike. It works really well. I proably can't comment on whether I have enough of the in between gears (I have a 10 spd on that bike and don't need the real easy gears so an 11-25 is great in the back for that type of riding). With only 2 rings in the front I've found I can get away with cross-chaining pretty far (the bike has bar end shifters though so I can fine tune really well).

Another thought- how you'll be riding the bike may have an affect on how well you want to fine-tune the middle gears. On my road bike I like to be able to be in exactly the right gear. On my ancient touring bike (the one I use for commuting and dirt roads- kind of like a cross bike to me) I'm not so picky- it has a triple with a really big middle ring- I spend almost all my time in that middle ring- the back has 7 speeds and quite a range. On that bike I don't mind at all that I don't have every possible gear available in the middle (I tend to meander more on that bike and my whole mindset is different).