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Father/Daughter Project - Part 10

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Father/Daughter Project - Part 10

Tuesday, January 14, 2020   |   Chris Blandford

Back at it:

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15. Top tubes / Seat stays!
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Lots of considerations for these top tubes/seat stays. Before I started this bike, I knew I wanted these tubes to be swoopy-curvy. So, I impulse-purchased a Harbor Freight tubing roller and some small-diameter dies from Swag Offroad. When I got the roller home, I ran a couple test pieces. I’d never rolled tubes before. Super fun. I’m definitely going to use this in the future, for a variety of things.

I wasn’t able to find any definitive information on how to achieve this style of mixte-esque stay, so I had to wing it a bit. I began by rolling the tubes (went with .5”) across their entire length, holding them up to the frame to check the radius as I went. I rolled little by little until I was satisfied. No drawing here, just building in space.

Next, I compared the curved-to-my-liking tubes to my drawing. I determined that if I raked the ends of the tubes on my fork blade bender (using a 5” radius die), the dropout tabs would line up nicely. So, I raked the ends.

Next came the tricky part. I knew I wanted the stays to pinch in towards the seat tube (for little-kid-inner-thigh clearance), as opposed to running in straight (wide) lines from dropouts to head tube. But, I was unsure how to put that kink/bend in the middle of (and opposed to) my long, swoopy bend. In addition, I thought it would be nice if the seat stay portions of these tubes were “twisted” inward a bit (for brake stud spacing and aesthetics purposes). I thought about just putting a slit on the outside of the stays and nudging everything inward, but in the end decided to split the long tubes entirely in half and miter the four different sections individually. (My fall-back plan was to just throw a single top tube in the frame, and side-tack the seat stays.)

Once split, I tabbed, slotted, and mitered everything up. As I normally do with seat stays, I started with the dropout tabs, then slotted the seat stays (using my drawing, just like the chain stays). Then I mitered the seat tube ends of the seat stays. Finally, I mitered the two little top tubes. All of these miters were done the same way as the chainstays-to-BB miters, using BikeCAD’s paper templates.

This all took two full evenings. Definitely the trickiest mitering I’ve done.

Once mitered, I was left with (anticipated) little half-butt joints on either side of the seat tube. I’m guessing this could be addressed differently on a welded frame, but I decided that--since I was brazing this all together--I’d use an internal splice to connect everything back together and avoid having two tube ends running right into one another. I’ve spliced the ends of rack decks together like this, but I had/have no idea if this is a kosher way of doing this here (so certainly don’t copy me!). The end result, though, is essentially a solid-ish plug of steel and bronze, about an inch long, right at the side of the seat tube. (These spots are also going to get little reinforcement bridges/plates ala Retrotec in my next installment.)

Speaking of, if there’s a better way to execute--in general--these types of top tube/seat stays, I’d love to know what it is! It seems achieving multiple bends in a single plane (?) is pretty simple… But putting multiple bends in two different planes is tricky. I’ll give this some additional thought for the Father version of this frame…

Anyway, I next drilled vent holes at the "tops" and "bottoms" of each of the four tubes. Then I cleaned everything up and tacked everything together, in the jig. At UBI, we tacked seat stays in place out of the jig. However, I’ve found that--as long as the frame goes back in the jig easily--the “universal tube holder” that my Anvil came with is really helpful for holding seat stays in place for tacking. Once tacked, I removed the frame from the jig and brazed all six joints in my Park stand. Same tips and fillers and methods described previously. Once brazed, I let everything cool, soaked off the flux, and finished all of the joints. Again, all as described previously. A quick check on the alignment table told me that everything stayed put, for better or worse.

Bridges, reinforcements, and braze-ons next. Fork after that. Need to move quickly now… Might even take a day off of work-work to get the fork built. In any case, back soon!