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Braze-on derailleur mount is too low

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Braze-on derailleur mount is too low

Old 08-09-21, 01:42 AM
  #1  
karldub
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Braze-on derailleur mount is too low

Hi,

I'm in the last steps of finishing up a build on a Batavus Professional from the late 70's. When mounting my braze-on derailleur, I noticed that the highest mounting point of my derailleur mount is too low to give full clearance for my largest 52 tooth chainring (about 2-3 mm too low). After reading up on this before purchase, I was only worried about the opposite being a problem!

Even though I can slide the derailleur further up in the slide, when I tighten the mounting screw it just kind of settles in a bit below the highest point of the slide.

I need it to stay at the maximum slide point. Any ideas on how I could solve this problem?

I'd rather not switch my largest chainring.


Last edited by karldub; 08-09-21 at 01:50 AM.
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Old 08-09-21, 03:03 AM
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Have you tried pushing the derailleur up to the highest position, while you tighten the bolt?. If that dosenīt help, I suspect that something on the other side of the brace-on, is pushing it down. A picture of the other side of the brace-on might make it easier to suggest a solution.
Another possebility might be to find a derailleur that is "lower" (Shorter distance from mounting point to cage).
If the frame is from the late seventies it should be possible to make it work, as it is my understanding, that 53 teeth was pretty much standart at the time.
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Old 08-09-21, 03:47 AM
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Rotary tool?

50t ring and smaller rear cog?

Drew?
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Old 08-09-21, 05:47 AM
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I know I've seen a gadget to allow a braze-on FD to be mounted lower, I presume for teeny chainrings; but unsure of source. I wonder if such a part might be reversible, to allow mounting the FD higher.
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Old 08-09-21, 06:01 AM
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You could also drill and tap a lower hole for the mounting bolt. Looks like there's plenty of meat on the mount.
Also suggest you clean up the back side and even extend the hole upwards with a file or Dremel bit.
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Old 08-09-21, 06:12 AM
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@karldub - I ran into this problem with the De Rosa build. It didn't take long to see that I was using a triple derailleur instead of a double. The issue was the location of the 53 t from the center of the bike. If I had used a longer spindle, it would have cleared, which is the configuration with a triple. The location arms of the triple are longer than the double FD. That is how a triple is able to move the distance without a significant change in the shifter location.
How is your chain line?
The location of "settling in" of the bolt looks like a widening spot in the bracket. Make sure the bolt will move freely while in the slot top to bottom with the head against the bracket, not just the treads. If not, file or dremel time.

If you have other bikes with a braze-on FD mount, measure the distance from the BB on both to determine if it is truly in the "wrong" place.
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Old 08-09-21, 06:29 AM
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I've never been a big fan of braze-on front derailleurs. Not only do they limit your chainring choices, they also are brazed on the thinnest part of the seat tube. I've seen cases where a chain jam in the front derailleur twisted the mount and tore a hole in the seat tube, like a can opener.

The only place they make sense is on non-round or non-standard diameter tubes, where a clamp-on derailleur wouldn't mount.
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Old 08-09-21, 06:57 AM
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Braze-on front derailleurs vary a lot in where the attaching bolt is in height above the bottom bracket. This is one reason I gave up using front derailleur braze-ons (as well as the reason JDT mentioned already). I remember a new model of a Dura Ace front derailleur needed to be 4 mm higher than the last model. So one answer is to look for a different model front derailleur. As a frame builder I would look closely at the boss and perhaps take a file to the slot to make it a bit longer going higher (and perhaps it needs to be widened too?).
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Old 08-09-21, 07:26 AM
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And this must be at least a mid eighties frame.
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Old 08-09-21, 08:13 AM
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Just looking at the photo, my first thought is the paint is pretty thick! I don't like that, but be that as it may, I assume you don't want to take a file to it, because that will expose bare steel and possibly lead to rust.

So what I would do, first, is cut a little block of aluminum (or wood or plastic or... well, almost anything except peanut butter would do the job) to jam into the slot below the bolt. It should completely fill the slot in the derailleur mount except for a 5 mm hole at the very top. Press that into place in your derailleur mount, and then go about mounting the derailleur.

If that didn't work, I'd look for another derailleur.
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Old 08-09-21, 08:15 AM
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I agree with Doug Fattic, some re-shaping of the slot should be possible. I had this on one frame of mine. It looks (from the front side) like your slot is narrower at the top than the rest of the slot, certainly at the middle. I would try to widen the slot so the curvature at the top of the slot matches the curvature of the threaded area of the derailleur clamp bolt. Needs to be done delicately to prevent marring the paint.

I could see trying that minor surgery with a round file which matches the bolt, or a tapered hand reamer. Both tool types can be extremely helpful or cause irrevocable damage, so go carefully. In both cases you're going to expose some steel, so you need to find some method of touch op to restore protection of the underlying steel.

This ignores whether there is an additional blockage on the concave side of the braze-on, perhaps caused by brazing material. I would take a look at that. If that's the problem I'd look for a real expert like Doug to approach the problem. If there is a blob and it is forcing your front mech to move downward as you snug up the bolt, there should be some kind of witness mark on the derailleur mounting surface itself, indicating that the softer aluminum has "lost a battle" with the harder steel or perhaps braze material.

Doug, are small frame braze-ons like this for steel frames case-hardened? My guess would be no, but if they are then amateurs like me should ot even do a careful trimming of the slot.
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Old 08-09-21, 08:20 AM
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Can you slide the derailleur up then put a piece of something(metal, plastic, rubber wood) under the derailleur/through the slot so it can't slide down any. Maybe with tabs you can bend down on each side of the bottom of the mount so it won't move. It would act as a bumper stop. Something like an old piece of cable housing or a chunk of rubber from an old brake pad. Anything that would fill the space in the slot under the bolt. Try various sizes of BBs. That would work.


Edit: Here's a BB I just put under one. There's the concept. Find/make something that works.


Last edited by seypat; 08-09-21 at 08:54 AM.
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Old 08-09-21, 11:02 AM
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Originally Posted by Road Fan View Post
I agree with Doug Fattic, some re-shaping of the slot should be possible. I had this on one frame of mine. It looks (from the front side) like your slot is narrower at the top than the rest of the slot, certainly at the middle. I would try to widen the slot so the curvature at the top of the slot matches the curvature of the threaded area of the derailleur clamp bolt. Needs to be done delicately to prevent marring the paint.

I could see trying that minor surgery with a round file which matches the bolt, or a tapered hand reamer. Both tool types can be extremely helpful or cause irrevocable damage, so go carefully. In both cases you're going to expose some steel, so you need to find some method of touch op to restore protection of the underlying steel.

This ignores whether there is an additional blockage on the concave side of the braze-on, perhaps caused by brazing material. I would take a look at that. If that's the problem I'd look for a real expert like Doug to approach the problem. If there is a blob and it is forcing your front mech to move downward as you snug up the bolt, there should be some kind of witness mark on the derailleur mounting surface itself, indicating that the softer aluminum has "lost a battle" with the harder steel or perhaps braze material.

Doug, are small frame braze-ons like this for steel frames case-hardened? My guess would be no, but if they are then amateurs like me should ot even do a careful trimming of the slot.
No the ones I've used were cast out of mild steel. Because I'm a painter as well as builder I wouldn't mind filing any of the boss to make it fit the derailleur. Then I apply a little paint in the area I filed. In the OP's case it looks like there is extra material (maybe paint, maybe metal) that needs to be removed so the boss sits securely. That is easy for me to do but not everyone has developed framebuilder and painter skills.

Original Dura Ace braze on front derailleur bosses had uneven thickness along the slot area. There were for the Dura Ace EX models. These were some of the 1st ft der bosses ever available and in the era of this frame. I'm not in the shop to check but the bottom part was thicker than the top part. This changes the angle of the cage to the chainrings. What later derailleurs were attached to the boss might have squished material out of place when it was tightened down.
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Old 08-09-21, 11:40 AM
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Werenít Campy and Shimano braze ons different both height from the bottom bracket and adjustability in the mount?

I havenít had a Shimano road bike in decades
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Old 08-09-21, 12:33 PM
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I only have two bikes new enough to have braze on FD mounts. both have slots wide enough to rotate the FD about the center of the curvature. neither slot is exactly or even within the expected tolerance of a hole for the 5mm fastener. Is this unusual for a braze-on?
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Old 08-09-21, 01:49 PM
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As others have mentioned it seems as it the opening is tight. The loose bolt should move freely by hand throughout the slots range. I have feeling the bolts 'walking' down the slot when tightened. No harm will come from opening it up with a Dremel or file. One could always take a longer bolt, wack the head off it and use it as stud.
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Old 08-09-21, 02:00 PM
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Originally Posted by Germany_chris View Post
Werenít Campy and Shimano braze ons different both height from the bottom bracket and adjustability in the mount?
Both (and more) were delivered with a Batavus Professional.
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Old 08-18-21, 02:41 AM
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Thanks for all the good input! I went with idea of using a file (flat) and made the area on the bolt side of the derailleur flatter, this made the derailleur stay up higher when the bolt is seated.

The next problem (which I've never encountered with clamp-on derailleurs) is that the derailleur won't shift to the big cog even the shifting action is manually moved by hand. See video on this below.


Link to video of shifting attempt. https://vimeo.com/588746743 . Do you see anything strange with my set-up and do you have any ideas what is causing the issue? List of some possible causes:
1) The mounting of the derailleur gives the derailleur cage a faulty angle
2) This derailleur and frame/clamp combination can't handle the size of the largest chainring (Big one is 52t, the small is 40t) or the tooth difference between them
3) Other causes
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Old 08-18-21, 03:01 AM
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Old 08-18-21, 05:21 AM
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Yeah, it’s at a funky angle. The outside of the cage should be parallel with the big ring. Is the braze-on mount causing the FD to be rotated a bit?
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Old 08-18-21, 05:24 AM
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Yeah, that was my feeling too.

However, since the braze-on mount is kind of a half pipe and the derailleur mount just fits snugly in there, I don't know how I would be able to adjust that angle with any kind of precision - any ideas?
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Old 08-18-21, 06:01 AM
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You're not done with the files yet.

Remove sufficient materiel from either or both of the mount and front mech casting to allow the mech to rotate. You may have to widen the slot, and/or remove a part of the casting where it bears against the mount, limiting the amount of rotation.
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Old 08-18-21, 09:51 AM
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Before filing, I would mount another FD to verify it is the mount causing the problem and not a crooked FD.
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Old 08-18-21, 12:36 PM
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I have, in the past, had good results by GENTLY bending the braze-on mount in the proper direction using a long handled crescent wrench.
Doesn’t take much to move it into the proper alignment. Never had any problems with the mount during, or after, such treatment.
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Old 08-18-21, 01:41 PM
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You can get an angled shim to correct the tail sitting too low/high. The rotation always settling in a funny place is a little more annoying. I usually keep swapping derailleurs until one sits right.

Cheap version, SRAM has made one too: https://www.ebay.com/itm/35355193564...Cclp%3A2047675
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