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Brazed cargo frame

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Brazed cargo frame

Old 02-02-20, 02:31 PM
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Pleux
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Brazed cargo frame

Hi all

I'm planing on making my first frame or rather my first conversion from a 26" MTB frame to a cargo frame.
i only have access to a ****ty flux core welder, which based on my research wouldn't do the job right and i don't have the budget to buy a proper tig welder, my only option is to go with brazing.
is brazing going to be strong enough for a cargo bike?
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Old 02-02-20, 02:55 PM
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Originally Posted by Pleux View Post
Hi all

I'm planing on making my first frame or rather my first conversion from a 26" MTB frame to a cargo frame.
i only have access to a ****ty flux core welder, which based on my research wouldn't do the job right and i don't have the budget to buy a proper tig welder, my only option is to go with brazing.
is brazing going to be strong enough for a cargo bike?
A brazed joint should be as strong as a welded one if done right. If fillet brazing you make up for the fact that the filler has less strength by making a thicker fillet. If using lugs the braze gets to stick to quite a large area giving you back the joint strength. What sort of brazing do you have in mind?

​​​​****ty flux core welders work quite well on anything between 2mm and 3/32in thick which puts you in the ballpark for a heavy mild steel cargo bike so you might just get away with it. Welding anything to the existing bike frame is going to be sketchy but you could get away with it on the HT or BB shell. Depends how redneck you plan for this project to be

Another option for low cost TIG is to do scratch start with a stick welder. You don't need much power for bike frames (80 amps maximum) and DC for steel so a cheap stick machine plus a TIG torch, a bottle of Argon, and plenty of practice will do the job. The main challenge with such a rig for bike frame purposes would be the lack of a foot pedal.
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Old 02-02-20, 03:53 PM
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i was thinking silver brazing without lugs since i was planing on using non standard / bike tubes, for the head tube 48mm outer and 34mm inner diameter should fit 1,1/8 headset and for the down tube maybe something bigger to support the weight.

my problem with welding is that im probably going to burn holes in the frame and brazing seem easier to get right and looks better as well.
in terms of wight performance and considering that it is my first time, is it worth to try the cheap flux core welder or should go with thick brazing?
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Old 02-02-20, 07:19 PM
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Originally Posted by Pleux View Post
i was thinking silver brazing without lugs since i was planing on using non standard / bike tubes, for the head tube 48mm outer and 34mm inner diameter should fit 1,1/8 headset and for the down tube maybe something bigger to support the weight.

my problem with welding is that im probably going to burn holes in the frame and brazing seem easier to get right and looks better as well.
in terms of wight performance and considering that it is my first time, is it worth to try the cheap flux core welder or should go with thick brazing?
I think you need to do some more learning about fillers, their strengths and their weaknesses. Silver is not the usual choice for a fillet joint. The most common, 56%, can suffer from internal to the fillet cracks. It shrinks as it cools and the outside cools/solidifies first. Then the insides have no where to draw material from and the crack opens, hidden like a bad seed. Fillet Pro silver can be built up without this issue but it's the only silver filler I know of that will.

Bronze/brass is the choice for filleted tubes for a variety of reasons, if welding is out. Strength is then about the fit of the miters, the proper brazing techniques and how big a fillet. The rule I was taught is the fillet size should be about 4 times the tube wall thickness. No reason why a bronze/brass filleted bike should be as strong as a welded one and might have other advantages too. Andy
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Old 02-02-20, 09:54 PM
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I've seen a bunch of tall bikes and what not made with stick welders, so as pointed out before, it's apparently not unworkable when working with thick heavy tubing.

Most industrial literature points to big fillets actually being worse and weaker than small fillets, with worse fatigue strength, increased difficulty in inspecting penetration, increased porosity, etc. Natural concavity is proof of flow, a convex bead is generally considered bad. The filler material doing the work is the filler material filling the gap. The guy behind Fillet-pro says 3:1 fillets work well on thin tubes. Bontrager felt that the smallest size fillets he tested and felt were reasonably possible at ~2.5mm were the strongest, in large part based on HAZ analysis. The mechanics behind braze strength are more complicated than just multiplying UTS by 4, but those size fillets tend to work reasonably well in practice.

Silver also has been used for fillet brazing, notably in construction 753 frames. Raleigh SBDU made some, so did Herse. SBDU supposedly had a 2 step process, one alloy for what some people call tinning, or the penetrating part of the brazing, and another to build up a fillet. I'm not sure what Herse did. These examples seemed to have moderately small fillets. It's certainly possible, but the process is enough of a mystery that I'm not sure there's any good reason not to just use fillet pro. It should also be noted that silver may actually produce weaker joints than brass due to weakening the tubes directly at the joints. Silver brazing severely tempers the steel reducing strength, brass brazing goes above critical and allows the rest of the frame to cool the joint when cooled in air, allowing the steel to slightly harden, but with weakening beyond the joints.

There's a few reasons for large fillets. There's tradition. It helps reduce stress risers at the joints. It can make the joint stronger in the case of poor braze penetration, what is now called "bronze welding" in other industries often has little or no penetration but instead relies on a built up bead for strength. Bontrager claims fillets larger than the smallest size rotary tool are easier to finish, although this stops being the case with big lumpy fillets. Most people also think large fillets look good from a purely aesthetic standpoint, and small fillets barely look different from steel welds at 5 feet away.

All that muddying the water is just to point out that despite numerous exceptions and supplementary and seemingly contradictory information, 4:1 in brass is generally safe and sufficient, there's no particular reason to deviate from this in terms of material or size if you aren't interested in experimenting, which you probably shouldn't if you're learning how to do it.

Last edited by Kuromori; 02-02-20 at 10:09 PM.
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Old 02-03-20, 02:25 AM
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Originally Posted by Pleux View Post
i was thinking silver brazing without lugs since i was planing on using non standard / bike tubes
If you mean the kind where you heat the joint up then touch it with the rod and it kind of sucks the filler in, that's fine for a bottle cage boss, maybe a bridge, but too weak for a main junction of actual frame tubes (unless you're using lugs). Andrew Stewart knows much more about brazing so see his answer. What sort of torch do you have? You might need oxy-acetylene to do the kind of brazing you need (but I'm not an expert).

Originally Posted by Pleux View Post
for the head tube 48mm outer and 34mm inner diameter should fit 1,1/8 headset and for the down tube maybe something bigger to support the weight.
That sounds like 7mm wall thickness which would actually be too thick for your flux-core welder! If you're using normal cheap rolled and welded mild steel tubes about 1.6mm or 2mm wall should be about right which will be doable with your welder.

34mm is the right ID for the headset in theory but basic rolled tubing will have a weld seam on the inside that will need cleaning up. Bicycle tubes are often supplied a little bit too small on the ID with the idea that you ream them out to the exact diameter, typically with a hand-reamer. The proper tool for this is rather expensive but you can manage with a cheap adjustable reamer. I would say see how you get on and how well the headset looks like it's going to fit. You also should face the tubes so that the ends are parallel (another expensive hand-tool although the reaming tool is often combined with the facing tool). Alternatives are to use a lathe before you build the tube into the bike or just to get close enough with sanders and files. It's not the proper way but it's a cargo bike, your first frame, and often there's a lot to be gained from just diving in and having a go without too high expectations from your first project.

Originally Posted by Pleux View Post
my problem with welding is that im probably going to burn holes in the frame and brazing seem easier to get right and looks better as well. in terms of wight performance and considering that it is my first time, is it worth to try the cheap flux core welder or should go with thick brazing?
I've never tried fillet brazing but I don't think it's particularly easy. Not that you should be discouraged-- these skills can all be learned with enough practice. The first thing is to get some cheap round tube of about the right size from your local steelyard and start practising some joints. Mitring is important-- the better the fit-up you can get, the easier the welding (or brazing) will be. Yes it will be very easy to blow holes with the flux-core machine but anything 2mm and up should not be too hard, 1.6mm at a pinch. Then you need to strength-test your joints by breaking them-- the tubes should be completely buckled up and deformed before any welds break. This test doesn't make any guarantees about fatigue life but is a sensible minimum. You need to do the same kind of testing with any kind of brazing you might be thinking of trying.

If you do get into using proper bike tubing at some point in the future (0.9mm wall and thinner for the main tubes) flux-core is not even an option and you will need TIG or (the right sort of) brazing at that point.

BTW you should also check out Phil Vandelay on YouTube as he has designed and made quite a few cargo bikes (taking plenty of liberties with things like reaming of head-tubes in the early days .
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Old 02-03-20, 11:25 AM
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fillet brazing is a good skill to have, but it isn't particularly easy. I have done some fillets with silver (fillet pro), but I think you will find it difficult. LFB fillets with collapse with a little too much heat, but nothing like silver does. Bronze (LFB) is far cheaper as well. But you should practice.
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