cycles per second
Join Date: Oct 2003
Bikes: Vitus Aluminum, DiamondBack Apex, Softride Powerwing 700, "Generic" Ishiwata 022, Trek OCLV 110
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Yes, the 39 and 53 Ultegra rings for double will fit on your 105 triple. However, the 39 will not have the ramps and pins for shifting up from the granny. It should still work though.
Why not just restrict the usage of your granny the equivalent low gear of a 39x53 combo? A 39/25 low gear is about the same as a 30/19 and a 39/27 is about the same as a 30/21. So if you need to use the 30/21, you should go with a 12-27. If you need to use 30/23 or 30/25, you should stick with the triple.