peugeot.ash

Hello, I hope this is the right forum to ask some advice about an idea I would like to try. I have an old steel touring frame made from Reynolds 500. It's heavy and worthless but fits me like a glove and has sentimental value.

​​​​​​IT currently has bosses for cantilever brakes and I would like to have a go at attaching disc brake mounts to the fork and rear. I have researched and most of the poeple doing this seem to use an arc welder. I was wondering if sliver brazing the mounts would be strong enough? I plan to reinforce the fork and the rear triangle with a brace to compensate for the added force from the rotors.

I am a decent home bike mechanic but have zero experience either arc welder or brazing but am keen to learn and this seems an ideal first project. Before I invest in the tools required for either technique I would like the advice of experts to decide which would be more suitable

Thanks in advance

bulgie 

There very fact that you would ask that question of strangers in a forum makes me think you're not up to it. Disk brakes are difficult to do right, and the stakes of failure are very high including death.
I wouldn't ride a disk brake attached by a "decent home bike mechanic", and neither should you.

But then, I've often been accused of being too negative about things a talented beginner can achieve on his own without expert teaching. Maybe so, but I can't assume you're one of the 2% (or whatever) of beginners who have all the right stuff to do a great job. You probably need a teacher. The people who can figure this all out on their onw are few and far between. Just by the numbers, I'd say you're likely to fail spectacularly.

Sorry to be so negative; maybe show us some evidence that you're better than most. But until then, "helping" you might lead to your death.

peugeot.ash

I appreciate your response. However, I think maybe you are being slightly negative in your opinion. You may well be right that I could fail spectacularly, especially if I try and attach the adaptors as my first effort.

I think I could have been more detailed in my initial post. Upgrading my frame is my ultimate goal. I intend to learn a new skill along the way and will obviously practice the skill before attacking my beloved frame with an OA flame or arc welder!

I would prefer to learn to braze the disc brake mounts, part of my question is whether this will be strong enough? Surely this is the right forum to ask this?

Cynikal 

I doubt the fork will be strong enough for the change in forces. Forks built for disks generally have thicker tube walls to address the twisting forces.

bulgie 

OK good, that's the kind of extra info about you we would need to judge whether you can pull this off. It's starting to sound more like you have a good attitude. Get real good at brazing before you try a disk brake, don't jump right into that first.

Lots of disk fittings have been brazed successfully by master framebuilders, so we know it can be done. Whether yours will be strong enough, I wouldn't hazard a guess. I strongly urge you to get some coaching, or at very least an inspection of your work after it's done but before you ride it. Good luck!

peugeot.ash

Several of the videos I have seen address this issue by adding strengthening sections to the fork. I have a genesis croix de fer made of 725 and this has a factory installed section of tubing over the fork as reinforcement. I would plan to do the same.

peugeot.ash

I am grateful for your concern over my safety. It is obviously worrying for you to give too much advice lest I follow your advice, do a shoddy job, then crash and die. Rest assured, I am a middle aged doctor with young children. I have no intention of putting my life in danger so your conscience is safe. 😁

bikeaddiction1

As mentioned above, the fork would not be designed to resist the moment applied from the force at the disk, much lower down than those on the rim. The same would apply to the frame. Assuming you are welding the rear mount to the chain or seat stay, they would not have been designed to take that load down at the disk height either. You might get away with it on an overbuilt heavy frame if it is not pushed too hard, but I would think it would be questionable on a lighter frame designed to restrain braking loads up at the rim height at stiffened locations, and not down by the hub. Failure may not happen on early rides, but repeated loads that the frame was not designed for could result in fatigue stresses which can ultimately lead to failure, especially as you may have effected the temper and strength of the metal at the location from the heat of welding. The heat effected zone right next to the weld is the most likely place for cracking. Failure is not as likely to happen at slow speeds with light braking loads, but at higher speeds where braking is harder and therefore loads the frame, and your welds higher. If you do decide to go ahead I would recommend regular inspections of the welds and tubes at the mount location for any indication of cracking.

Good luck with whatever you decide to do.

bulgie 

Cool. Then I will give this advice with a clear conscience
Silver is best for joints with a lot of surface area with very close fit, say .001-.003". Most disk fittings are not like that, they're intended for welding, but if you make your own and miter them to be a close fit on the blade, then maybe. Fitting that tightly to a blade, over enough surface area for silver, is do-able but difficult. Especially if the blade is curved and tapered at the same time (as most are), and not even a fully round cross-section (they distort from being curved). So fettling the disk braze-on for proper fit is usually more of a hand-filing operation, not really a precision-machining one.

Silver with a lower silver content (say 45% vs 56%) is less "runny" and fills gaps better, but at a higher melting point and lower strength.

If you're not stuck on silver brazing, this will probably be better suited to brass (some call it bronze), which gives good strength even when filling gaps. Nickel-silver, a filler alloy with zero silver, is even stonger than brass, in fact it's well into the steel range strength-wise.

I stongly suggest making some dummy joints, with pieces as close to the dimensions of your real blade and disk fittings as possible, then destructively testing them. They need to be really hard to break. The steel should bend substantially before the braze starts to fail. A braze that fails before the parent metal is evidence you're not ready, keep practicing.

Watch some Paul Brodie videos on Youtube, he's a good teacher and has a good videographer. You can skip the ones about motorcycles! He like brass brazing (he says bronze) and is also a big fan of nickel-silver for when extra strength is called for. I can't remember whether he has brazed on a disk mount in any of his videos.

Here's a blog post from Alex Wetmore, an amateur hobbyist who's turned himself into a pretty decent brazer. He brazed his disk mount on with brass, over 10 years ago and it's still holding in there. It's for a road-utility bike though, not a MTB. MTB is a whole 'nother ballgame. YMMV based on your weight, typical speed, how agressive a rider you are, how steep are your rides, etc.

peugeot.ash

This is fantastic advice and exactly the kind of content I was hoping for, thank you. I just had a quick go at it on the front fork and it seems fine so I'm off on a hilly 100k to test it out 😝.

Being serious, I intend to use the bike for commuting on city roads and leisure rides, nothing too aggressive. I'm a tall 85kg and the frame is tough Reynolds 500 chromoly.

I think that I'm definitely leaning towards brazing rather than welding as it'll be a more useful skill going forward. Next step is getting an OA torch and some rod to practice with. I'll definitely check out those resources on the tube 👍

Doug Fattic 

peugeot.ash, I've taught hundreds of students since 1976 how to braze in my frame building class. I can tell you with certainty that the likelihood you will be successful without lots of preparation is very very small. While the chances that you will fail is very high. I know what it takes to do it right, There are all kinds of obstacles in your path. It starts with the expense of getting the right equipment. And some students lack enough talent no matter how much they practice. While your frame may not have much monetary value, you will most likely be really disappointed to destroy it (since it has sentimental value) in an attempt to improve it. My recommendation is to look into the cost of brazing equipment and the fixturing required to accurately hold your attachments and see if those costs are within your budget. There is no point in proceeding unless you are willing to spend quite a bit of money and time to do this right.

peugeot.ash

Thank you for your post. In your opinion, would the weakness introduced by heat close to welds also occur if I heat the tubes by brazing the mounts on? I intend to fit the rear mount to the seat stay and then place a brace between the seat and chain stay to reinforce the whole area.

The fork is less of a concern for me as I can always replave the fork with one that includes factory disc mounts..

bulgie 

Yes the fork is more dangerous for a couple reasons,so I like your idea of buying a fork that was made for disk.
Here's what happens all too often when people retrofit a fork not designed for it:


Since this happens under max braking, like a panic stop to avoid being run over by a garage truck, it will almost certainly result in you flying over the handlebars.

bikeaddiction1

Although I don't think brazing will produce as high of heat as welding, it also is not as strong in tension. Adding another structural member between the chain and seat stays will also stiffen both of them setting up different load paths and stress risers and possibly stiffening the ride a bit. You count be trading problems.

guy153

Reynolds 500 is chromoly so in theory this can be welded as opposed to brazed.

An arc welder (unless someone is exceptionally practiced with it) is suitable for anything about 3mm thick and up. Your tubing is less than 1mm thick. I would not recommend any welding process for this except TIG (and I would personally use TIG brazing with silicon bronze, although welding would be fine). Is the fork also Reynolds 500? It may not be strong enough for a disk brake anyway.

I know people do spatter these things on with arc welders and flux-cores. It's really not a good idea, and neither even is MIG. This is thin metal and safety-critical. If you do want to learn and invest in the tools there's no reason why you can't but if you go with welding you need a TIG. They cost a bit more, but not much, and you will need a bottle of argon and practice. It's not rocket science but you also can't pick it up in an afternoon.

Or you could try a gas-brazing process, with others in this thread will know much more about.

squirtdad

FWIW,

I recently got a custom lugged steel bike from a well known builder.

this builder embraced disc early on and still builds them, but when discussing them he noted that he has to build the front fork much stronger/heavier than for rim brakes and it impacts the ride.

disc brakes are also the reason for thru axles for wheel retention.

so the comments about the fork likely not be strong enough are serious consideration

maybe build a frame as an alternate project.

guy153

If you look in the Reynolds catalogue now they have "disk specific" fork blades and regular ones.

zandoval 

Boy! These replies look difficult to decipher.

Of course Welding or Brazing would work. I think I would go for Brazing because when you weld its permanent. In both cases setting up a proper jig with very close tolerances is the rule.

There are however, a few parts of the bicycle and its set up that I simply do not mess with. One area is the Forks and Head Tube components. I work around older economical steel framed bikes and love to tinker. Over the years I have had more then a few failures.

Yep... That Front End of the bike I have learned is a BUGGER!

Even though your quest could work well, get yourself a proper Fork. It's not worth the risk...

unterhausen

Brazing isn't permanent? Live and learn

duanedr 

These projects have all kinds of red flags for me. Technically, is it possible for a skilled framebuilder to accomplish the changes? Yes. Is the likelihood of improving the bike high? No.

Red Flags:

• You'll likely need to spread the rear spacing for the new hub,
• the fork dropouts may not be facing in the right direction which risks the wheel coming dislodged.
• The fork crown and 1" steerer aren't beefy enough to resist the forces which leads to shudder when braking
• the blades aren't thick enough/large enough to take the forces leading to failures as above or poor handling when braking

If you care about this frameset, leave it as it was designed and buy or build a new one that suits your new requirements.

I'm also suspicious of the 'arc welding'. MIG wouldn't be suitable but I suppose TIG is technically arc welding so that might be ok.

Kai Winters

Go for it...what good go wrong...remember to submit lots of photos

guy153

"Arc welding" is a common UK English term for stick welding. Highly unsuitable for this job.

Have seen more than one video on YouTube of people spattering on caliper mounts with flux-core welders etc. Driven ultimately I suppose by all the marketing that makes people think they "need" disk brakes. I'll tell you some things you need more than disk brakes though. Stays and fork blades that haven't been overheated and aren't full of holes. Brakes that are attached to the frame in places where it has enough metal to support them. Your front teeth.

unterhausen

I always wanted to know what happened to the gravel bike in the viral video where someone "updated" a nice vintage bike to gravel. They didn't blast any holes in any tubes while joining, but everything was done improperly and the brazing was awful. I'm a big fan of disc brakes, but my rim brake gravel bike worked fine.