Building lugged CX/touring bike
 

I've decided to build a cyclocross/touring bike. I've bought all the tubes and lugs, and have started practicing silver brazing on a pile of pressed lugs and cheap tubes.

I've drawn up the frame in rattlecad, and printed out a 1:1 copy of it, so that I can measure directly.

I plan on making a rough jig out of MDF or plywood, and shimming up the tubes to level.

The plan is to use the jig to hold everything in place while I drill and pin all the lugs.

I will first cut, mitre, and fit up the main triangle in the jig. Then drill and pin the lugs.

I will then dismantle the main triangle and re pin the ST to the BB for brazing, braze, check for movement, and straighten as required.

Next I plan to re pin the DT to the HT, braze, and check angle.

Then re pin DT to BB, braze and check alignment.

Then re pin TT to HT and ST, braze, and straighten.

Does this sound like the correct order of operations?

Should I be cutting, mitering and pinning the main triangle all together before any brazing? Then working on the junctions in that order? Or would I be better off mitering and pinning each joint as I go?

Any help with the order would be much appreciated. Cheers.

I have done the bb/seat tube joint first. Then the hockey stick (head tube/down tube) then put the front triangle together. I think this was a pretty common construction method before the advent of fixtures.

I think the best construction method is to put everything together but the seat stays. I am not sure I would disassemble. that's a lot of (hopefully not moving) parts though.

I was wondering about the possibility of pinning the whole front triangle at once, and brazing all the joints while pinned. I haven't heard of anyone doing this though, so I assume there's a reason it's not commonly done. Maybe stresses in the frame from heat distortion? Although I suspect this would be less of an issue with lugs than on a fillet brazed or welded frame where you have filler metal contracting and pulling in as it cools.

I am pretty sure that most people that pin do the entire front triangle and braze it at the same time. Same for people that tack the frame.

have you found pictures of people doing pieces parts separately?

Ok thanks, I might try pinning the whole thing and then brazing as one peice. I haven't found many photos of pinned frames, only details of pinned lugs, making it hard to tell in what order it is done in.

Pinning serves a few purposes. What many talk about is with a few pre joint/lug one can hold the geometry with no tacking or jig. This opens up both the set up options (jig or drafting determining the geometry as example) as well as makes jigless brazing so possible. Less often mentioned is that with a pin per lug the tubes become indexed to their position. So removing a tube or lug from the set up and replacing it can be done with assurance of duplicating the tube/lug's orientation. If you're the type to want to do a number of building steps to the tube before final in frame brazing (adding braze on bits is the typical need). Pinning also traps many dimensions from shifting when brazing, like fork blades in crown.

When I do a lugged frame I both pin once per lug socket/tube and tack before I remove the frame from the jig, correct any alignment drift then final free braze. Andy

Thanks for the info Andy, I think I'll be going with pinning.

I was planing to post my progress as I go, but I can't seem to attach images to my replies in this thread, it says I do not have permission to use URL's. I'm on an Android phone, and trying to attach an image from my gallery. It uploads, but then says I don't have permission to post?

I'm pretty sure you need 10 posts to attach an image. Anti-spam measure. You're almost there, sorry for the inconvenience. We'd love to see pics.

On my bike with similar objectives, I'm going to use a filleted bb shell. It's just a little too tricky to stuff big tires into a frame built with a lugged shell. And I figure nobody really looks at the bb anyway :)

Funny that I've decided to make my next city/workhorse bike with lugs, including the BB. How the shell will go I have no idea yet. Will I have to build that too? The saving grace is that I'll use 559ISO wheels and this will make squeezing in 38mm tires much easier. Andy

I think 38's might fit with oval chainstays. Depends on your tolerance for clearance on the sides

Ok makes sense, I think I've got 2 more posts to make.

Ive done a couple of practice pressed lugs with some spare tubes I bought. Using cycle design light stainless flux, and 56% silver.

https://cimg6.ibsrv.net/gimg/bikefor...170edd7988.jpg

Joint cooling down, and flux beginning to crack. Looks like only a very small amount of burnt flux. Pretty happy with that for my second lug.

https://cimg1.ibsrv.net/gimg/bikefor...dafc1dc473.jpg

Here's the same lug after a quick scotchbrite. Bit of over filling with the silver, but would be pretty easy to clean up.
https://cimg7.ibsrv.net/gimg/bikefor...25e9a8d0f7.jpg

I bought a stainless lugged headstem set, so I thought I should have a go with some stainless. I've heard it's a fair bit harder to solder. Here's some 316 SS that I sleeved over itself. Took a bit longer, but didn't seem too difficult. Will be a bit more difficult with the headstem lugs and the thin walled connector tubes I imagine.

https://cimg2.ibsrv.net/gimg/bikefor...421afb1c62.jpg

I've started cutting and mitering the tubes, and noticed the gap between the top HT lug and the HT is sort of largish. I can fit a 0.6mm shim inbetween the lug and HT in some areas. Not sure that silver will fill this. I'm considering cutting and re welding the lug back together, or could i shim it, and let the silver fill in around the shim?

That is a big gap. I probably would fill it with 45% silver.

One thing I noticed from your lug is that you are filling from the shoreline. Leave the seat tube long and fill from above the seat lug. Then pull the filler through to the other shorelines. Similarly, leave the head tube long on top and bottom and fill from there. Makes for much nicer shorelines and you can be sure you filled the lug.

The stainless looks good. It is more difficult because it's easier to overheat. Just a little bit of extra heat and the joint is cooked and it's difficult to flow silver. Looks like you have your heat control down pretty well though. In that case, it's no different than other steels.

Thanks for the info. All the seat lugs I got have an inside lip that stops the seat tube coming through any further. I will leave the HT longer though like you said, and feed silver into the HT lugs from the outsides.

I hate that lip. For one thing, it's keeps my fixture from holding the seat tube properly. I put the lug on the lathe.

I know the other guys here have provided some info on order of operations, and I'd like to add my own experience learned from making mistakes to the mix. Like unterhausen suggested I've also found that building ST-BB and then HT-DT as two units is the way to go with limited tools. This way you can more easily correct mistakes in the front triangle before getting the TT in place, because once the TT is brazed in making corrections to the front triangle can be VERY difficult as this assembly is super strong. Also, a word of warning regarding pinning: Do practice joints and measure them. I've found that pinning isn't all that easy to do properly. Getting the pins placed so that they lock the tubes in place can be quite difficult and if anything moves on your during brazing or while moving the frame out of the jig and your joint is crooked it can be a lot more work to make a correction since your joint is full of holes that might be in the wrong place. I've found tacking at the lug tips to be faster and easier than pinning, but for the newbie like myself there are dangers here too. If you get enough heat into the joint so that all the brazing material is liquid and your jigs aren't properly setup (or if your completely out of the jig) the joint can slip and move out of alignment which means reworking the joint. Andrew's advice of pinning and tacking is quite good (he tends to give good advice!).

Thanks for all the insights, it's very much appreciated.

I think I'm leaning towards pinning the whole frame in the jig, taking it out and brazing, despite the potential issues of alignment, and the trickier straightening after brazing. My thinking is that, if I can get 3 pins per tube end, I can not really see very much movement being possible. With 2 pins on oposing sides of a tube, I can see movement around that pin axis being possible, but not with 3 pins. I'll do another test lug with pins, as I still have some tube and lugs left over, and see how it goes. I may still change my mind depending on the test lug.

I would test out a pinned joint before committing. If your jig is not rigid enough there is a chance you'll throw the mitre off inside the lug when you drill for the pin. I've tried it and was disappointed to see a gap when I cut the joint up. Brazing looks great, well done.

I would not overthink alignment unless you have a reliable method to inspect it. What I like to do is braze the HT/TT then add the ST so I can check for twist. Then add the DT and BB and check BB alignment.

https://cimg4.ibsrv.net/gimg/bikefor...168cce6037.jpg

Found some steel pipe that fits pretty well into the BB, squared off the end, and welded it to a plate. Then drilled and countersunk some holes so that I can screw it down to the MDF. I had planned to reference the BB off one of it's faces, but found that they are not parralel to each other.
https://cimg5.ibsrv.net/gimg/bikefor...bd6612b8ff.jpg

Roughly shimmed everything up to have a look. Found some pipe that fits nicely into the HT, and figured a wider reference area would mean more accuracy in keeping the HT in line with the ST.

​​​​​​

Regarding the ST lug, I had already cut the ST to length before reading your comments, so I can no longer file out the lug to fit the ST through the top. I'm assuming this isn't a big deal? Will i just have to file it close to the ID of the ST before reaming it out?

no, it's not a big deal. I think it's a really good idea to feed filler from one location and pull it through to the other tube just so you know the junction between the tubes is filled. Even then, you might need to fill some gaps. But it looks to me that your heat control is good enough you should be able to fill from one place and move the filler around. Try it.