I have some rear triangle process questions for the experts. I currently have the front triangle completely finished and the CS’s are tacked on the BB shell. Normally, I will go ahead and fully braze the CS’s then finish/clean everything up before getting the SS’s tacked in and then fully brazed. Once however, after I brazed on the CS’s they had actually angled up towards the ST and I had to do some cold setting to get them back to where they should have been. It’s only happened the one time.
This got me thinking about the process as a whole. Maybe I should go ahead and tack the SS’s in place, then fully braze everything? Are there any pros/cons for completing the rear triangle in this fashion? Will doing it this way cause any undue stress on any of the joints? What does everyone else do?
Thank you in advance, and hope everyone has a wonderful Holiday.
after the main triangle is completed i place it back into the fixture making sure that all clamps and cones fit nicely. because heat and sequencing can change dimensions slightly, i leave the holding devices on the loose side in order to get a sense of how the triangle is versus how the original setup was.
then i take a pair of chainstays with the rear dropouts already installed at the proper angle, and insert them into the shell. the dropouts are clamped. a pair of seatstays are prepared and then fitted to the rear dropouts.
then, i pin the top of the stays to the seat tube, the bottom of the stays to the dropouts, and the chainstays to the shell. i disassemble all of this, add a surrogate axle, tape the seatstays together, and braze the seatstays to the dropouts.
i now have a left and a right rear stay assembly.
after cleaning up all the brazing, the assemblies are re-pinned back into the front triangle but not in my fixture. bear with me. after the right and left sides are pinned to the front triangle, i once again add a surrogate axle.
the next step is critical. with my straight edge i make sure the side to side sway is spot on. when it is i then lightly tack the chainstays to the shell. then i carefully braze both seatstays to the seat tube. and then i finish brazing the chainstays to the shell.
For this reason I ready up the SSs beforehand; and do not remove the frame out of the jig after tacking the CSs, and continue with tacking the SSs too. That way the SSs constrain the CSs from moving up or down. This produces very good rear ends for me.
Of course there will be some internal stresses this way, but I think that is insignificant. The situation is not very different from tacking the front triangle in one go.
My process is somewhat similar to what you initially describe - after finishing the main triangle and filling all the fillets, I then tack the chainstay and dropout subassembly to the BB in the frame fixture, then pull it out (leaving the dummy axle in) and fully braze the CS. I will sometimes check the side to side alignment between tacking and brazing, just to get a sense of how much my tacking sequence is working or not working. AFTER brazing the CS to the BB shell, I will check alignment again and coldset the stays. With thru axle, I do this by placing the frame back in the fixture with a 142mm long 1/2" .035 tube stock where the dummy axle would attach to the fixture, and sight through the dropouts to see if one or the other has raised/dropped. Once I cold set it, I then proceed to hand miter the seat stays, tacking them at the ST first, then braze the dropout end, then pull it out of the fixture to finish the SS/ST junction. It’s a laborious process in some ways, but it gives me peace of mind that the CS are cold set first, and not being brazed under stress/tension. This was both the process I learned at UBI and one that was used at BWC when I worked there. And I know many people tack the rear end all at once to help keep things in alignment, which makes sense too.