<< my forte is building shifters >> I love shifting, it reminds me of the cars I have had; nothing fancy, just stick shifters.
<< that anything over 20 MPH was just too crazy fast >> Yes, I tell interested by-standers that my trike is an exciting little death trap. With the new diff, it handles well, but you need to be careful on corners, and don't twist the throttle too quick or it will throw you off the back. Don't tell my wife I said so.
<<in 1st or 2nd they would promptly pop a wheelie and spin like a top around the left static back wheel.>> Yes, that's what mine did before the diff. I recon that it could look like an elegant ballet if done right, otherwise it may look like a calamity that is fascinating for everyone to watch.
Interesting about the Nuvinci 50% failure rate; I'm glad I didn't get one. Also interesting that the Nexus is so successful. I scoured this forum about shift hubs, and the Nexus seems to be the most popular. I used a Sram iMotion 3 speed on my chopper, because it was available locally, and I didn't have room for the Nexus click-box.
When my Sram gives up, I plan to mod the rear end to accomodaye the Nexus, because the click-box is a much better arrangement with indexing at the hub, instead of indexing at the shifter as with the iMotion.
<< will guess your failures were caused by the same thing I have seen eat a couple of 7-speed derailleur shifters, downshifting too low when you think you'll have to stop but don't and hit WOT causing a high power hi torque hit on the back wheel.>>
Actually I think I have the freewheels set up so that it is immune to that treatment. I destroyed my hubs in the following 3 ways:
1. When I bought the first hub, I asked the guy at the bike shop if he had one without a coaster brake. He said to just take the coaster brake out, so in blind faith, that's what I did and looked no further. It turns out that the brake shoes that I removed were important spacers for a pair of pawls, so when those slid loose, they jambed and the hub blew.
2. The second hub also had a coaster brake, so this time I made a spacer out of copper pipe to hold the pawls in place. All was well until the shift cable housing slipped. I did not have a proper cable stop, so when the housing slipped, it changed the cable adjustment and I applied power between the gears, and it blew. That is when I learned why indexing at the hub is better than indexing at the shifter.
3. The third hub was a Frankenstein job that I made from the first 2 hubs. I slathered and packed it in grease so that it would last forever. Well the grease caused one of the pawls to stick, forcing the other pawl to take the whole load. Now I know that when that happens, it will break, jamb, and blow out.
For the fourth hub, I found one without a brake. I put it in without messing with it, and it is still working well. As long as I make sure that the cable is correctly adjusted, I think it will hold up.
Your pic of the cranks is interesting. On the trike, I have the spindle and cartridge shown on the top, with the cranks shown on the bottom. The spindle is from SBP and it is longer on one side than the other. Since the engine is wider on one side, I put the wide part of the spindle on that side. As a result the threads are backwards, but it works OK with some Locktite.
The specs say the sprocket takes a 420 chain.
Will it take a 415 or maybe a 41?
Good question. The first thing I did when I got the engine, was to see how the chain fits. I'm using 41 chain and it fits good. I tried heavy duty cruiser chain, and it is too narrow (not sure what size it is).
Happy to share as others here have helped me so much. I appreciate your interest. I am such a gear head nerd that I can ramble about mechanical toys forever.
I will post pics as she progresses, and maybe a video.