it’s been awhile since i’ve bumped this thread. still modifying old frames, still learning a lot. picking up new tools as i need them and getting a little better at this with every frame.
i have a question though. on this cross roads i’m shortening the rear a bunch but also reduced the bb drop, so for the first time i have a frame where the original seat stays are too short to reuse as they are.
i have two options, or three i guess but i’m not a fan of the third.
first option is to lower the point where the seat stays land on the seat tube. they would still be within the external butting on the seat tube, but probably about 20-25mm below where they originally landed. i feel like this would be fine since it’s still on the thicker part of the seat tube butting but i’d love someone who knows things to verify that.
second option is to do my first segmented seat stay frame, which is something i’m kinda excited to do. would a chunk of down or top tube from another frame be adequate to use for the main part of the wishbone? i’m thinking about using a single length of tube for the cross bar rather than two segments. i hope all of that makes sense, i’m not really sure what the individual parts of a wishbone/segmented seat stay are called.
third option that i’m not a fan of is to just buy new steel for the seat stays. i’d rather use what i’ve got though and don’t really see a reason to have to buy new parts.
i feel like i’m getting closer to building my first frame from a pile of tubes. my mitering situation isn’t the best, but as long as i take my time and test fit a million times i get them about as tight as i could any other way i think. still don’t have a proper way to align finished frames either.
I was in your same situation, I am making a 20in minivelo from an old Marin MTB and the seatstays were too much of a paint to adapt so I save them and in their place I have bent a tube that I had, a normal steel 1/2in tube, I have a manual vevor bender and I gave it a 180d bend, then I will have to weld a bridge to the seatpost but so far so good.
it’s the only fixture i’ve ever used but there’s a couple things i’d change about it if i could. that said, it’s been awesome and pretty easy to set up!
Yes a bit of top tube or down tube will be fine for the “wishbone” of a segmented rear end. You want an end piece so it has a butt on it.
It sounds like you’re then thinking of doing a kind of T junction with what I suppose you could call the “rear fork crown”. If they’re individual legs you can angle them a bit which will make them a bit stiffer, but it probably doesn’t matter.
For segmented forks at the front I use sections of 1" steerer, which is overkill, but it’s a fork, which is unsupported and safety-critical, and they’re short sections so weight is not an issue.
There is much less stress at the top of the seatstays because we also have chainstays. They are often pencil-thin. Although your “crown tube” will be in bending, it’s very short. So I don’t think it will matter too much what you use. I can’t see another piece off the end of a TT or DT being a problem. But I haven’t run an FEA or anything.
The gravel fram I built last year features both of your preferred solutions, a segmented rear AND dropped stays.
I think 20-25mm of drop will not cause any major issues as long as there is still some distance to the beginning of the butted section. Of course, depending on what you’d like to do with this frame… Seated 20ft huck-to-flats might not be the best idea
I actually did some FEA to compare the different possible configurations of how the segmentation can be done and couldn’t find a significant difference in stress for any of them, so you should be fine on that one.
this is gonna be a polo frame for a buddy so it does need to be a little on the tougher side, but it’s a lower model crossroads and the scrap frames i have around for tubing are like old 4130 straight gauge frames so strength wise it should be fine.
i don’t think i’ll drop the seat stays, but i’m going to go with a wishbone stay arrangement for sure. i think i’ll be doing that this weekend so maybe i’ll come back with some photos of the results.
this was a lot more work than i expected! i know one would usually build the seat stays and wishbone as a unit then put them on the rest of the frame but since i’m reusing the original seat stays and don’t have a good fixture to keep everything straight and where it belongs, putting the wishbone part on the frame first seemed like the better move.
i didn’t get the actual stays on this weekend as things are crazy around here (we’re fostering a couple of kittens with ringworm that need a ton of work and our water heater basically exploded) but i’m gonna try and get out there sometime after work this week to wrap up the rear end. i’d like to get the head tube replaced next weekend and start soaking the flux off.
i did cut a chunk of seat tube from a frame to use for the main tube of the wishbone, but i didn’t really care for how it looked, being roughly the same size as the other tubes in the main triangle. i ended up using a chunk from some steel handle bars for the main tube, and a bit i cut off my 70’s derbi moped for the cross bar. i’m gonna miter the seat stays to fit up under the cross bar similar to the image luniz posted above.
