jchabalk 

Hi there - i'm working some things out here and wanted to get your opinions on this.

I started spec'ing this out a couple of months ago and had the gearing set up mostly as a traditional touring bike. Since then i've been second guessing myself on the touring setup (i got bounced off my bike by a car and kind of broke both of my arms which is keeping me off the bike for a few weeks and giving me time to think about this sort of thing)

I'm going to use the bike for unloaded day rides (mostly) and potentially some short touring rides mostly around the bay area. I want gearing that can serve as a solid road bike but still have enough of a granny to be comfortable hauling some extra weight uphill. I have two different configurations that i'm playing with right now:

More Road:
30/39/52 + 11-34 = 45t capacity
Mountain (XT) RD, Road FD (undecided, whatever fits)
Max Gear Inches: 126.5
Min Gear Inches: 23.6

More Tour:
26/36/48 + 11-34 = 45t capacity
Mountain (XT) RD, Road FD (undecided, whatever fits)
Max Gear Inches: 116.8
Min Gear Inches: 20.5

Overall the Gear Inches differences are small, but the 45t difference seems rather large for something that i want to use as a road bike a good deal of the time. (It looks like i exceed and road oriented RDs' capacity pretty well) Am I expecting too much trying to use a long-cage mountain bike derailleur at the max advised tooth capacity for road riding? (not "racing" mind you, not even "competitive riding" either, more akin to randoneering/killing-time/commuting/exercise/beer-runs-30-miles-away/grocery-getting-in-the-next-county all that sort of stuff)

Should i roll with a more traditional road-triple configuration? I'm kind of leaning towards the "More Road" configuration above but would love to know if there's a reason to stay away from a mountain derailleur for road use. (weight is not a primary or even really a secondary concern for me). Function over form, function over weight. Compatibility over incompatibility. Shimano STI Brifters

I thought about posting this in the touring forum, if you think mechanics isn't the right place for it please let me know

thanks in advance for any opinions and pros/cons!

dabac

A short cage RD can possibly be a bit crisper in its gear changes, but then again as long as it's reasonably clean and lubed I'd happily use anything down to Deore level w/o feeling that I'm missing out on something.

My suggestion is that you start by figuring out what top gear you'll actually be likely to use most of the time. I find that even on a very road adapted MTB(26" wheel and 46T biggest ring) I'm rarely at the risk of spinning out unless I'm doing powered descents.
Sheldon Brown has a nice gear calculator that lets you see which speed you get for a certain gear and cadence. That's a good starting point for determining what top gear you need, and then you can see how low you can get from that set-up.
Depending on the amount of handiwork you're comfortable with another option is to go with a quad in front to widen the range. Or if you're willing to accept the price you can go for a Schlumpf BB, which'd let you use a regular road set-up and still get a whole range of climbing gears below that.
Or there's an SRAM rear hub that holds 8 external gears and 3 internal gears.
Combine this with a triple of your choice, set it up around the middle internal gears(no drag there) and dropping to the low internal will cut a further 27% of your gear ratios.
If you're not interested in coasting downhill you can go to the high internal and get a 36% higher range at the cost of some drag.
Do note that changing up on the internal and changing down on the chainring is likely to pretty much cancel each other out, giving duplicate ratios.

HillRider

I'd use the road crank for everything and change the 30T granny for a 26T ring. Yes, it works and I've done it on many Shimano and even Campy road cranks. A road fd will be needed with a road crank since MTB fd's are designed for a much smaller radius big ring.

Also, do you really need an 11T cog with a 52T big ring? How about a 12 or even 13T small cog?

Retro Grouch 

If you are planning to use a 34t big cog on your cassette, you're going to need a mountain bike rear derailleur. A road derailleur, even a long cage road derailleur, will rub the biggest rear cogs.

jchabalk 

i was trying to see if i could make it fit with a road derailleur and it looks like i'm exceeding all of them by quite a bit, so i'm totally fine with a mountain RD - with these configurations i'll need to use one with a 45t capacity.

The published capacities i've found are:

Shimano (mtn) RD: 45t
Shimano (road) FD: 22t

i'd be right at the end of each.

i'm definitely thinking of increasing the size of the rear cassette. i just want to make sure i have enough top end. My last bike was a 48/12 at the max and i always felt like i could use a little more.

The road crank with a smaller granny gear is interesting to me but in the configurations i initially posted i'm already at the recommended max of both the front and rear derailleurs and changing out the granny would exceed both by 4t. I have little experience with oversubscribing the derailleurs but 4-teeth sounds to be too much from what i've read.

Overall it are there any negatives associated with using a mountain RD for road riding (and i don't know why there would be really other than compensation for pushing more chain through the system).

On the detailed specs for the mountain RDs (shimano.com) they don't seem to publish the cassette speed range. Is it safe/recommended to use the mountain RDs with 8,9 or 10 speed cassettes? The crank-sets specify compatible speeds - i'm assuming that it's the chain-link width/chain-ring width that's the real concern. Any problems with the RDs?

Retro Grouch 

Naw, it's commonly done. That's the traditional gearing spec for the majority of stock tandems.

HillRider

Despite "exceeding" the fd's recommended tooth capacity, a 26T granny ring does work very well on a road crank. I've substituted 26T rings for the stock 30T on 7,8 and 9-speed Shimano cranks and even on a 10-speed Campy Chorus triple crank. In every case (105, Ultegra, RX-100, Chorus and Veloce derailleurs) the fd handled it just fine.

If 48x12 wasn't high enough, I'm pretty sure 52x12 will be and you still get the same 26T granny for your low gear. Try it, you'll like it.