Saturday, January 28, 2012


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.


  1. You could also get a simple piece of aluminum 90* angle (say .25"x .25".) Rest it on the tube and draw your line. Or you could get some layout dye, paint the area for your line and drag the tip of the height gauge along the side of the tube while sitting in the V blocks. Much easier than holding tubes together IMO.

    1. I tried using a piece of extruded aluminum (not a 90, but that fit over it pretty well), and had issues with it shifting around. Might just need a bit of practice ;-)

      I'd seen people use the height gauge as a scribe, but the place I'm renting use of equipment at is pretty picky about how you use tools(for good reason, way too many people use it not to be). Using the dye makes perfect sense though, because I wouldn't have to actually apply any real pressure at all.

      Thanks! I'll re-do the top tube using that technique, since it'd be a bit more precise and that's where the miters have the largest chance of being out of alignment with each other.

  2. If the height gauge thing doesn't work, get an inexpensive machinist square and drag the blade along the tube to scrape the dye (while held in v blocks.) It's not as thin of a line, but it works well to verify an existing line (like when you've already joined the HT & DT and need to miter for the BB.) The height gauge will give you a straight line, but not necessarily where it needs to be once the phase of the tube is set by being attached to another.


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