Repositioning a Brake Bridge
 

My old (steel) road bike has a brazed brake bridge. I'd like to raise it a couple millimeters to get some more room for bigger tires.

How hard would this be to do (for someone with zero experience)?

From what I can tell (after googling), strip the paint in the area being worked on, clean the frame, apply flux to the areas you don't want to be affected by the heat, then...

Heat until the brazing softens/melts? Could you then simply reposition the bridge and let the old brazing take again? Or do you need to remove the bridge, clean the area again, and then braze with new?

While anything is possible, especially of you prepared the tube by sanding it to bright metal before you started, it's still best, to remove the bridge, then braze it fresh in the new location.

Know also, that since the gap between the stays tapers, you might need to shorten the bridge slightly if moving it more than 1-2mm.

IMO, this is a poor choice for your first job. Taking apart a brazed joint without damage is much harder than brazing it in the first place.

All of that makes sense.

As for shortening the bridge, I think I could get away with just spreading the stays a bit. (Old spacing for the dropouts so it could do with a cold set, but my hub is newer wider spacing--while I can squeeze the wheel in without cold setting the frame, I assume the geometry could work out ok as a bridge higher up spreads the stays ever so slightly.)

So just melt the brazing isn't going to be good enough to move or remove it, huh?

I strongly suggest that given your questions you either gain a bunch more experience with a torch and steel or employ someone who has said experience. I wouldn't try to reuse the old bridge for a few reasons. I wouldn't remove it using heat. I wouldn't modify the stays to fit a bridge (new or recycled), any drop out width mods should be done after removal of old and before installing the repositioned bridge. Flux is used to better allow the filler (brass/bronze most likely) to wet out, so if the heat is proper where there's flux there easily can be filler there when done. Flux does not protect the steel from the heat, it keeps the hot surface of the steel from oxidizing which inhibits the filler from adhering to the steel. Andy

Two points.

1- the problem with unbrazing a joint is that the steel becomes soft at the temperature that braze melts. So, unless you're careful to heat uniformly to controlled temperature where you can take it apart, you can end up pulling a bit of steel where it's still attached. I suspect that this is less likely with a bridge than where you have to slide a tube from a lug, but it's still something to be careful about.

2- Yes, you can spread the stays slightly, but you can't do that by forcing the bridge into place. Once you heat the area, the tube walls will soften and the natural spring in the stays will bring them back in causing the bridge to push the wall in. Again, not a structural issue for a millimeter or so, but can be a cosmetic one because the distortion may be visible once the frame is painted.

So, if you plan to spread the stays, do that cold so they are set at the right distance before brazing.

Also, if you opt to cold set the stays, do so after removing the bridge, and be sure to buttress the seat lug joint. I've seen a number of instances where someone trying to set seat stays ends up pulling them off the seat lug instead.

Cool. That's why I ask.

Thanks. That all also makes sense.

+1,, on get more experience .. you don't have to heat it to take off the old one,

Because a hacksaw and hand files can take off the old one,, then get more material to make a new one.

Another potential problem is that the seat stays may be drilled for the bridge, which means moving the bridge would require replacing the seat stays as well. Some early Treks were built this way.

I kept myself in beer back when I was in college re-brazing brake bridges. These were originally brazed by people that brazed many per day, but they still broke. So it's not as easy as it looks

As someone who has taught a lot of students how to braze in a frame building class, I don't recommend learning on a brake bridge. It is not especially difficult with the right knowledge but you want to practice with something easier 1st. The proper procedure is to cut the bridge off each side close to each stay (with 2 cuts) and then file the remaining bridge away. Be careful not to to file into the stays. Finish with finer cut files and emory cloth. Miter a new bridge to the higher height you need. Spot braze one side and check level. It is easier to braze each end of the bridge with them sticking straight up with the stays being nearly horizontal. My strong recommendation is to have someone experienced do this for you.

I am a hobby builder, and sometimes frame repair guy for others. The first brake bridge I removed went well. This is what I did: Removed rear wheel from bike, I set a spring between the seat stays to apply an outward pressure. Evenly heated the joint and pop! it separated when the filler reached melt temp. For the other joint I held the loose end with a pliers and evenly heated the joint. As it reached melt temp the joint became loose and pulled right off. If there are vent holes, no biggie. Either fill with silver or if the hole is small, which it should be, fill with putty. Filed down the filler stuck to stays and shortened the bridge ever so slightly to fit higher up. Rode it for 5 years and then built another frame!

Hacksaw and files sounds like a ridiculously more difficult method for a lugged bridge.

Interesting to know (not that it really matters since consensus seems to be it's not a good project for a complete noob).

Any difference in technique between lugged and non-lugged?

This is more what I would've expected for removal. I wonder if the same trick could be used to rebraze.

you hid those details before..

if you had brazing skills , already , you would not be left to wonder in this post.

Sorry. I didn't mean to hide any details. But I have no brazing skills, which is why I didn't realize that was important information. I'm just trying to learn a thing or two.

you may burn holes in the frame, with excess heat application, you may need to hire it out, like to a frame builder, already skilled.


So don't start on this thing first,... take a class in the community college , I Learned a lot from Jewelry, Art metalworking classes..







....

By "lugged," do you mean a bridge with the decorative reinforcements? That's more metal to file off when you remove the original bridge. A tedious job at best.

http://www.os2.dhs.org/~john/paul/track0004.jpeg

I'm curious if the frame really has more room at the chainstays than at the brake bridge.

Yeah, a bit like that.

I have at least 5mms at the chainstay (near the bottom bracket).

I only have about 2 business cards between tire and brake bridge. (If the road is clean, it's fine. If there's any grit, it sticks to the tire and catches at the bridge.)

Anyway, it all sounds like a PITA, so I guess I'll just size down on the rear tire.

Honestly, brazing skill is not something that should stop you from trying it. My brazing skills come from high school class 40 years ago. Picked up the torch 30 years after putting it down and built my first frame. The Paterek manual was indispensable, however heat control was acquired during the process. You won't burn a hole in the tubing unless the flame sits in one spot and turns it molten. Just do it.

Sure just do it. You are running a front brake after all, so there's a redundancy if a joint should crack/fail.


Seriously that is said in all satire.


Don't "just do it". Be smart and get some experience/practice before you change a bike to be ridden in a manor that's of no return. Then do it. But not "just".


Why am I so focused on that word? Because it implies so little preparation or planning. So I say "do it but verify" (And that's a take on a Ronald Regan quote). Andy

I'm a planner to a fault, so that's unlikely to be a problem. Some things, however, really require more experiential learning.

Oh... I have another frame that is pretty much scrap at this point. I could practice on that...