Chronicles from the deep end: Cargo bike

[bushtrucker]

@Matt

Nova S-bend chainstays

Is there wall thinckness info on these anywhere? Like what are you getting down too if you do cut into the thinner middle section?

Also, based on what otheres have said on this forum that seatube should be ok as it without a sleeve but it’s probably as thin as you’d wanna go on a cargo bike. That’s assuming you don’t have to ream out too much material after welding/brazing too…

@anon91558591

You can TIG right over a silver brazed sleeve, it works fine as long as you’re moderately competent with the welding.

The entire seat cluster area should be at least 1.2mm wall (you can accomplish that by sleeving something thin if you’d like) extending below the seatstay and toptube joints. Otherwise expect failure down the road, generally from the seatstays cracking the seat tube.

Once you have a lot of experience you can go down to 1.1 or even 1.0 wall if you want to geek out on weight, but the handful of grams you’ll save is IMO not worth it. Failed seat clusters are almost impossible to repair.

Your existing frame, assuming you’ve already welded it, is not fixable, but it might ride ok for a while and you’ll have plenty of warning when it does fail (IMO).

-Walt

It seems like next time you mighy be better using a MTB seattube next time. That way you can get a beefy enough top section without a topper. I’ve also found dropper posts on cargo bikes to be really handy and a 31.6 seatpost diameter would open up option there.

@Daniel_Y

@evan.trem

Anything to be done now? Could I do some sort of crazy external collar or side gussets from seat stays to top tube? Or do I just ride it until it breaks.

That bike looks great, especially considering it’s your first frame!

I think there is a high likelihood that it should be fine. Wall thickness is only half the equation, the tube diameter dominates the strength equation, and 35x.09mm is beefy. I think most SS-ST failures you see are on 29mm tubes with thin .7 butts.

Total guess: I think half of the frame failures are engineering mistakes, and the other half are fabrication errors. If you try reinforcing and gusseting the area, you are probably more likely to introduce fabrication errors (heat, contamination, etc…)

I also recommend using a pre-butted seat tube instead of the topper. IMO seat toppers are an outdated process: 35MM Ø SEAT TUBE DEDICCIAI BUTTED FOR 31.6 SEAT POST 1.65/.9 X 600MM — BICYCLE FABRICATION SUPPLY

The first time is never perfect, I have to constantly remind myself of that. You already know what to do next time (place the stress on the thick butts), so if it breaks, just build it better!