The 2 main issues I can see with the build will be belt alignment and the eccentric bottom bracket.
Thanks all for the interest and feedback. I gave it a go and pretty much hit it on the head, and I have given up altogether on this bike and am moving on to another bike. EBB just wont work, chainline is a mess, adapter makes it worse.
Since I was unable to make the combination work, I thought I would document my struggles and findings so others in the future can have a resource when attempting to pair their Bafang BBSXX motor with a Gates Carbon Drive System and an IGH. A seemingly ideal combination, or at least I thought so...
For reference there are three key criteria to make a Gates belt drive work well and in a way all have become struggles when pairing with the Bafang BBS. Having a frame split, perfect chainline with an IGH or SS drive, and being able to set belt tension to the desired level.
1. Frame Split - You need a way to get the continuously looped belt through the chainstay and seatstay triangle on anything resembling a traditional frame, so you need some sort of frame split either on one of the drive-side stays or in the dropout. If your frame has a spit, proceed, otherwise you will need to see a local frame builder and convince them to take the work and cut your frame. This was quoted at $150 locally (Seattle) but would "seriously screw up" my paint and also require a teardown of my bike. Properly done (with paint and prep) expect ~$500 for the split and a new powder-coating.
2. Chain Line - Gates CenterTrack Belt Drive Systems supposedly have more tolerance for a wayward chainline than the older CDX, but my experimentation of moving my crankset laterally in the bottom bracket and trying to pedal showed significant resistance and noise beyond ~2mm in either direction. For a belt to make any sense at all, chainline must be perfect.
3. Belt Tension - Greater than 50 lbs. tension for IGHs in high torque applications is needed to prevent slippage. In order to do this you will need either horizontal / sliding rear dropouts or a eccentric bottom bracket to adjust the distance between the belt's cogs. Not only is this more difficult than it sounds to do precisely, but a frame that flexes will have slippage as well. I dont know for sure, but I can only imagine that flex would be exacerbated on a frame that was not designed around a split, but rather was literally cut, welded, and bolted back together.
With that preface, the process that ultimately led to the abandonment of my Bafang / Nuvinci / Gates concept ride. Here is my story....
My first clear complication was the eccentric bottom bracket which is used to adjust belt tension. I was fairly confident that I could manage to both fit the Bafang and set the tension properly as the desired tension actually put the eccentric hub in the front-bottom of the BB shell. With the 13mm of motor to BB spindle clearance provided I was confident that I could make a fit despite some passive mentions I discovered claiming that EBBs were not compatible with the bafang. Unfortunately little further detail was provided, which is why I am elaborating now.
To make it clear, EBBs are not compatible with the Bafang BBS.
My, like every other EBB I have seen has a spilt on the bottom which allows for rotation of the eccentric hub chainline length and latitude. The split is held together with tensioning bolts which protrude downward form the BB's center and effectively att 10 mm to the width of my BB. These bolts along with the protrusions on the motor's casing for the attachment plate and even the screws holding the unit together blocked the Bafang from even getting close to on the bike. Before quitting I even contemplated disassembling the motor then reassembling it on the bike frame, but even then tolerances would have been so tight that I would have been relegated to a fixed chain tension and no room for adjustment, which means it would be impossible to get the belt on or off; a showstopper.
My next line of logic was that I love this bike, especially it's drivetrain; how great would it be if I could just find a frame with rear-horizontal dropouts and just strip down the Gotham and its icky EBB frame and rebuild the new frame?
Or the logistics and feasibility of a Bafang / Gates Belt / Nuvinci bike
I am convinced that it is not possible to pair the Bafang BBS and a Nuvinci N360 IGH with a belt drive connection without seriously altering your bottom bracket or other deep fab.
Chainline, or the distance of a particular drive component from the center line of the bike is the crux of using a belt drive efficiently. Here are the respective chainline differences for all of the elements I encountered, in the most ideal situation front and rear drive element chainline measurements would be equal. For sprockets the chainline is measured to the centerline of the sprocket.
Chainlines in 135mm dropouts
Bafang BBS (stock ring): 50mm*
Nuvinci w/ Gates Sprocket: 46.45mm http://www.g-boxx.com/pdf/pdf-23-03-10/ ... ec-drawing
Rohloff 500/14 w. Gates: 54.75mm
*The stock bafang configuration considers the dished chainring that comes wit the kit. This chainring effectively removes 4mm from the overall chainline from the chainring's mounting point on the face of the reduction housing. As a belt is clearly not compatible with a "chain"-ring, an adapter will need to be used...
No matter what, with the 130 BCD adapters currently available you can only increase the BBS's chainline by utilizing one. First you add ~4mm from the loss of the dished chainring, then depending on the adapter used can easily add another 2.5-6.5mm from the face of the housing to the center of the sprocket's spine.
http://california-ebike.com/product/ba1 ... g-adapter/
I tried the 130 BCD California Ebike Adapter which is a perfect fit for the drive and is very well machined. The adapter itself is 6mm thick and the outer bolt points are recessed so that they serve as 3mm thick mounting points. With the mounting recesses facing in, the mounting surface is out 3mm further (2.5mm effective w/ spine) from the surface of the drive than the original chainring. Adjusting for the stock ring's 4mm dish, with an adapter you have at least 6.5mm increased chainline distance from stock, or a actual chainline of 56.5mm.
** All of this assumes a 68mm BB, if you have a different size BB then take half the difference over 68 and add to the total chainline at the BB. EX: A custom chainring on a 73mm BB would be 59mm (56.5mm, per above + 2.5mm (73-67)/2) from the frame centerline.
A 9mm (21x tolerance) difference in chainline alignment from the nuvinci to the bafang is too much to overcome. The only options to use a belt drive & Nuvinci IMO would be:
Have enough bottom bracket (78mm?) to grind off 10.5mm on the drive side to bring the chainline in. Unfortunately you would risk ruining your frame to yet gamble with tire clearance with the wide belt, whether or not the reduction housing will actually have enough room to mount against the BB and clear the chainstay expansion.
Build/beg for a gates sprocket that is dished to move the chainline in and around the reduction housing.
To the later option of a custom chainring, the likelihood is slim as well since the inset would need to be on the perimeter of the ring to clear the reduction shaft and gears much like this guy:
http://www.alcedoitalia.it/shop/accesso ... rale-bbs01
With Gate's proprietary CenterTrack sprockets they look difficult to mill or render and can not be legally replicated without license. Poop!
So, no dice on the belt drive N360. I am currently investigating a Rohloff as well... I love the belt drive that much. A Rohloff with a Gates Sprocket conversion has a chainline of 54.75mm. The stock Rohloff is only 1.75mm closer to centerline than the modified BBS would be. 1.75mm is a more more realistic to shave off a BB or shim in my front chainring than the 10mm of the Nuvinci would have been. Most likely outcome here is the upside of the Nuvinci will lead me to a different chain drive rig, and I am pretty much OK with that since the Surly ECR that I have my sights on is a 73mm BB frame and my logic indicates that it is impossible to use a bafang/belt/IGH combo on a 73mm BB without throwing some sparks.
Interestingly Rohloff's decision to place their cable shifter interface outside of the rear dropouts rather than inside like Nuvinci, Shimano, etc, is what allows the drive such a large chainline distance and it's Bafang compatibility. It is actually Rohloff's defiance of a 50mm chainline convention that makes it unconventional and pair well with the extended Bafang, all the other IGHs I have looked at come in or slightly under 50mm which is pretty much the single speed frame convention that they were designed for.