Spindle
The square taper spindles I found at the bike shops were all too short, so I hacked up a cartridge bottom bracket, removed the spindle, and then cut it in half.
Spindle (cont.)
A piece of steel tubing was then turned on a lathe to have a 20mm OD, and a bore that just fit the spindle halves pulled from the bottom bracket.
Once assembled and welded together we had a square taper spindle that fit right inside the 20mm thru axle hub and which was long enough for the cranks to be properly clear the frame.
Nuvinci Drive Cog
The nuvinci hub comes with a splined interface that a threaded adapter slides onto on which you then attach a screw-on freewheel. This had to be modified quite a bit to in order to fit a drive cog and the one way ball bearing. I decided to use a standard ISO 44mm disk bolt pattern, since there are fix-gear bike types who use the disk mount of a bicycle hub for a fix gear cog.
In my case, I had a set of 16, 18, and 20 tooth cogs with a disk mount bolt pattern from tomicog that I got in anticipation of this project.
So a disk was cut out of 1/4" steel plate with a hole saw, then drilled and tapped with a 44mm ISO bolt pattern, and then silver soldered to the splined freewheel thread adapter from the Nuvinci.
Here is the set of drive chain parts lined up how they will all fit together.
First drivechain test
And then after sandblasting and making a few shims and spacers for all the alignments to work out, we finally got to test out the entire drivechain assembly just last week!
go JUSTIN
That frame is beautiful! Brilliant work! 
Beautiful work!
I did not realize you could braze stainless steel. Do you need special brazing rods?
Scott
Wow!
Holy cow, what an amazing series of photos showing the evolution of an even more amazing project! Looking forward to seeing more results as you post them, keep up the brilliant work!
Over the years I’ve been a member of multiple car and motorcycle forums that have never had a post with such detail and commitment to the final product. Kudos my friend. Oh the world domination devices I would make with all that machinery at my finger tips…
BUT it looks like its going to be ridiculously heavy. I still wanna ride when you get done though.
Oh, it’s heavy that’s for sure! Heavier than a lot of bicycles even. I’m leaving in about 4 hours on a train ride to SF for the weekend, so I might not have time to get many more build posts in before the middle of next week.
However, if anyone wants to brave riding it during this time, it will be meandering around the Maker Faire in San Mateo on both Saturday and Sunday. Well, odd’s are with these things it will probably be broken by day 2 but hopefully we’ll get to have some fun with it before that happens!
Maiden voyage was last Friday at the Kris Holm Book release party, and I was a little shaky on it. But we’ve since got the hang of things pretty well and Pinefresh really showed us how it’s ridden at the vanuni wednesday meetup yesterday.
It’s not stainless, just standard DOM steel tubing. That said though, I’m pretty sure there are no issues brazing stainless with normal rods and fluxes.
crazy, great, very cool!
the frame looks really very nice, and i like the way you’re trying new stuff and bring our sport further! we need people who have ideas and even go for it, thanks for that!
What else to say but insanely awesome… Please keep us updated with any future developments or projects. If you ever bring it to the East Coast, let me know. I want to ride it!
This is an awesome accomplishment! Thanks for taking the time to post everything to share with the uni community. 
So the shifter is under the saddle? I don’t see it in any pics so far.
That is a beautiful machine. Your fillets look perfect.
The one thing I don’t like about the Nuvinci is the twist shifter. If you come up with a lever style shifter I’d love to see it. I have an idea to make a geared shifter since it needs to wrap something like 10" of cable altogether. Also, I think a lever would be easier to use on a uni.
In a world full of square edges, the curves on your frame are a thing of beauty.
Thanks for sharing!
Thanks as always for giving me the chance to break your creations Justin. 
At this point the shifter is the grey knob on the right side of the nuvinci. (see the second picture of me). I managed to shift on the fly, but it involved doubling over completely at the waist and reaching down, it was very awkward.
I’ll make a post with my initial impressions if I have time after work, when I’m not on my phone.
If I understand it correctly there should be very little backlash in this setup. Maybe a bit from chain looseness, but the sprague should engage almost instantly.
How is the transition from being geared up/down going forward to the 1:1 in reverse?
Awesome! I can’t wait to see Justin tomorrow and ride it 
Jason, you should come to the maker faire!!
corbin