Bodging a very compact double
 

At the moment I have a 50/34 double on my 'cross bike using a hollowtech II bottom bracket.

I understand that with a 110BCD crank the 34 is my limit. What I'm wondering is if I could buy a mountain bike triple, say a 44/30/22 and remove the 22 ring and use it as a very compact double for my 'cross bike.

I'd prefer to stick with the current BB as when summer comes round I'll be back on the road more and wanting the 50/34. But off road and with my love of spinning low gears the bike is completely overgeared.

It might be easier and cheaper to put a low geared mountain cassette on the rear and not worry about switching the crankset out, because with that you'll have to adjust the front derailleur a good bit every time you switch.

Changing the cassette would give me the lower ratio. But having the big ring as being a 44 would remove the fact that I never use anything above 50/15 and give me a more usable spread of gears.

Cost wise I'd expect it to be the same as one is mech + cassette and the other is crankset. The more I look at it the more I think I'd probably use the 44/30 on the road as well as I'm more of a long distance cyclist in a hilly area.

34/25 = 1.36
30/25 = 1.2

34/32 = 1.06
30/32 = 0.93

50/12 = 4.16
44/12 = 3.66

I understand what you're trying to do. I bought a used 5-arm "micro" triple crank (94/58mm BCD) and put Ritchey 44x29 "2 x 9" rings on it and use it on my commuter/tourer with an 11-28 cassette (or 11-34 for mountainous touring).

But can you do it with 104mm BCD Hollowtech II cranks? I dunno. Can you even get a 30T ring for one of those? If you find a Hollowtech II crankset with those rings, do they even sell them without BB cups, and if so, would the crank even work in your current BB cups? And can you lower your front derailer enough for a 6T drop in the big ring? It's about 2mm per tooth so a 6-tooth change would require lowering the FD by about 12mm (almost 1/2 inch).

If you have gears low enough, I'd stick with what you have. If you need lower, I agree with Rick on the wide range cassette.

you'll probably want the road actuation MTB FD if you go this route.

can't you just buy a 44T 110BCD chainring? unless you really want that 30T.

Does anyone make a 30T middle ring MTB crank? The ones I've seen are 20 or 22/32/42 or 20-22/34/44.

Unless you can find a 30T ring for a MTB crank, just replace the 50 on your current crank with a 44 or 46T ring.

You will have to replace the bb no matter what other crank you use.

IT's not very compact unless you run a 46/24 using the middle and triple of a 110/74. It actually works remarkably well, and can be built with almost no overlapping gears, while still allowing you to climb most anything.

I recommend going the route of getting a standard road triple crankset and putting custom chainrings on the inner and middle positions of that. This will make it easy to get something that matches your current bottom bracket. The BCD on those is normally 130mm for the middle (and outer) rings and 74mm for the inner ring, so you'll be able to get any combination you want. I do this and run a 26-42 setup on my commuting/touring bike (with a 12-27 9-speed cassette) and a 28-46 on my racing/road bike (with an 11-26 10-speed cassette) and lovely the gear choice I get on it with the simplicity of a double cransket (I spin at a high cadence all the time and do a lot of long climbs, so the gears on the compact are just not in the right range for me).

EDIT: If you go for a big difference in tooth count between the inner and next ring like this then make sure you get some sort of chain retention device to prevent chain derailment off the inner chainring when doing fast down-shifts.