The Sora 2x derailleur mech that came with my gravel bike broke in half. Is the 2x Tiagra/105 front derailleur compatible with the R3000 shifters? Thanks!
You can get the Shimano specs and compatibility info here (expand Technical Documents on left hand side).
The front drivetrain compatibility chart says the ST-R3000 lever is only compatible with the FD-R3000 front derailleur.
If you study the compatibility chart you'll see there tends to be a lot of compatibility between drivetrains that match in number of speeds, but not between speeds. For instance 2x10 Ultegra 6700, 105 5700 and Tiagra
4700/4600 are all compatible with each other but not 2x9 or 2x8 group components.
I recalled a question where the asker had a Trek bike with a Sora drivetrain and 105 front derailleur. Obviously Trek thinks this works and Shimano are known to be conservative in their specifications and compatibility recommendations.
2But if you already own the FD in question you could always give it a try.– CarelMay 24, 2019 at 17:10
24700 Tiagra isn't compatible with any older 10 speed groupsets as they changed it to be like the 5800/6800/9000 11 speed groupsets. Officially, it's only compatible with 4700, but the FD works with Shimano 11 speed shifters. May 24, 2019 at 21:11
Any Shimano road shifter 10 speed and below (except Tiagra 4700) works properly on any Shimano derailleurs designated 9 speed and below. This includes designated mountain mechs. Obviously a 10s road shift will work with a 10s road derailleur, but things get too different here as mountain 10 speed systems (DynaSys) significantly changed the length of cable pull/derailleur movement. Also, some older duraAce series can only be used together. Spacing of the cassette sprockets differ as the number of them increase, so that's involved too but derailleurs don't know or care what "speed" they are– JeffMay 26, 2019 at 1:41
@jeff That's true for rear derailleurs, because it's known that Shimano used a 1:1.7 actuation ratio for MTB 9 and below and road 10 and below. Front derailleurs are a different matter. I've never seen any comprehensive information on their actuation ratios and cable pulls. May 26, 2019 at 10:39
@Argenti Apparatus I've found in practice that the front shifting systems are less finicky than rear. For instance, my Giant mtb front end, combined a Deore 9 speed shifter--a left 3 speed actually, SL-590, controlling a Shimano 7 speed/triple front mech (model number in the 100's) perfectly. Now I have no probs after changing to a Deore FD-M611, which is marketed as 10 speed. My Stumpjumper got a new 10 speed triple crankset (FC-622) which meshes perfect with the stock 9 speed system. I did assume the same relationship exists up front that is documented for the rear, but maybe not.– JeffMay 26, 2019 at 11:31