So Miguel and I are having a debate over this topic. I am fairly strongly opinionated and as he is designing his new CX frame, this topic is rearing its head again. I figured I's reach out to y'alls: the officianados.
Q: How do you, crossers, prefer your cables for the front dérailleur to be routed?
A: Across the top of the top tube! (or so is my heavy opinion).
Where this gets into all kinds of hairy debate revolves around the need for a pulley as most (if not all) road dérailleurs are bottom-pull-actuated. So to circumvent this (unless you are using an MTB dérailleurs if it can provide enough clearance for your cross needs to the rear tire) frame designers have long worked up a system that routes the cable over the top tube, down the seat stay (a la MTB's) and around this pulley which is affixed to the seat tube sending the cable back up to the front dérailleur. Just like you see on this Sycip below:You can see this on the Ridley image I posted to the right as well. I think Sven's Colnago is the same but I have no solid pictures.
The problem is, you guessed it, mud.
The routing debate relates to lots of things but primarily is is rider feel believe it or not in addition to humping the geometry by needing to lengthen the wheelbase too much for that precious extra clearance if you need a pulley and need it to clear mud. I've ridden Felt's, Scott's and Kelly's with cable routing which sent the front dérailleur (and rear in some cases! (see Kona pic below) down the down tube and when grabbing the down tube (which is the style I use versus top tube unless its REALLY muddy) to shoulder/port, it always felt shitty. Often, you get your gloves caught between the cable and the frame and it just doesn't feel right.
When I switched a couple-a years ago to a single 42 or 44 up front, this all changed and I am cable-less on all my down tubes (so it moots the debate), but I have been considering going back to double rings because of the non technical/roadie courses out here. So I am stuck in this debate....and trying to help out my bro.
Anywhoo, opinions would be great and help Mike out.