From what I've tried, it was that road shifters had less pull than mtb mechs, so the case of Deore shifters with road FD should be fine, unlike road STIs with mtb FD - it just doesn't reach the outer ring, doesn't pull the cable enough to make it.
Anyway, have tried several times, and the safe way to do this was for me to use a road 3x front derailleur (with pulley if needed, as road and top pull don't go together). However, sometimes the road shifters worked fine with mtb FD - and sometimes not. Also, asked some people with these conversions, varied from "I don"t use outer ring" and "yeah, outer ring rubs the chain" to "set it and everything works fine".
Magic, probably. Got a stack of pulleys and road triple front mechs for me, but apparently other people can make it work.
(still not sure how to quote properly on the forum, will learn one day...)
Anyway, have tried several times, and the safe way to do this was for me to use a road 3x front derailleur (with pulley if needed, as road and top pull don't go together). However, sometimes the road shifters worked fine with mtb FD - and sometimes not. Also, asked some people with these conversions, varied from "I don"t use outer ring" and "yeah, outer ring rubs the chain" to "set it and everything works fine".
Magic, probably. Got a stack of pulleys and road triple front mechs for me, but apparently other people can make it work.
(still not sure how to quote properly on the forum, will learn one day...)