When I made my order with Luna, they did not list a 100 mm as being available, or at least in stock. So I opted for the 120, figuring I could always shim it out as needed, plus it made the difference in the clearance being adequate between the cranks and the chain stays. A 100 mm would need offset crank arms I'm pretty sure. I ended up with just a hair of clearance between the motor and the chain stay, 1/16", I didn't like the thought of it being up tight against it. That's a 11.5 Panasonic battery, just 6.65 lbs! I have another identical to it for the Trail Viper, and also a 11.5 Samsung for the Montague. This should give me lots of options range wise, depending on the days "mission." I really like not packing a bigger battery around for my in town zipping around of just a few mile. epic rides though will require an epic battery, or several less then epic ones.
My 15.5" Bullet, after a few brief (AND exhilarating) test rides on the thawing out gravel road (I need fenders/mudguards.....) I live on, has been at my (what's the male term for someone who sews, seamstress can't be right) guy who works for me on the crane from time to time, and also knows how to use a sewing machine (tailor, that's it), getting a stealthy cover for the frame mounted battery box. The last short ride I took, about 1/4 mile, I rode up a short steep slope, about 20 %, in 2 or maybe 3rd gear, and it just ate it up without even breathing hard
I will post some pics and my building experience thoughts when I get it back and have time. I have had an unusual super busy time on the crane, good thing as I have been spending money on E bikes big time, being busy and the crap not quite winter and not quite spring weather has also slowed things down. I did not use the brake sensors, partially because I wasn't sure how to install them, and mostly because I am riding my Montague with PAS 0, and using the thumb throttle as needed. Backing off the throttle when shifting has quickly become second nature and even with the 10 speed rear cassette and no brake sensor on the Bullet shifting so far has been trouble free.
Here is one shot, (the prerequisite one without using a kickstand) with the battery box still uncovered and the wiring just temporary. I think the fact that I have a 15.5" frame makes those tires look even bigger, ha, ha! That hill in the background is 9275' and I live at 5600', with the valley below at 4500', so lots of vert terrain, for better or for worse. The Bullet presents no particular issues with a Bafang conversion, though I must say my Montague folder came in lighter (not a fat tire so no surprise) and the frame layout on it allows the motor to tuck up super high. I have not yet have all three of my E bikes in the shop at one time, the Trail Viper (currently undergoing an upgrade to 52 volts and a new controller), the Montague, and the Bullet. I did find the time to make a bike rack, working off my spare tire rack on the rear door of my 2000 Toyota RAV4, as it occurred to me that I had no way to get it into town to my tailors shop. Having two folders makes a bike rack somewhat archaic, but once I completed the simple RAV4 rack, I realized I could easily fit both the Viper and the Montague, folded, inside, AND the Bullet on the rack. This will be great fun, being able to not only show others my new E ride but also put their asses on two others I just happened to have along. I didn't plan this or realize I had this option until it just kind of fell into place, my first use of this new capability will be with my neighbors who live up the hill. I'll show up at their workplace in town around noon, and say "let's go to lunch", and break the bikes out. I'll supply the rides, and they get to buy me lunch. I post a few more specific details on my build in a few days.
I just saw tomjasz's post re: frame size and how it effects battery selection. I have to agree, somewhat. I was sort of bummed out when I first had the bike in front of me and started playing around with different battery options. I really had my heart set on a 20 AH'er, (as a kid I had a 5 gallon gas tank on my mini bike, nowadays I have a 8 to 10 hrs duration on my homebuilt plane, I've always liked lot's of range in anything) and even made a cardboard mock up of Luna's 20 AH cube, but no way would it fit. Only after grudgingly ordering the 11.5'r did I get the brain fart of having the versatility of having in effect a 11.5, 23, or even a 34.5 AH battery as needed, while not have to lug around a heavier battery all the time, even when it was not needed. The only disadvantage to this approach is it costs more, but screw it. As a very crude way of determining how far "out" one can go, running out one pack and then switching to an fresh identical one of the same size should make it pretty easy to get a handle on range capability. I was at the upper range, 5'8", suggested for a 15.5 frame, according to the info I had, and really didn't even think of the the effect on battery selection, if I had to do it over, I'd go the next size up I guess. But being stuck with what I have, I think I stumbled on a pretty good alternative with a lot of options. BTW: I did determine that a 11.5 sharkpack would NOT fit on the 15.5" frame.
That's cool, as about the time I figured that out, I did research on hot rodding the TV (another ongoing project, waiting on my controller, but the Bullet comes first) and will build the Bullet without the inherent compromises that the Montague folder/plane combo and the folder/crane ride TV. So yeah, I'll have 3 E-bikes..... but all will be 52 volt, and the Bullet and Viper will have the same exact battery box for the Panasonic 11.5 6.5 lb. high performance battery. The plane/Montague combo uses a 11.5 ah Samsung. 
It occurred to me that if I could lash my snowboard to my Camelback, I could jump on the Bullet and ride up to the ski area above my place. This is a funky smaller area, but steep and challenging, don't think big time high dollar Sun Valley type of place! This would be my first ride of any distance on the Bullet, 1.5 miles down a rolling up/down dirt road, to a paved road, with a 1400' elevation gain with slopes of 12% in long stretches. I tried real hard to keep the speed down to 10 mph or so, as the aerodynamics of the snowboard strapped to my back were not the best. I was in a lower gear of course but not the lowest, as I really wanted to avoid lugging the motor, and I couldn't keep up pedaling so I let the motor do all the work. I could have gone much faster then 10 mph even up the steepest parts, but was in no hurry, had the board on, and wanted to see the effect of low speed on the range. Once I started up the steeper parts I only "ghost pedaled" (love that term) as I was wearing my snowboarding boots and dressed for the top of the mountain, 8800', and really didn't want to sweat too much, or at all. The ghost pedaling was for the benefit of the few people that drove by, just for the hell of it. Halfway up, going past a neighbors place, I saw him out and turned in his driveway and he took this shot. He had noticed the speed I was going by and it got his attention, NO ONE, no non E assisted biker anyway, by the time they get to this stretch of the road, is making that kind of speed while sitting upright in the saddle. More a head down grind, pumping away at about 4 or 5 mph, especially if they have started from the valley, another 1,000' below my starting point. I let him take it for a spin and he was super impressed, as was another neighbor I met earlier who was out walking his dog. I'll be taking all 3 of my Ebikes to his place at some point and let him and his wife check them out, he's a pilot also so I'll let him ride the Montague folder.
