Working with Columbus' ZONA12750001110E Bent Down Tube

I am using Columbus' ZONA12750001110E bent down tube on my Fill It, Chubby! demo build. I ordered two with the anticipation that something would go wrong (and secretly hoping that the second tube would be a talisman against bad luck!). I ended up needing that second tube.

Let me back up a little bit. Columbus does not provide a critically dimensioned drawing for the ZONA12750001110E. Although the drawing provided does provide most of the important dimensions, some dimensions required educated guessing. Most noticeably absent are the tubing or center line lengths from the end of the tube to the point of the bend. As a point of comparison, True Temper's HVERUSDTMAG bent down tube has a drawing that is critically dimensioned. I used the True Temper down tube multiple times with zero problems; it always mitered as spec'd in the drawing.

The result of the under-spec'd Columbus drawing is a frame drawing that may or may not be completely accurate (i.e. may be off by a couple of millimeters). This turns out to be a big problem. I ended up being 3mm short on my first attempt at mitering the tube based on my initial drawing.

For the second attempt, I measured the actual length of the tube along the center line of the top and bottom of the tube. I did this with duct tape (which stretches), so, again, I wasn't super confident in the directly measured dimensions. However, these dimensions provided me with a slightly better guess of the true tube dimension.

Despite the better dimensions, I got scared. Consequently, I just mitered the tube a little long and then finished it by hand. This worked fine, but I definitely prefer being able to cut the down tube correctly the the first time on the mill.

I recorded the under miter and the amount that I filed off to get a better dimensioned down tube. That drawing is provided here in PDF format. That said, I am still not 100% confident in the drawing, so USE IT AT YOUR OWN RISK. I am very curious to hear what you find while using Columbus' ZONA12750001110E, please email me (ryan@pornstarbike.com) with your results.

PS: When I order my next ZONA12750001110E, I'll take some very close measurements and then post a DWG of the more accurate drawing along with a revised PDF.

Columbus Bent Downtube.JPG