The track hub on the back wheel allows me to do some interesting things.
This side has about 1/2" of threads so I used that for the 2 sprockets since it would provide the most support for the motor power.
This side has 2 sizes of threads that are about 1/4" deep each. The larger size fits a standard brake rotor adapter.
The rear brake rotor mounted.
The drive side uses 2 brake rotor adapters locked together so that all the threads are engaged on the track hub. Conveniently I found 2 with the right internal threads and that the M5 threaded holes lined up perfectly. If this doesn't hold up, I may need to make my own adapter.
I machined the SOMA track cog sprocket to have the 6 bolt pattern of the dual rotor adapters. The steel in these sprockets is VERY tough and hard. Typical HSS drills will not cut it. I used a carbide drill to make the 6 bolt holes. The threaded flange was milled off with a carbide end mill.
I got a bunch of these adapter plates laser cut. The outer bolt pattern is the same as is common on 219 wheel sprockets. The inner bolt pattern matches the brake rotor adapter bolt pattern. I then mount the 219 sprocket to the adapter plate and then bolt it to the brake rotor adapters. This is the rear sprocket I'll use. It's so large since I'm gearing in a single stage from the 74kv motor to the wheel. It needs to opened up a good bit to take out weight and to make it into less of a speaker cone so it doesn't amplify chain noise.
The bike chain clears the bolt heads by about 2mm. I think this will be OK. Worst case, I mill them down a bit like I did on the crank bolt heads.
Test fit of the 219 sprocket next to the tire. I have a straight edge in there to make sure the 219 chain will clear the tire. It's close, but OK. This sprocket looks huge on a 20" wheel. It will look lots better once I open up slots in it. It is a custom made sprocket from Azusa Engineering. You can't find ready made 117T wheel sprockets!
The final sprocket set up. The derailleur is just keeping the bike chain tensioned.