I want to change my 130BDC Shimano Exage Motion double crankset to a 110BDC double crankset. I have a Shimano RSX road triple that I was planning on useing without the smallest ring. Am I right in assuming the existing bottom bracket leangth will still work? thanks.

Don't see why not, just adjust your lower limit and you're set.

I searched a bit and all the disscussions where about dropping the small chain ring from an existing set up.

I guess it's not clear what your objective is. Are you looking for a 44-48 t and 34-38 t double versus a 52-38? Do you need to drop the granny gear for weight?

Swapping cranksets from double to triple shouldn't affect your chainline much, but I'd imagine that an indexed shifter would be tricky to set up that way. Piece of cake with friction though.

54+40t, Swapped for 48+34t chainrings, with out changeing bottom bracket is the objective. I have the rings and the RSX triple crankset.

The biggest issue is probably your front derailleur. At a minimum you'll want to slide it down your seat tube to position it 1-3 mm above the large ring's teeth when the outer plate is directly overhead. Less likely, the front derailleur's arc isn't wide enough to accommodate the slight increase in distance to the outer ring on a triple crank.

Shimano has technical documents for their equipment going back years that covers installation, adjustment, and specifications. Other manufacturers are the same. BB width is more defined by the frame than the crank. Read up on chainline. Your swap is likely to shift your effective chainline to the right a few mm, since shimano uses the center of a crankset to determine chainline. That's over the middle ring in a triple, and halfway between the two rings on a double. The threads about chainline should give you an idea about what to look for and expect.

Looking up all the specs, it looks like the tripple is speced for a 118mm bottom bracket and the double a 115mm. Figuring 8mm chainring spaceing theory put everything in the 1 mm close enough zone. I was hopeing to leave the deraileur height alone as this might be a temporary change. The new large ring will have 4 less teath. Im not sure how much that changes the radius.
PS. Google tells me the diferance is 8mm, so dropping the deraileur is probably a good idea.

PSS. I found some more info and i'm going to drop off speculating till I get all the parts in hand.

From your above posting your big chainring is going to be 6 teeth smaller (54T to 48T) and that is a huge change in radius. Lowering the front derailleur will be essential to get decent shifting.

I was using a phone the last couple of days and mixed up some of the numbers. I'm going from 52t to 48t, so I think thats about an 8mm reduction in radius. I also wasn't looking at the bottom bracket specifications correctly. I haven't dug out the RSX crank yet to confirm the exact model, but the Shimano literature is listing a 8mm bottom bracket difference between a double and triple RSX crank sets which is basically the same as the chain ring spacing, so that's promising.

Got a follow up question: eyeballing the RSX crank against the present Exage road crankset, it looks like I will need to rmove the small ring mounting posts to clear the chainstay. Any reason not to take them off? Also I was comparing the RSX crank to an Exage R500? mountain crank and it looks like they have about the same Q factor. Besides weight, is there any functional difference between road and mountain cranks with the same Q factor and crank leangth?

Sounds like a lot of extra work, and what will you do if you're not pleased with the appearance after grinding off the granny bosses? Just buy a 110BCD double, they're not hard to come by.

There is no guarantee at all that mounting the RSX's crank on the Exage's bottom bracket will result in the correct chainline, or satisfactory Q factor, either. BB's aren't expensive or hard to remove/install.

Just wanted to follow up on this. The 110bdc RSX tripple works fine as a double. Chain line seems to be the same as the original crank set. Curiously the original cranks chainine was noticeably different when I swapped the original 130 bdc steel biopace rings for sugino and willow rings