For my mountain bike frame to road bike conversion, I planned to use my 'old' Shimano Sora 9-speed road bike parts. But I had to replace the front derailleur as the road bikes are designed as pull-down but the mountain bike shifter cables are routed for top pull actions.
I could opt for an MTB (mountain bike) front derailleur but, for example, the Shimano DeoreFD-M6025-L 2x10-speed is only designed for top gear of 34-38T (teeth), much too low for my intended road bike chainwheel of 50T. I could climb nicely but on flats and downhill I would spin out of gear if using a 38T MTB chainwheel.
Checking the road front derailleurs on Shimano's homepage, they are all down-pulled except for the cyclo-cross designed "top-pull cable routing, "for best performance". It has top gear teeth from 46-52T, matching my needs. I was quite relieved that I found this part because I would not have liked to use a 1x drivetrain instead of my familiar 2x setup.
Shimano offers two different versions which are quality-wise in the Tiagra range:
a. FD-CX70-F-T:
SHIMANO (T) Top Pull Front Derailleur (Direct Mount) 2x10-speed
b. FD-CX70-F-D
SHIMANO (D) Down Pull Front Derailleur (Direct Mount) 2x10-speed
According to Shimano, both are compatible with road drivetrains and should work with their own STI shifters, other good news for my build. Without long consideration, I've ordered the Top Pull version for my conversion online. It came in an original blue box with an insufficient multi-language manual. I've bought a cheap band-on alloy adapter for my MTB frame and mounted the front derailleur on it. I've routed the cable from the shifter and
1. Tried the cable routing from the outside to the inside (frame) around the fixing bolt. But while the upshift worked ok the downshift worked only with a big 'bang' sound.
2. I've changed the cable routing from the inside (frame) to the outside abound the fixing bolt - the downshift felt much smoother.
Possibly because of the first routing, I noticed that the inner cable began to spit around the bolt after a few rides. I replaced it with a new cable as soon as I discovered it. Since then, I've changed from Sora 9-speed with STI levers to a Shimano 105 (R7000) 11-speed with Microshift CENTOS XPRESS PLUS SHIFTERS 2×11 flat bar shifters. The cable, and the front derailleur both worked and work fine on both different systems.
Verdict:
Even though the SHIMANO FD-CX70 (FD-CX-F-T) Top Pull 2x10-speed front derailleur is an older design, it works well for a niche product with 9-. 10- and 11-speed front derailleurs with STI road and flat bar shifters. Set up rather by experience than with the insufficient enclosed leaflet, I experienced smooth and reliable up-and-downshifts with a reasonable shift cable life.
Plus
+a real problem-solver from the Japanese giant
+not mainstream, therefore good deals might be possible
+compatible with 9-speed, 10-speed, and 11-speed shifters
+easy to set up, easy to shift
+Shimano quality
Minus
-the only (!) Shimano-offered top pull front derailleur for road bike compatible with STI and flat-bar shifters
-could 'eat' cables if not routed properly around the fixing bolt
-may be difficult to source
Ride carefully, Gerhard