guy153

I have a braze-on front derailleur so am planning to make or buy the hanger that goes on the frame and braze it on. Does anyone know the standard for where it's supposed to go? Obviously I'll make sure the derailleur I have fits but it would be better to stick to any published standards if possible (in case the owner of the bike wants to try different chainrings etc. and runs out of wiggle room).

unterhausen

I don't think there is a standard exactly. I think that page of the shimano frame requirements might be online

Doug Fattic 

Eric is right there are too many different kinds of front derailleurs to have a standard. And of course their height changes based on how many teeth the front chainring has. Even Shimano's Dura Ace had a different height depending on the year. I have a very long list of front derailleur heights based on who the manufacture was and the year it was made.

To be safe you will want to mock up the position before brazing it on. That means installing the BB and crank and then placing the front derailleur where it is supposed to go compared to the chainring size and then marking that position. Don't forget that position is not only its height but also rotation.

I would definitely encourage buying an IC braze-on. I like the top of the derailleur part that attaches to the boss to be at the same height as the top of the boss. That just looks right to me. Because what chainring size I use may get smaller and how much the front derailleur height changes based on model and year, I didn't like to use a braze-on front derailleur boss. For example the old heights based on a 53 front chainring may or may not work with a compact crank and even if it does, it looks silly to have the derailleur at the far end of the slot. Of course if neither the chainring size or FD aren't going to change, a braze-on front derailleur looks nice and clean.

bulgie 

Two small points to add:

Remember that the der. cage rises up as it moves outward, so the height of the cage has to be measured with the parallelogram extended outward. Undoing the spring can help with some models; others don't allow that or not easily. you might have to rig up a cable and shifter if you want to be precise.

Be aware that the brazing heat, applied to only one side of the seat tube, will curve that tube a bit. If you're a stickler for perfect alignment, it means you can't use a "frame stick" or string method for centering the rear dropouts, unless you account for the bow in the middle of the tube. Other than that, it doesn't affect function, so most people ignore it, but I feel it's worth being aware of at least.

Would you consider the clamp-on part that gives the interface for a braze-on derailer? Looks a bit like a kluge, but it keeps all your options open for chainring size, derailer brand etc. Some of the clamps available are better looking than others, sorry I don't remember which ones I liked.

unterhausen

clamp on adapters are a good idea. Unfortunately they don't make them for 27.2 anymore

guy153

Thanks for all the advice! I could just get a clamp, but it's an unusually large seat tube (32.7mm OD). I was planning to TIG braze a tab on with basically a fillet on each side so it probably won't warp the tube (TIG brazing is very gentle with the heat input). I do have the derailleur and have ordered the chainrings and BB so I can make sure everything fits and then I guess I'll basically put it roughly in the middle of the range as it's a 50T big ring, which is itself middle of the range (sometimes people use a 46 or a 53).

unterhausen

I like to use a 46. So I put them a little lower than other people. It's a balancing act

e-RICHIE

27.2?!

https://richardsachs.com/product/peg...6mm-f-c-clamp/

bulgie 

That is a nice lookin' part, thanks for the link.