mid-drive free-wheeling cranks: need some undertanding...

i-Zapp

New Member
Was planning on building my own ground-up ebike this winter to keep busy and quickly realized there's ALOT of stuff out there ready to go! Spent considerable time watching videos and reading websites and still undecided about build vs buy, and still have a fundamental question...

It seems that most (all?) crank-wheels (the part with the pedals) incorporate a one-way clutch bearing which some designate as a "free-wheeling crank". My initial presumption was that the clutch was there to allow the motor to be stopped and still allow the rider to pedal without feeling the drag from back-driving the motor. (the motor is disengaged from the crankwheel when the crankwheel is spinning faster)

However it seems it is the opposite: the clutch is installed such that the motor can drive the crank without the rider pedaling. In this case the rider would feel pedal resistance due to the crank driving an unenergized motor. (the crankwheel is disengaged when the motor is spinning faster, but engaged when the motor is spinning slower.) Am I understanding this correctly? If so, is the drag significant?

My intention is to supplement my human effort, not replace it. So I'd like to be able to pedal unassisted, AND unimpeded by the parasitic drag of an unenergized motor so it would seem that if I built my own (or modified a kit) that I'd need to swap the one-way clutch bearing around?

thanks!
 

rick-n-ns

Member
i-zap sorry you got no reply. I assume "most" kits will have double clutches / freewheels.

I just got a bafang BBS02 mid-drive, and this is how it works.
1) there is no (noticeable anyway) drag when you pedal it like an ordinary bike (without any motor help), there is a one way clutch (or freewheel) inside one of the internal gears.
2) when the motor is running on throttle the pedals do not have to turn, as the front chain ring has a freewheel inside it.
3) and of course rider, and motor work together in PAS, both working to turn the cranks.
 

i-Zapp

New Member
Thank you! Exactly the feedback I was hoping for. I didn't consider the idea of a SECOND clutch inside the motor/gearbox. That would prevent back-driving the motor when it's off, and having a clutch at the crank wheel would prevent the motor from driving the pedals. Got my cake and eat it too!

Funny, I was also considering the BBS2 for my Specialized Rockhopper. How do you like it so far? what battery are you using?
 

i-Zapp

New Member
You should go to the Bafang forum (8Fun) and read all about it....

Court J.
ok dumb question, but where exactly is that (on THIS forum?)? although i was casually considering this conversion LAST winter, i'm now sitting at my computer with a foot of snow on the ground outside and another foot coming, and i'm going to get serious about this conversion (Rockhopper-->mid drive) and just now rekindling my research efforts. pointing me in the right direction would be super appreciated!
 

flymeaway

Well-Known Member
ok dumb question, but where exactly is that (on THIS forum?)?
The website has been redesigned since your original post. Ask your questions in the DIY forum that is where the conversion crowd is. Many many experienced Bafang mid-drive posters.

Court J.
 

Ann M.

Well-Known Member
@i-Zapp, browse around in the DIY Forum here to learn about a bunch o' options for converting a bike; either mid drive or hub motor and for general knowledge about Bafang motors (which are the core of many mid drive conversion systems but customized for each particular company) check in the Bafang Forum here. Just send me a message if you need more help sorting out the Forum; always glad to help.

You've got a great bike to ride; now let's get it electrified :D. BTW, there is no such thing as a dumb question, except for the one you don't ask! So ask away.