Setting up a Double Crankset on a Triple FD
 

So I got a new double crankset and I'm having a hell of a time trying to get it to work with my triple FD. I have heard that it is possible, and most people have little problem with it. Right now I have it set up so that the limit screw blocks out where the granny gear would have been. It works, but shifting is harsh and far from perfect. Downshifts especially are rough. Any tips? Anyone have experience doing this? How did you get it to work?

The triple FD should work same as the double. Don't worry about guestimating where the granny gear would be. Try disconnecting the cable then reset the low limit screw so that there is a two millimeter gap between chain and the inside of the derailleur cage. Then reattached the cable and adjust tension as needed with barrel adjuster. Should work.

That is essentially what I did and by doing that it completely eliminated the lowest step of the FD (where the granny gear would be). Is this right?

For any problems with a FD, I recommend:
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Take the cable off the FD. Now install it from scratch as per Park Tool Repair:

http://www.parktool.com/repair/readhowto.asp?id=75

And BicycleTutor:

http://bicycletutor.com/adjust-front-derailer/

When you have it properly trimmed and are ready for the cable - a new one wouldn't hurt - you want to pull the cable taut - not too tight, taut. Then apply 48 to 60 inch-pounds to the pinch-bolt. Then put it through it's paces.

It's always easier and faster to install a FD from square-one, than it is to make adjustments with it already attached. This tends to fix one thing - while throwing another out of kilter. Start fresh. You'll get it.

Adding a barrel-adjuster can solve many problems with the cable coming loose - or being too tight.

Thats right, but to clarify from my earlier post, the low limit screw should be set with chain on small ring in front and big ring in back to ensure cage is positioned properly relative to the chain when on small ring. There are no "steps" per se for the derailluer. For double or triple, FDs are pretty simple mechanisms that slide inward and outward as cable tension changes; triple FDs have bigger cages needed to work properly with the granny gear but the larger cage does not affect performance when the derailleur is used with a double crank; the limit screws determine how far in board (Low) and out board (high) the cage will slide. Since you say your's is shifting harshly mostly when down shift, my best guess was you had low limit screw dialed down too much so derailleur was prevented from moving inboard far enough to shift cleanly (too much cable tension also will do that). If you set the limit screws while chain was on big ring, you should redo. Otherwise, tweak cable tension until derailleur is functioning properly.

By the way, when you swapped cranks, did you change the BB to one with a spindle length compatible with new cranks? If you are still using the BB that came with triple crankset, with the double, your chainline will be off, which also could affect shifting.