Making Stays

The seat stays connect the dropouts for the rear wheel to the junction of the seat and top tubes.  On the dropout end, I attach a cap that slots into the rest of the dropout.  The inside of the cap has some curves to it, so I started by filing the stay to match.  I used calipers to determine the maximum distance that it could sink in, then measured that on the stay each time I made an adjustment.

Now, since the cap doesn't have a second open end, I had to do things a little differently to see if I'd filled everything properly.  If I were a pro, a blind braze like this wouldn't make me blink, but I'm still a bit paranoid.  On the suggestion of the internet, I curled up a little plug of silver, coated it in flux, and dropped it in the cap.  Then I added the stay and heated the whole end until the silver melted and was sucked up into the joint by capillary action.

For the other end, I wanted to make an abrupt but rounded cap.  I've already done this, so it didn't hold a lot of surprises.  It was a bit harder than last time because I spaced on drilling vent holes before brazing the end up.  That leaves heated air trying to push the filler out.  It worked out alright, but I'll be more careful next time.  I used brass so that the filler wouldn't melt when I braze them to the sides of the seat lug.

Also note that I notched the lug itself to increase the surface area that will hold things together.

The dropout caps go on the chainstays in about the same way as they go on the seat stays, but you have to line them up so that they're in the same plane as the oval in the stay.

Mitering!

I just finished preparing the new tubes for my main triangle.  This time around, I marked the center lines a little differently than last time.  Instead of using a Sharpie to mark the center of the tube, I slathered it in layout fluid (a kind of easily removable paint) and dragged the height gauge along it.  That left a nice clean silver track through the coat of blue.  It was a much finer line than the sharpie left, quicker to place, and more accurate.

I also re-measured the butts on the tubes, as I'd swapped a couple of them out for new ones.  I'm glad I was careful; one of the tubes had the thinned section shifted about 30 mm towards one end.

I'm using some guidelines recommended to me by a talented builder (who coincidentally designed the slant-six lugs that I'm using) about the location of the butts and lugs.  He said that I should make sure that the lug tips don't come closer than 20mm from the end of the butt on the top tube and 40mm on the downtube.  There's not a ton of agreement about how much of a safety margin is necessary, but I think that it makes good sense for me to err on the side of caution for this first bike.

I marked the tubes at the end of the butt and at the beginning of the "safe area" to avoid confusion.

Mitering

Mitering is when you cut the "fishmouth" into one tube that lets it rest flush against another tube (the base tube) at a specific angle.  Figuring out where to cut can be tricky.  Luckily, Nova (where I buy my tubes) has a great little app on their site.

After you set the tube sizes and angles, the app gives you a printout that you can cut out and tape around the tube in question.  By lining up the center lines of the printouts with the center lines on the tubes, you ensure that everything is in the right plane.  Even better, you get a reference line that helps you space the patterns out.  In this case, the square end of the paper is 101mm from the center point of the base tube.  If you take the total distance from one intersection to another and subtract the offsets from both of the templates, it gives you how far apart they need to be.

I roughed out each miter with a Dremel, then knocked off the last few mm with a half-round file.  That kept me from going too far or heating the paper until it browned, which makes it hard to see what's going on.  Notice that the template has two cut-lines on it.  One represents the outside of the tube, the other represents the inside.  This helps you figure out which way the edge of the tube should be slanted, and how much.

Note: When you join two tubes of the same size, it gives you a pretty huge "outside" line.  This assumes that you can file the sides infinitely thinly so that they go all the way to the middle of the other tube.  You can't.  I just knocked mine down to a bit over the "inside" line.

I finished each miter by wrapping the corresponding base tube in sand paper and dragging it through, taking care of any remaining little issues.  Then I balanced the base tube on top of the miter and checked the angles of both tubes with my little Wixey Angle Gauge.  In one case I was within .1 degree of the correct angle on the first try.  With the others I had to do a bit more filing to touch it up.  

This weekend I should be able to finally start brazing my front triangle!

Ignore This Boring Post About Butts

One quick bit of advice for anyone else embarking on this whole frame building thing.  Always ALWAYS measure your tubing.  Most of the tubes we use to make a custom bike are butted, meaning that they are thicker on the ends and thinner in the middle.  This lets the thicker section soak up the higher stresses near the joint, while the thinner section makes the finished bike a lot lighter.

This instructive daguerreotype, found in my great grandfather's memoirs, should help to illustrate what's going on inside the tube.  

You have to be careful though, because you can cut too much off of one of the ends and end up with some of the thinner tubing forming the joint... which may then fold like a coke can.  Even worse, tubing often comes with the butts shorter or longer than the spec sheets tell you they should be, so you really have to measure for yourself to be sure.

I held my tubes up to a bike light and stuck the depth measuring tool on a caliper into it, set for the point where the transition was SUPPOSED to start, then made sure that it lined up with the actual change in reflection indicating the start of the slope towards the thinner spot.  It can be hard to see what's going on in those things, so I had to bracket the measurement (-10mm is DEFINITELY falling short, +10mm is DEFINITELY going over that line).

I found two discrepancies: my seat tube started getting thinner about 1.5 cm lower than it should have and my chainstays (which are ovalized) are about 2.5 mm too wide and a bit too short.  The first won't be an issue provided that I leave the tube as long as possible when shaping the end to fit into the bottom bracket lug.  The chainstays will probably need to get just a kiss from the vise to be the right shape.  I'm a little nervous about that, but it shouldn't be a big deal.

Now I have everything marked, so I will know EXACTLY when I'm in trouble as I trim stuff to length and shape the ends.  Next post will likely be about some more practice welds, which will hopefully be more interesting than all of this measuring.