Bafang mid-drive for dual use: singletrack mtb + commuter ???

I've got the ebike bug and would love to be able to convert my mountain bike where I could use for both singletrack trail riding as well as my work commute. My initial thought was to install the BBS02 and simply buy a separate wheelset for swapping between road and mtb tires.

As I got further into the purchase process, I realized I need to think about the front chainring size.

My question is simply this - do you think it will be practical to choose a front chainring that will suit both singletrack mtb AND road riding? The trails are relatively flat in my area but a lot of switchbacks so A LOT of acceleration/deceleration (and some mild climbing). My commute is very flat, on pavement and I'd like to maintain moderately high speeds (~25 mph) to minimize commute time.

Please let me know your thoughts if I can achieve the best of both worlds with one chainring or if I should be thinking about converting two different bikes?

Thanks!

if you're using two different rear wheels, then use a cassette or freewheel on each one that gives you the gearing you're after for each situation, using just a single size front chainring that's the best compromise between the two needs.

you may have to use a gearing calculator or spreadsheet like those listed at sheldon brown's bike info website:
https://www.sheldonbrown.com/gear-calc.html
or the various ones found on google, to get the right speed range vs system loading.

if you end up needing such radically different speed ranges that the rear gearing can't create this on it's own, then you would have to change the front chainring for each application, too. but i doubt that's going to be necssary.

Thank you much, amberwolf. That sounds like a good idea, but wouldn't that be a problem for my shifter/derailleur calibration? i.e. could I expect the shifter/derailleur to work seamlessly with both cassette or freewheel sizes?

it depends on how you have to do the gearing.

if you have something like a "megarange" derailer, and use the same number of gears on the cluster on each wheel, and they all line up exactly the same (same spacing from the dropout), then as long as the derailer can handle the chain length "change", and can physically move down/back far enough to accomodate the largest gear on the one (slowest) cluster, and the smallest gear on the other (fastest) cluster, it should still work fine.

it's only if the derailer doesn't have enough range to "reel up" the chain length needed for the largest combination of gears, that you'd have a problem.

or if you have different numbers of gears on each (cuz the derailer won't move between them correctly if it's not got the right shifter "spacing", though you can use a friction shifter instead of a click shifter and fix that particular problem).

if the gears don't line up because the hubs are different, you can fix that by using spacers on the drive side of one hub so it caused them to line up with the other. or you can readjust the derailer's inner and outer limit screws each time you change the wheel, but that would be annoying.




if you can find (or make) a cluster with both the slowest *and* the fastest gear you need, without changing gears at the front, you don't even need to change clusters between applications. if you use a cassette type of hub, then it's reasonably easy to make your own custom cluster, as long as you can find the right size cassette sprockets for it to do the gears you want, *and* your derailer has the range to accommodate the change "length" change from tallest to shortest gear. (which it would have to do anyway even for the wheelset change).

I have a BBSHD on a cross-country MTB that I use 80% for commuting and 20% for trails. I'm running a 53T chainring with two different wheelsets and re-arranged cassettes. If I remember correctly, it's 36-32-24-18-15 for trails and 30-23-17-13-11 for roads. That works well for getting up to 30 MPH on flat roads and decent climbing on trails. If I use the same chain for the MTB and commuter setup, I'm getting a lot of skipping with the 13 and 11 sprocket. Using a shorter chain for roads seems to to reduce the skipping. So all in all switching wheels, chains, re-aligning disc brakes takes me about 30-40 min.

if your derailer has a tension adjustment (usually a third screw) it might fix that chain jumping problem.

if not, adding an extra spring pulling on the cage from your frame in the right way might fix it too.

Switching sounds like a PITA to me. You might try a 42t or so ring with a 9-speed (8 will work too and is what I use) 11-34 and don't change the wheels (IMO, you won't need to with the motor). Five or so years ago when I built my BBS02 hardtail, it had a 52t chainring and 11-17-28 cassette spaced 8-speed (to improve chainline). Worked fine in the dirt and got to 30 mph on the street.

Thanks everyone for your feedback.

Also look at the Sram EX1 drivetrain, it has enough range for dual duty without swapping parts, and made for mid drive systems.