Welding a 4130 Chromoly Frame Crack
 

I have a Burley Duet Tandem. The frame has a crack at the base of the back seat tube, near the bottom bracket. This bike is made of 4130 Chromoly and I am wondering if it is wrong to attempt to get a professional to weld the crack.

I once repaired a Burley that cracked at the base of the seat tube. It was kind of a Big Hairy Deal, not economically feasible, did it as a hobby so I didn't mind not getting paid for all the work I put into it. Most qualified pros would have turned down the request, or charged more than the frame is worth.

The problem was , the crack was caused by rust. It had rusted all the way through from the inside. Burley didn't put any vent hole at the bottom bracket, so water that got in had no where to go, just puddled there.

And water will always get in, if the bike is ridden in the rain or carried on a car rack through rain etc. It needs a way out.

No way to know if that's the problem in your case. Pictures are 100% essential as a minimum whenever asking "can this be fixed". They need to be properly focused close-ups with good lighting, from several angles. Look at your photos closely, delete and re-shoot if they're blurry or don't show the crack properly. I only say that because so many of the pics that people upload are useless. Sorry if that seems harsh, but you're asking for help here, so help us help you.

But if rust played a part in your crack, then consider that the other seat tube might be just about as rusted, and just about to crack. So repairing just one crack might only get you a little way down the road before the next crack.

As a general rule, welding a crack in thinwall tubing almost never works, it will just crack again, even if rust-through is not your problem. A proper fix might include doubling up over the cracked area with a sleeve or gusset, and fillet brazing (not welding) which allows the sleeve to be properly "unitized" with the tube. But I'm more skilled at brazing than welding, so that biases my opinion on how to fix it. Maybe a really good welder could do it as well as my proposed brazing repair.

Another good option is shop for a new (used) tandem. Vintage tandems often sell for amazingly low prices. You might even be able to upgrade to a better machine for less than what this repair might cost.

Here's my repair, before and after:

https://cimg8.ibsrv.net/gimg/bikefor...6ced3184b3.jpg
https://cimg0.ibsrv.net/gimg/bikefor...41b24697f5.jpg

The sleeve is tapered, thick at the bottom, down to vanishingly thin at the top and faired-in with brass, so it disappears when painted. WAY fanicier repair than was needed or justified, but it was "fun" for me. sort of.

Mark, your collection of pictures must be massive :) I should take more pics, I don't think I have taken any of the repairs I have done.

That is just great!

Just curious if you could have accomplished similar with a big fillet? I realize the brass or silver wouldn't be as strong.

I would have had no way to know how high up the fillet would need to go to be fully above the part thinned by rust. My guess is "way too high".

Main reason this repair was so much work was I replaced the BB shell with a larger-diameter one, and offset the miter upward so as much as possible of the rusted tube was mitered away.

Mitering:
https://cimg2.ibsrv.net/gimg/bikefor...b574b05cfb.jpg

New shell fitted:
https://cimg5.ibsrv.net/gimg/bikefor...22ef822ab3.jpg

Taking the old shell off obviously made more work, but I wouldn't have done the repair otherwise, because I wouldn't have been able to know if it was all a waste of time. With the old shell off, I was able to see inside, remove the rust, measure how much thickness was lost, and how high up. Only then was I convinced to go ahead with the rest of the repair.

Replacing the shell also allowed me to get away from the abomination of setscrews for locking the rotation of the eccentric insert. They suck. Some may say a Bushnell internally-expanding insert is the best, and they are very clever, but expensive. Plus I still prefer this old way, which has always been reliable and easy to adjust, anywhere including mid-ride, no matter how old or corroded-in-place everything is.

Finally, the eccentric that came with the bike wasn't large enough diameter to have enough eccentricity (I call it "E") to adjust to proper tightness, on any chain. The bike came to me maxed out, eccentric all the way forward, but the chain was still a bit loose. But we couldn't take a link out, because then the chain wasn't long enough, even with the eccentric all the way back. To work with any length of chain, you need at least a half-inch of E, and this Burley had less than that. Yes I know about half-links, a sad kluge to have to use when the correct diameter shell can avoid needing one. So replacing the BB shell had at least three advantages.

Note: by length of the chain, I'm not talking about number of links, I'm talking about chain wear, or "stretch" as some call it. The whole reason why you need an eccentric in the first place. The interplay between keel tube length, chainring size and chain length, chain wear and E, has been well known for well over a hundred years, but somehow Burley didn't get the memo. They're not alone there; some other brands of tandem have been made with less than 1/2" of E as well. That requirement seems obvious to me, but then I've been a tandem specialist since my first FB job in '76 at Santana, so I've spent a lot of time thinking about these things.

Note, I liked the Burley company, mourned it when it died, and I liked almost everything else about this bike — other than the lack of vent holes (causing rust-through) and the too-small BB shell with setscrews to lock down the eccentric. Everything else was amazing, how great a bike they made for the money. Tremendous value.

But like I say, buying a new tandem might well have been cheaper.

Did you cheat the shell up from its old center to take out more seat tube? Or is it located on the old centerpoint?

Cheated it upward, I forget how much, probably 5 or 6 mm. That plus the larger diameter shell ate up most of the worst-rusted tube, and the sleeve reinforced the rest.

There are those who claim to be able to feel the difference 5 mm of BB height makes, but I don't believe them!

I don't notice BB height 99.999% of the time. But every time my pedal hits, it becomes an instant obsession.

Definitely not on a tandem when the BB center is in an eccentric!...sheesh.