Andy_K 

I just got a new frame and I'm trying to figure out what front derailleur I should get for it.

The seller (Competitive Cyclist) said it takes a 34.9mm front derailleur, but it also came with an adapter for a braze-on. Beyond that, it has a flat spot on the seat tube as seen here:



Is there any reason to choose a braze-on type front derailleur and use the adapter versus just getting a 34.9 clamp-on model? And what is the flat spot for? Do I need to do anything with that?

fietsbob

My guess : the flat spot is clearance for the FD to swing inward farther than if left round

With a Braze on FD and Adapter, you can use a K Edge chain catcher and so not take a chunk out of your
carbon chainstay when you drop the chain off the inside chainring.

bobotech

I guess you could use the adapter if you already have the derailleur? I have used a few of the braze-on adapters on my bikes because I happen to have had a few braze-on style front derailleurs.

I imagine that the non-braze-on style derailleur is a little lighter than the same derailleur in braze-on style with matching adapter if that matters to you at all, it doesn't to me.

HillRider

I would use a 34.9 mm braze-on clamp adapter and a braze-on front derailleur. I would not use the bolt-on tab or buy a dedicated 34.9 mm clamp-on fd. The clamp adapter will allow you a very wide range of height settings so you are not limited to which chainrings you use and it will allow you change front derailleurs more cheaply as drivetrains evolve in the future.

Werkin

An adapter that captures the derailleur in a fixed position offers no advantage over an integrated clamp, unless you have a braze-on derailleur to use already. An adapter clamp with a slotted & curved braze-on interface will allow guide cage adjustments (micro for toe, and major for vertical) without having to loosen and tighten the clamp bolt after initial installation. When a clamp is tightened chain guide toe position can change, which is no big deal to plan for, but when swapping chainrings often, only having to loosen the derailleur mounting bolt to adjust, and cage position remain static when tightened, offers more convenience. Not all clamps & tubes are concentric; one set & forget install for the clamp applies less stress to the tube in the case of a slotted & curved braze-on interface. There may be an advantage to using a conforming carbon fiber clamp (aesthetic aside) for this reason also.

HillRider

Not immediately perhaps but in the future if you buy a replacement or different make or model front derailleur it does.

Andy_K 

Thanks for the help. I like the chain catcher idea, so I'll probably go braze-on.

HillRider

There are several chain catchers that clamp right around your seat tube below the front derailleur. The Third Eye Chain Watcher, the N-Gear Jump Stop and the Deda Dog Fang asr among these.

fietsbob

CNC , anodized and in colors,, even RWB stars N such..
https://www.acecosportgroup.com/shop/...n-catcher.html

Pro level stuff..

another thing you can do with a braze on shifter, is rotate the derailleur itself .

by adding wedge pieces between the derailleur and the mounting adapter..