Bicycle Mechanics - Why FD will work with say a 46 but not a 50T chainring?

Just curious what makes a front derailer work with only a specific amount of chainring teeth? Is it the curve of the metal cage? Or is the throw different? Was wondering b/c I was going to buy a cheap one off ebay to use with my compact crank, but many of what I was looking at must've been for mnt bikes as they were maxed at 46 and 48T. Was looking at top pull FD's, they must be more common in mnt bikes I take it. Building a cx bike and need one for that application.

Curve of the cage is the biggest reason. Roger

CX , a 46t outer will be fine, given mostly soft dirt courses..
46:12 should be high enough..
If you are going for the popular 50/34 double, Note,
front derailleurs being specifically made for those .

I'll have to look into compact specific FD's. I wouldn't even need a big ring if all I did is race it, but I often ride on the roads to impromptu club "races." Nice to have a 50T for the road. Have a nice 36/46 crankset, but put it lives on the wifes bike for now..

The curve of the cage is optimized for the size of the chain ring. But it's not like there's a really sharp cutoff on what sizes of ring will work ok and the recommended size range can frequently be exceeded. My folding bike has a 60t large chain ring and uses a standard Shimano 105 front derailleur which certainly isn't designed for that size but works fine.

I'll have to look into compact specific FD's. I wouldn't even need a big ring if all I did is race it, but I often ride on the roads to impromptu club "races." Nice to have a 50T for the road. Have a nice 36/46 crankset, but put it lives on the wifes bike for now..

As of 2010 Campagnolo double front derailleurs were officially compatible with chain rings from 45 to 57 teeth although using the braze-on version with a traditional sized mounting slot instead of a clamp-on would preclude using all that range.

Their specs tend to be conservative and I can't see why a Shimano or SRAM front derailleur wouldn't work similarly provided the chain line was correct.

Just use a standard road double.

Note that MTB and road front shifters use a different cable pull ratio.
Normally you just don't mix them. It is done though, especially by tourers. Google CTC front shifting for an excellent explanation. They have an excellent page on unconventional choices in rear shifting too.

I've used a MTB FD for a 42 with a 48 and it worked OK.

Get a clamp on cable pulley (problem solvers has one) to run a bottom pull FD with top tube routing.

Get a clamp on cable pulley (problem solvers has one) to run a bottom pull FD with top tube routing.

The original poster does not want a mountain derailleur regardless of where its cable runs unless he's running a mountain bike crankset with its intended bottom bracket length.

Front derailleurs work best around the chain line they were designed for. Road doubles run 43.5mm, road triples 45mm, and mountain bikes 47.5-50mm.

The cage configuration for doubles and triples is different too.

I tried a Campagnolo Racing-T road triple front derailleur which had run perfectly with a 50-40-30 road triple when I switched to a 50-34 compact double and couldn't configure it to reliably shift with the rear derailleur in all positions without periodically dropping the chain.

Setting up the appropriate double derailleur was painless.

The original poster does not want a mountain derailleur regardless of where its cable runs unless he's running a mountain bike crankset with its intended bottom bracket length.

I'm aware of that. That's why I suggested he look into a pulley so he could run a bottom pull [road] FD.

Thanks for all the great info guys. For now I am going to use a DT shifter so will be able to use my current setup afterall. Nice to get the information for later though for if/when I get an integrated brake/shifter and decide to go with a top tube cable run and top pull FD.

If you are going for the popular 50/34 double, Note, front derailleurs being specifically made for those .
Campy sold specific "comapct crank" front derailleurs for a couple of years but has abandoned the idea and now says their standard fd's are suitable for any of their cranks. Shimano never made specific compact crank fds and always said they were unnecessary.