Bicycle Mechanics - annoying, hypothetical question re: f.der and chainring compatibility

hello. please, pardon my obtuse title. i'm trying to pre-empt the ogres who berate ppl for adding new threads. i searched, i looked around on google, i checked manufacturer's sites. I'm sorry, but i think we can discuss s'more.

here's the hypothetical question:

let's say i'm intrigued by the 2x10 gearing that SRAM is offering for off-road use. but, let's say that i'm disturbed by the expense, i despise all of the proprietary stuff in terms of actuation ratios, and i really don't think i need 10 cogs or a 36t in my cassette. (for the record, shimano is offering a 12-36t 9speed cassette, but i digress.) So, what i'm really saying is, i like the idea of running a 40/28 or 39/26 crankset, with an 11-32t cassette. Which is the best option for Front Derailer?

-road double? all the sites everywhere list only the max ring size for front derailers, not the minimum. Would the typical road double, with a 16t capacity and a 53t max ring size, work for a 39/26 crank? drawbacks i see: would need to keep the chainline tight to the frame, would need to run a frame with cable routing for a bottom-pull f.derailer.

-mtn double, as in the slx? The shimano site lists 36t as the max ring size, but i suspect that it could squeeze in a 39t. fwiw, a buddy of mine is running it on a cross bike, with a 48!-39, but i suspect the shifting is yucky. I'm suspecting this is best, but what do y'all think?

-mtn triple, with the limit screws set up drastically? i'm not too sure with this one, but i'm curious.

Now, again, i'm sorry to waste anyone's time with a hypothetical query, especially when i could conceivably buy some 39/26 rings for my old sugino crank, swap cranks on my old hardtail, try it with the mtn triple that's on there, make notes. then, swap the same crank onto my old road bike, and see how it does with a dura-ace 10 speed front derailer. And make more notes. Finally, i could get that slx double deraqiler from my buddy, and try it on both bikes. But, that'd be a lot of swapping. I thought it would be easier to ask here. 'Twould also be easier than searching google and bikeforums with phrases like "front derailer minimum ring" + suck?", which is what i was doing before i posted this.

Thoughts?
-rob

What are you planning to use for a shifter? You may have trouble mixing road derailleurs with MTN shifters.

Also a road derailleur probably isn't going to give you the best performance with chain rings that small. The bottom pull thing is something you can work around. You can get a pulley designed to solve this problem for $20 or so.

You're usually bette off staying within brand if possible, so the lever and FD travel are well matched. This is far less critical for a double front than it is for RDs, or even a triple front, provided the lever pulls enough cable. (most do)

As far as road or Mtb FDs, I'd let my chainring selections be my guide. If using something similar to a typical 39-53t combination use a road derailleur. If it's more like a compact drive, use a compact drive FD, or if it's a classic Mtb combination use a Mtb front. That's because the angle and curvature of the FD cage is matched to the intended chainring sizes. Also the inner cage plate is sculpted based on as assumed inner ring size.

So it isn't about the label as much as it is about matching the chainrings.

More information needed. Start off with exactly what kind of shifters you plan to use.

you can go down to around 36T large ring size and a road FD will still work. It just needs finer adjustments.
Most CX bikes will have a road FD and road shifters and they have no problem with the small chainrings.

I've a 26-36-46 for my CX and it works fine with a road double FD.

I'm running 26-42 chainrings on my tourer/commuter bike. I'm currently using a MTB triple front derailleur, and it works fine. On a road bike with 28-46 chainrings, I'm currently using a road double FD, but have previously used a road triple FD and a MTB triple, they have all worked OK for me but the latest road double that I have on there is the stiffest and so is the best so far. I only ever use front shifters that give me continuous control: down-tube, bar-end, old MTB thumb shifters, or twist-grip, and so compatibility with the pull ratio is never an issue for me but will be if you have indexed front shifting (which I hate). The important thing is that you can get away with using quite a large variety of FDs when you only have two rings.

I'm running 26-42 chainrings on my tourer/commuter bike. I'm currently using a MTB triple front derailleur, and it works fine. On a road bike with 28-46 chainrings, I'm currently using a road double FD, but have previously used a road triple FD and a MTB triple, they have all worked OK for me but the latest road double that I have on there is the stiffest and so is the best so far. I only ever use front shifters that give me continuous control: down-tube, bar-end, old MTB thumb shifters, or twist-grip, and so compatibility with the pull ratio is never an issue for me but will be if you have indexed front shifting (which I hate). The important thing is that you can get away with using quite a large variety of FDs when you only have two rings.

Along with down-tube, bar-end, and MTB thumb shifters, most Campy Ergo front shifters are not indexed and will work with a variety of FD/Crankset combinations.

Along with down-tube, bar-end, and MTB thumb shifters, most Campy Ergo front shifters are not indexed and will work with a variety of FD/Crankset combinations.

that's not entirely true.

the low end campy shifters are indexed. The ones with no indexing are 2009 series and Chorus/Record (one year only for Centaur) for the older generation.

I would go with any mountain derailleur you can find. I know that shifting between small- and middle-chainrings is lightning fast and consistent, and the only compromises in set-up result from the need to shift past the middle ring to get to the big ring. Any front derailleur can be set to slam back and forth between two positions very reliably.

It really should be a mtn front derailleur if you are using mtn (flat bar) shifters, in my experience.

.... and so compatibility with the pull ratio is never an issue for me but will be if you have indexed front shifting...

Indexed levers usually pose no added for doubles because as long as the amount of cable pull between the clicks is enough to make the shift (and it usually is), you can pretty much ignore the index and use the limit screws to set the inner and outer trim. The drawback of index FD levers is that with most you won't have the ability to fine tune the trim when riding slightly crossed over combinations.

that's not entirely true.

the low end campy shifters are indexed. The ones with no indexing are 2009 series and Chorus/Record (one year only for Centaur) for the older generation.

I thought it was only 2007-2008 low-end campy that was indexed. Everything before and after isn't (unless I'm mistaken). That's why I said "most".

sorry, re: the lack of info, but as i said: this is at the moment a hypothetical question. I'm thinking of going with a 2x9 set up on my next mtn bike, with the 39t104mm5bolt and 26t74mm5bolt up front and a 11/34cassette. but, i'm just as likly to go with a regular 44/32/22 triple. who knows? the only shifters i use are shimano barcons, typically mounted to paul thumbies when on mtbs.

i'd be using a sgs old xtr rear derailer, but i'm not worried about that much.

mostly, i'm thinking the new slx double, which is designed specifically for the 36/22 crank, would work best. that my buddy could even get it to work at all on his 48/39 crankset is amazing. I'll bet the 3teeth will be easier to handle than the 12. My friend confirmed that it does work, but that it does suck, and that he will be switching to a road fd very soon. So, i'll have easier access to a lightly used slx double fd.

thanks for the ideas thus far. being that this'd be a trailbike, campy ergo shifters aren't even on the radar. like i said, i'm a strictly barcon type of guy.

any more thoughts?
-rob

I run a 24-38-48 with a 12-32 on the back on my commuter with XT thumbies and a Sora front derailleur. Works great. It would work just as great with a double crank.