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You can. I'm not entirely sure it will work. The chainline won't be that good with such a long spindle.

You'll have to keep the old rear der to manage the slack between the two chainring sizes. A chain tensioner won't have enough slack to allow you to move between the two rings.

And you're going to want something up front to keep the chain on as well. Probably easiest to just keep the front der up there as a retention device.

An 8spd chain will work fine for this proposed drivetrain.

You didn't ask, but I don't think this is a great idea. Why not build it up as a single speed? Or a 1x8 or something? Having gears in the back is more useful and easier to build. What you propose is unwieldly and ultimately not going to serve you very well.

Interesting idea, sort of like a home-made Schlumpf drive.

You could do it, just to try it, for very cheap, keep your mtn crank, just remove the low ring, and use a 16 tooth rear, a 44-32 x 16 would give almost exactly the same gearing as your 50-34 x 18 (Sheldon Brown gearing calculator).

Keep the front deraileur, for convenience.

Keep your rear derailleur as a tensioner,adjust the screws so it's fixed, can the cable.

You'll have to install spacers on the cog so the chainline is right, as Mayhew said, that is important.

You can try your idea and all you've spent is for some spacers, and time. And everything can be put back if you change your mind.

Thanks yinz guys. Both of you pretty much summed up my feelings on this project. It's an interesting idea, not a great one. I'm just tinkering with the whole thing now. I did find a Paul Components tensioner that will act like a rear derailleur. They claim it can handle a 20-tooth swing. I think I'm going to pick one up to play with and save a little bulk.

My main problem with the whole thing is that I'm new to this and the number of different styles of bb's has me baffled. I'm looking at them and I can't seem to figure out what ones will work to replace the square splined one that I'm comfortable with because I know it.

Anyway, my motivation for this project is that, though I ride mostly flat rail trails and need something like 50x18 or 44x16 that will keep me pedaling for exercise, I occasionally get on something like the West Penn Trail that will require me to drop down to the 32x16 or 34x18 in spots. If I get it all built and it's a success (somewhat doubtful), I'll post some pics.

Also, any other input (like what fsa, octalink, isis, whatever bb will effectively replace a 68-113 square taper bb and not be a huge mistake) would be helpful. If I'm overstepping the bounds of advice seeking, I apologize.

http://urbanvelo.org/splined-bottom-brackets/

It's a really good article. All their articles of this flavor are. I could have kissed them for the "line your valve stem up with the label" one.

Anyways, the square tapers had a million lengths. The newer the BB interface, the less options in length. So there's actually less to deal with.

I still think a 1x8 would be cheaper and meet your needs better. Good range, less compatibility issues, etc.

ISIS is junk, as the article points out. Otherwise they are all pretty good. You'll have to change your crank to match as well.

Also this;

http://sheldonbrown.com/harris/bottombrackets.html

Bottom brackets and cranks have a huge number of variable, that starts to explain everything.

I have a couple of bikes with FSA octalink setups, I happy with them. I've also ridden a lot of old square tapers and they work just fine for me.

Here's a crazy idea for you, how about using a road double crank and a rear wheel with 2 cogs and spacers, and size the cogs to take up the chain slack so the chain length is the same with either gear pair. You would still need a tensioner to be able to move the chain from one pair to the other, but you would have 2 straight parallel chain lines with a wide gear spread. That there would be a frankenbike.

Oh, and I meant to mention, the first thing to determine is what type of thread your bottom bracket has, and the width of your bottom bracket, everything else proceeds from there.

If you really want a frankenbike make a retrodirect: http://en.wikipedia.org/wiki/Retro-Direct

That retrodirect thinger is a pretty cool idea. I'd like to try that sometime.