I am using an XTR crankset. I want to use Ultegra STI shifters. The LBS says that I cannot use my current FD because it is a differential plate design and only a fixed plate design will work…
But of course. I’ve just had a look at the XTR derailleur on one of my bikes, and the 105/triple set up on my road bike, and here’s what I think is the actual problem:
First, I’m assuming Wendy is using an Ultegra STI left hand shifter designed for use with a triple chainset, an XTR triple and an XTR front derailleur.
The Shimano triple left hand shifters for road bikes pull a different amount of cable per shift than the double shifters. You therefore need a front derailleur that can be set up differently to compensate. The RX-100 triple front derailleur I use on my road bike has two grooves for the gear cable. One groove gives the right derailleur swing per shift for a double shifter, the other works with a triple. As it happens, the grooves route the cable on different sides of the clamp bolt.
The XTR differential plate derailleur simply doesn’t have this possibility. It’s not really anything to do with the differential plate mechanism, which is a way of moving the inside and outside plates of the derailleur separately to make shifts faster. The problem, on the VTR derailleur I have, at least, is that the bolt is positioned so that it makes no difference which side of the bolt you clamp the cable, the action of the derailleur will be the same.
Without knowing what rings Wendy is using on her XTR crankset it’s hard to make recommendations for a front derailleur that will definitely work in this set up, but my best guess is that an Ultegra FD-6503 is a good bet; it might not follow the curve of a 46T big ring very closely, but it will probably work well enough. An XTR FD-M953 might work but I am not certain that the road and MTB derailleurs require the same cable-pull-per-shift, and the XTR FD-M953 doesn’t allow you to messwith the cable position the way the Ultegra FD-6503 does.