Originally Posted by
Wiggles_dad
I currently have a Shimano 6503 FD. Actually, as of today it is working but not 100%. I have the derailleur set so close to the middle ring that the teeth on the middle ring barely touch (by a hair) the back plate of the front Deraileur when it is in some positions. I have TA chainrings. I was thinking that a double front Der would work because the only difference between a double and triple is that the back plate of a triple extends lower. I think the Shimano 6800 would work as well as a CX Deraileur.
I suspect that you will just be trading one problem for another one. The problem with the Ultegra triple (FD6503) is that the distance between the inner plate and the outer plate is fairly narrow. It is further narrowed by indentations put in both plates to stiffen the derailer. The narrow gap causes other problems but in your case, the deeper stiffener pressed into the inner plate leaves you with less space to nestle up to the middle ring.
This is a problem with all of the more expensive Shimano front derailers...mountain and road. The more expensive the front derailer the harder they are to set up properly. The inexpensive front derailers...Tiagra and Sora, for example...have a wider gap between the plates and the stiffing indents are shallower. They are more forgiving in gear combinations and, in your case, would allow you to run the derailer closer to the middle ring.
Going to a double front derailer may cause issues with lifting the chain off the inner ring. You'll have to overshift the derailer to force the chain to the inner ring. Overshifting isn't something that STI likes to do nor is it that easy to make it do it. The likely result will be that you'll shift towards the middle ring but the chain will move to the outer and you'll have to shift it back. Doable but futzy.
The good news is that the Tiagra is cheap...$15 to $30...so if it doesn't work, you aren't out much money. Ultegra isn't nearly as cheap to experiment with.