Filed In: Process
Father/Daughter Project - Part 5
Friday, November 29, 2019 | Chris Blandford
Moving on to the chainstays.
Steps for today:
3. Tab Dropouts; Slot Chainstays
4. Braze Dropouts to Chainstays
5. Finish Chainstay Ends
Before I dive in, one thing to note. For the first couple frames I made I did things one step at a time, with a sort of process tunnel vision. Now, however, I tend to do things whenever it makes most sense in the overall build. An example of this is with the chainstays. Previously, I would’ve brazed the front triangle together before even touching a chainstay or dropout. Now, however, I work on these as separate sub-assemblies of a larger whole, prepping everything in anticipation of finally joining stuff together.
Anyway. I begin by laying my chainstays on top of my overview drawing. I include the tire, chainring(s), and crankarm in these drawings, and check that the chainstay will clear all three. I also check that the outer diameter of the dropout end isn’t wider than the tab on the dropout. It’s a bit of a balancing act to choose the most appropriate chainstay (to this point I’ve only bought pre-formed stays--I haven’t bent my own) and get it ideally positioned. Once satisfied, I trim the dropout end of the stay to length and rough-cut the BB ends to length. Then I lay the chainstays on my surface plate and run a straightedge across their tops, scribing a “top” line onto both.
Tool note: I use my Anvil fork jig as a makeshift chainstay jig. A while back, I made a crude little adapter that fits in the steerer tube holder of this jig. It holds the BB ends of the chainstays in place while I work on them, kinda like a crappy chainstay mitering fixture. (I miter out of the fixture, but the jig is helpful for getting everything in its place.) I use the fork rake measurement and a little math as a makeshift BB drop/rise indicator.
Next, I put a tab on the dropout. For chainstays, I’ve been making these 8-10mm deep. I check the internal diameter of the small end of the trimmed chainstay, and take off an equal amount of material from each side of the dropout (hacksaw first, then files), so that the tab fits into the stay.
Now tabbed, I can place the dropouts onto their dummy axle and load them into the jig. I return the chainstays to my drawing, and trace their shape onto the paper. From this, I make note of the chainstay-to-tab angle. I also note the length of the stays (to the back of the BB) and distance between one another at the BB. I slide the chainstays into the jig and onto the tabbed dropouts, and then move the axle holder assembly back and forth until the spacing (on the BB side) is as measured. I use a straightedge held against the dropout face to double-check my slot angle.
Next, I slot the chainstay ends. Using a hacksaw, I cut a slot the width of the dropout, 8-10mm deep (for 16-20mm total overlap), at the angle measured and confirmed previously. I then finish the slot with a slotting file (?... the flat one!). Finally, I spearpoint the end of the stays using a large round file.
To prep for brazing, I first dry fit everything. My goal is a tight-fitting tab/slot along with an assembly that is held in the fixture without having to apply any force to the tubes. Sometimes, my slot angle will end up a touch off. If this is the case, I’ll nudge the dropout tab in/out a smidge, using my vise and a large wrench. This time around, though, everything played nicely.
I use the usual 80-grit emery and 91% isopropyl to clean the chainstay ends and dropouts. Then I flux everything inside and out, and load the chainstays back into the jig. I braze these vertically--in the jig--as I was shown at UBI. (I know some braze these out of any jig, horizontally, or on a flat surface. I haven’t tried that.)
Now might be a good time to show my brazing setup. I’m using Oxy/Acetylene, with a Smith A1WA torch, the little Kevlar hoses, and a few different tips. For bronze, I use Gasflux brand rods and type B flux. For silver, I use Harris brand silver rods with Gasflux type U flux. I’ve also used CycleDesign stainless flux/rods on occasion. I run my O/A tanks at 7 and 6 psi, respectively.
For tab-style dropouts, I use a Smith 203 tip, a neutral soft-rumbling flame, and 1/16th inch bronze rod. For each dropout, the brazing goes something like this:
1. Tack all four “points” of the chainstay to the dropout
2. Get the entire joint up to brazing temperature
3. Sweat the tacks down into and around the slots
4. Fill each side with bronze (hopefully to the bottom of the tab).
I try to shove 14-16”+ of bronze rod into each stay, depending on the volume of the joint (more for a large chainstay end, less here because the end was small).
After brazing, I let the chainstay assemblies cool in the jig, then give them a soak in my heated ultrasonic cleaner (water only). Once clean of flux, I finish the ends. (This is another thing I previously did what-now-seems-out-of-order. I used to wait and do all of a frame’s finishing work at the end of the build. It’s much, much easier to do this type of work, though, while the chainstays are loose from the rest of the frame. So, I now finish these before attaching them to the frame.) I worked on these with a little Dremel drum sander and some needle files. At the same time, I also dressed up the tab. These bikes are (hopefully) going to end up very swoopy/curvy, so I thought rounding off the tabs would look complimentary. This was done with small files.
That’s it for now. Next I’ll miter the chainstays, connect the seat tube to the BB, and get all those sub-assemblies prepped for the fillet brazing.