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.

Center-lines

A few operations in the construction of a bicycle require you to have a clear center-line for the tubes.  The top tube, for instance, needs to be cut on both sides so that it will mate up with the head tube and down tube.  If those cuts aren't lined up with each other, you'll end up with a very twisty frame.  If you have a center-line, you just match it up to your templates (more on that when I post about mitering).  If you don't, it gets more complicated.

Before drawing on the tubes, I needed to figure out which way they bent, since it was pretty unlikely that they'd be perfectly straight. To do that I used an alignment table.  An alignment table is a big slab of stone or metal that is ground carefully until it's perfectly flat across the top.  It seems like any table would work, but it has to be pretty damned precise to make this work.  I rolled the tubes on the table, looking at the gaps underneath, until I found the flattest axis... then marked those as the "sides" of the tubes.  The center-line would be 90 degrees around the tubes from the "sides", because the tubes being slightly curved towards the front or back of the bike wouldn't make as much of a difference as having them lean slightly to the right or left.

Actually making that line is one of those things that seems easy, but ends up being really hard.  Lining up a straight edge with the tube is difficult.  Keeping it there while you mark things is pretty impossible without designing some kind of a fixture to hold everything.  I basically held two tubes together, with the point that I wanted the lines to start from deep in the "V" between them and dragged a marker down the center... again and again until the line started where I wanted it to. 

I then used the alignment table to check if they were straight with a specially made caliper.  If both ends of the line were the same distance off of the table, then I was good.  Otherwise, back to the drawingboard.  

When finished, I taped over them so that I wouldn't accidentally rub them off when working with the tubes. EDIT: NEVER DO THIS.  Taking off the tape is a full time job.  Next time I do this, I'll just get the miter templates and bottle cage mounts aligned right away.  

That's it for today, next time I'll share the diagram that I'll be working off of.