Anyone have experience using a Shimano STI triple with a compact double crankset? My LBS built out my Ultegra SL gruppo that way, and is saying using the triple shifter is no big deal and can give me more trim ability. I'm skeptical, as it takes two clicks to shift, and the first shift is too much movement for 'trim'. Is this mixing of the components a common practice, or am I having the wool pulled over my (new the road world) eyes?

TIA for any help sorting this out.

Set it up so the shifter thinks it is shifting between the middle and outer chainrings. The cable throw there is the same as with a double.
But the throw between granny and middle ring is different, so if it's set up to use that then it won't shift well.

The trim should be the same as with a Shimano double shifter.

I did it for a while. It works, but it isn't ideal. If possible get the right setup. if not, just run this. I raced on it and was fine, but now that I am on a true double the diff is noticeable (also went from tiagra fd to rival, so that could be part of the difference.)

Shimano has made several shifters designed for use with either double or triple cranksets. Their recommendation has been to use the first two positions when used with a double. However, this leaves the shifter vulnerable to being overshifted and damaged should someone forget that they were in their big ring and push the shifter too (reference all of the 105 10 speed left shifter failures). IME, you can set it up either way, using the first or last two shifter positions. With 10 speed shifters, it's irrelevant; you have a trim position for each of the three main positions. With 9 speed shifters below Dura Ace level, you only have trim positions for the first two main positions so using them provides more flexibility.

It sounds like the OP's mechanic set his double up using the last two positions and he is shifting down further than necessary to get into the small ring. Switching to using the first two positions is an easy task if you know how to set up a FD. That would eliminate the problem but require some vigilance with not overshifting.

^^^^^^^^^^ Or he could just adjust the limit screws properly.

I've always wondered about the damage to the shifters. I have my 105's that were originally setup as a triple, shifting a double now and can't imagine putting enough force on it to break it. You would have to try to rip the lever off in order for it to break. I always forget that I'm on the big ring and push the lever, notice it doesn't move and stop.

actually now that I think about it more the real answer is..................................


depends.

what model shifters, derailure, crankset etc. you are running. Some may be more problematic with spacing than others, if you are running a mixed speed groupo. As far as shifters go it shouldnt be a problem. Earlier shimano shifters were not double tripple specific, it should work you just end up with an extra trim position. 1st and second position and second and third position are the same spacing, just adjust the limit screws properly.

i'm using a 105 triple STI with a compact double - no issues.

I'm (the OP) using Ultegra SL STI and FD. The crankset is current AFS compact double, but next week will be Ultegra SL compact double.

On my CAAD9-6 (Tiagra) I swapped out my crank to a double 105 crank. It wasn't a compact, but it shifted fine.

Just set your limit screws correctly and it should work.

I'm using a 105 10 Sp. triple with a compact double - works just fine for me and feels solid as a brick.