Shopping for a front derailleur: what's the difference?
 

I haven't shopped in 18 years. I've always used cheap Shimano models that pull down. I see some pull up. I see a $1,000 model (digital, probably connected to the Internet: Russian hackers could maliciously shift my gears!).

I think I paid < $10 for the current FD. I can't imagine getting better performance from another model: it shifted when I wanted it to. Weight doesn't matter. Clamp-on, alumin(i)um frame, triple, 24-52 tooth. Non-competitive

Is there a reason to buy a pull-up instead?

I used to shop at Bike Stop, which let me specify criteria. They've shut down. I'd like to narrow down my choices to one with a clamp large-enough to fit my frame, a triple that can accommodate my chainrings. Is there an on-line store that allows that?

Some bikes need top pull derailleurs. Some need bottom pull derailleurs. Depends on your bike and where the braze on cable stops are. (Many new derailleurs come with "dual pull" capabilities which work with either top or bottom pull bikes.) Triple and double cranksets have different styles of derailleurs. You also have to be weary that derailleurs come in all sorts of different sizes for different sized seat tubes.

If the FD cable comes up from the bottom bracket area, you need a bottom pull FD. If it comes down from the top tube then you need a top pull. You also need to know the seat tube size for the clamp. Some Shimano FD's can fit multiple sizes with the use of included shims.

New SRAM YAW FD shift angularly, not just translationally, to match chain pitch towards the cassette out back.

If you have friction front shifting, and your FD cage's curve matches the outer chainring curve close enough, then I imagine pretty much any FD will perform as well as any other.

Is your front shift controller an indexed one? Is it a mountain bike or road bike one (as well as the crankset type)? Ft ders have a few specs that should all coordinate to insure best performance. Already mentioned is the direction of cable pull, bottom or top. But also mentioned is that some Shimano ders have both capacities. The next design aspect is whether the parallelogram is above or below the cage as in bottom swing or top swing. This is more about the interference with frame designs like suspension pivots, seat tubes that get non round at the shell or bottle bosses located where the der clamp might be. The there's the obvious aspect on mounting interface. What tube diameter for a clamp around one (and again some ders have shims for the common tube sizes), braze on mounts, E bracket and now direct mounts are the usual versions issues. A factor not often mentioned is that some ft ders have different cage movement for the same cable pull movement. Road and mountain ft ders can be different in their cable pulling needs as well as the cranks can have different ring spacing (center to center) between road and mountain cranks. There's the cage shape. Double rings usually have an inner cage plate with less height then triple ders have. Some cages are curved along their lower edges to better follow smaller rings. Lastly there's the cage width. Wider chains or a lack of ft shift controller trim functions mean wider or more sculpted cages. Andy

One of my bikes has a little pulley near the bottom bracket. So the cable goes down the seat tube, over the pulley, and back up to the FD. The angles never seemed right, so I also put a twist in the cable to get the pull direction a bit better.

It seems more common for road bikes to be bottom pull, and MTBs to be top pull, perhaps related to the downtube shifters on road bikes from eons past.

Why that's easy. Where are the cable stops on your bike?

If they direct your front shift cable under the bottom bracket, you need a down pull front derailleur. If they direct the shift cable downward from the top tube, you need a top pull front derailleur. There are of course work arounds that make it possible to change from one to another but, unless you're trying to solve another problem, why would anybody want to?