Any problem with BBSHD being part of a 2x drivetrain?

[kiltedcelt]

In continuing my idea of a recumbent cargo trike (tadpole configuration), I've been thinking about the drivetrain. Right now I use an Alfine 8 speed internally geared hub on my current cargo bike which has the BBSHD installed on it. That bike will be going back to pedal power only and the BBSHD will be getting installed on the cargo trike project. Bafang mid-drives by design are only 1x drivetrains. I've already decided to make the rear wheel 170mm spacing to allow me to have a traditional rear cassette and derailleur and had planned on an 11 speed wide-range 11-46 cassette. Currently my Bafang has a 46 tooth Lekkie ring. Normally when you think about how Bafang mid-drives are installed it's only on a bike that has a traditional drivetrain. Even on regular recumbent tadpole trike you still have a pretty traditional drivetrain, it's just that the chain is longer than most regular bicycles and has a couple idlers put in here and there to manage that long chain. With my idea of a cargo trike it occurred to me that I could easily install a second bottom bracket shell halfway between the front boom where the BBSHD will be and the rear wheel. I could have that middle BB shell mount a triple crankset, and the BBSHD could drive the inner-most ring, and a front derailleur could shift between the middle and outer ring creating essentially a 22 speed drivetrain. If I wanted a slightly wider chain I could go with 8 or 9 speed cassette in the rear and have 16 - 18 speed drivetrain. Of course as with any 2x setup there will be some gearing overlap and I'm not even sure I would really need that sort of gear range. Just sort of spitballing here with ideas.

With my current Alfine 8 speed I find that I'm topping out around 20 mph max and I believe it's more an issue of the size of cog I have installed on the Alfine. I could get a little more top-end but since I don't have sliding dropouts on that frame I can't effectively take slack out of the chain so I need to use a Surly Singleator chain tensioner. The chain tensioner thus limits the size of cog I can run on the Alfine and without proper chain tension the Alfine skips like crazy. The gear range of the Alfine seems mostly adequate for the cargo bike, but I'm wary that it won't be robust enough for the cargo trike. Also, the cargo trike is going to be built with the intention of being able to switch between a nominal 26x3.0 wheelset and a more extreme terrain 26x4.0 wheelset. Having to build up two rear drive wheels means the additional cost of ~$200 for each Alfine hub.

A normal cassette drivetrain is far cheaper and easier to deal with shifting issues or damage on long trips. So, would it just be ridiculous to have a 2x 16/18/22 drivetrain running off a BBSHD? I know of some folks running a Rohloff 14 speed IGH on touring setups for the robust nature of the hub and no derailleur, but I don't know offhand of anyone running something like a BBSHD through a Rohloff although it's probably the one hub with a decent gear range capable of handling the torque of a mid-drive. It seems like a maybe an 8 or 9 speed 2x drivetrain could offer a similar gear range to a Rohloff, but again, I'm just wondering if I'm overthinking/overgearing this system, especially when the e-assist means you're often not staying in any one gear that often. I find most often with the way I currently ride my e-cargo bike with the BBSHD I shift from a couple low gears very quickly then settle into a cruising gear for a certain speed, then skip over a couple more gears to get to higher gear. Out of 8 total speeds I probably use three gears more than the other 5 which are just stepping stones between those other main drive gears.

 

[colsaunders2]

With 170mm spacing in the rear, I think you would be fine switching out the Lekkie for a larger, non-offset chainring. That reduces the need for a 2x drivetrain and will be better for your smaller cogs anyway.

A 55mm wide rim would support both 3" and 4" tires, unless you're looking to construct them differently. Though with 2 wheels, you could have a different cassette on each.

 

[kiltedcelt]

colsaunders - Your comment made me remember the Bafang mid-drive chainring adapters. I could purchase one of those and theoretically get just about whatever gearing I want by changing my chainring on the crankset to something else. An 11 speed, 11-46 cassette is pretty wide range. I was playing around a bit with the gearing calculator on the Sheldon Brown website and with my 46t Lekkie ring and that cassette in the back the gear inches were somewhere around 89-ish I think. That's pretty damn close to what is possible with a Rohloff 14 speed IGH. I mention the Rohloff because that was part of a discussion on the recumbent forum about best equipment for building up a touring trike. Not that my trike will be touring exactly, but some of the usage is intended to be very similar to touring and riding with a load of cargo is not a whole different than riding with a load of touring panniers and such, so gearing considerations are pretty similar. I found 26x3.0 to be a pretty damn good wheelset even just for city riding, especially when set up tubeless, so my default wheelset is going to be built around 40mm wide WTB Scraper rims. Not sure what the 4.0 wheelset will be built around but that's a project for much later on.