I started on this fixture a couple of months ago, then back burnered it. A couple of weeks ago I started to get active again and this weekend I was able to put the finishing touches on it. I’m really excited about how it’s come out.
It is highly insipired by the Arctos Frame Jig that was designed by Gary Helfrich. The details and implementation are different, but it is setup in the same way and I copied the basic overall shape and virtual BB pivot of the Arctos. I always liked the idea of the Arctos Frame Jig because it was made of T-Slot extrusion (in my case I’m using 15-series 8020). That makes the fixture a bit more modular (so that I can reconfigure it for different needs) and kept the largest pieces that I needed to machine relatively small. It doesn’t make it easier or cheaper to make the fixture compared to other designs, there are still over a dozen custom cut parts on here which required high precision and 8020 extrusion is not cheap.
A key feature of this fixture is that every adjustment is locked independently. My old fixture had many parameters locked with the same few bolts. With the old one if I screwed up a miter and made a tube 1mm too short I’d spend 20 minutes rejiggering everything and getting it all aligned again. On the new fixture I’d just need to unlock a couple of handles, slide one part to make the small adjustment, and lock them again.
It was really important to me to have a fixture that makes it easy to install and remove the frame. This fixture makes it easy, you just raise the upper head tube and seat tube cones, release the dummy axle using the quick release, and remove the shaft collar that holds the bottom bracket in place. Many other designs have solid rods which run through the head tube and use many more clamps around frame tubing, but I didn’t want mess with all of that.
This new fixture is much easier to configure than my old one. The basic measurements required are:
- X and Y from the center of the bottom bracket to the bottom of the head tube (438mm Y, 451.7mm X in the drawing below)
- Head tube angle
- Seat tube angle
- Bottom bracket drop
- Chainstay length
- Head tube length
- Seat tube length
Here is a drawing for the next frame that I’m building which shows these dimensions (in red). It also lists dimensions for mitering (in cyan). That is basically everything that I need to make the frame. This bike will be my new commuter, replacing my Novara Fusion.
The fixture will have scales which allow for direct reading of the first 4 items listed. Currently there isn’t a permanent scale for the X offset, but I have a simple solution of clamping a ruler in place. I did make direct reading scales for the head tube angle and seat tube angle, shown here:
The bottom bracket drop also is set from an easy to read scale. The alignment of the bottom of the dummy axle with the scale shows you the bottom bracket drop (74mm in this example):
It isn’t obvious from photos, but a tricky part of the Arctos Jig (and this one) is that the BB position stays static as you adjust the seat tube angle. The same is true for the bottom of the head tube. The seat tube angle adjustment has a virtual pivot point around the bottom bracket despite not having anything connected to that pivot point. That is done with two slots that are concentric around the bottom bracket. I cut these on my baby sized CNC mill, this would be a much harder operation to do on a manual machine. The HTA block has a real pivot which is aligned to be directly under the bottom of the head tube.
The rear triangle setup is pretty easy. There is a rear triangle tower that has a quick release dummy axle holder that is centered with respect to the tube cones. It can be slid back and forth by opening two handles, and then you can read the rear chainstay length using a ruler:
Arctos Jigs use solid aluminum for the standoffs to this rear tower. I hate drilling really deep holes in solid aluminum, so I used 80/20 which already has a hollow cavity down the middle. The end plates that I made keep the threaded shaft for the locking T-Nuts in place:
I used the bottom bracket post from my old jig, but I’ve always been proud of this design. The key is using an adjustable locking shaft collar (about $30 from McMaster Carr). That shaft collar gives me a fine adjustment for the offset so that I can handle 68mm to 73mm bottom brackets easily. Removing the outer locking shaft collar frees up the frame. The rod running through the bottom bracket is solid 5/8″ steel and is plenty beefy.
I’ve already been asked if I’m going to make this fixture as a kit. The simple answer is no for two good reasons:
- It isn’t my design. I wouldn’t feel comfortable selling anything that was so heavily copied from an existing source.
- There are a lot of parts in this fixture that were done on my manual mill and which require a good precision. I think that a realistic estimate would be 15-20 hours of labor per fixture. I just don’t have that amount of free time and would rather use what I have to build frames, spend time with my family and friends, ride my bike, etc.
However I am helping Alistair Spence make one of these fixtures for his shop. I’m excited about the possiblities of us both having compatible fixtures, it opens up options to make unusual hybrids for building tandems or cargo bikes somewhere down the line.
I’m writing this post while on limited sleep, so hopefully it makes at least a little sense. If you have questions leave comments here, on my smugmug (permanent), or my flickr (free account, so photos will fall off over time). There are more photos published in both locations than in this blog entry, and you can find high resolution versions of all photos there.