Mid-drive freewheel next to chainring

[richj8990]

Hello everyone,

I and many others would like to modify a mid-drive's crankset to put whatever axle / spindle and cranks on that we wish. Easier said than done of course. For example the Bafang BBSxx has the entire bottom bracket / axle built in with the motor housing; it's directly attached as one piece. So that's a no-go. Other DIY's however like CYC, Lightest Bike, and (yes it's a POS but I'll mention it anyway) L-Faster have separate motors from the crank axle, so you 'could' modify it if you wanted to. All of those either come with or require square-tapered bottom brackets, and all three also have a freewheel attached to the inside of the chainring, so that you can pedal without the motor being on.

There are a few alternate things that may supposedly happen if the freewheel is not there; please correct or elaborate on these below:

1. The cranks automatically turn by themselves whenever the motor is on. That would obviously be bad if they turned for example at 150 RPM!!! I can't pedal that fast for long. So you'd have to be in a higher (smaller) cassette gear to start out for the proper cadence and that would lug the engine and cause problems, correct? For example, if the motor itself wants to spin at 150 RPM and you want 1/2 of that at the cranks, if you have a 36t chainring then you'd want the cassette at 17-18-19t. That's pretty high for starting out. I'd need to use a really large chainring like 52T for cadence and then it would still be hard to main cadence for example if I'm at 52t/52t in the 11-speed cassette's granny gear. Even 52/40 in 3rd gear would be difficult to maintain up a longer hill. And that would basically be almost all motor propelling the bike up and you just are trying to keep up with the pedals; it seems pretty ridiculous for a pedaled bicycle. May has well get an e-dirtbike if it's going to be like that.

2. The PAS doesn't work at all; you can only throttle. This would also not be worth it, especially for a bike that is built to climb technical steep singletrack.

3. Everything is normal except that the motor drags when you stop pedaling, and you have to keep the motor on all the time in some power setting to move the bike. PAS does work (assuming the controller's connector has compatible pins and programming with the different crank PAS sensor you have for your other crankset). This would be worth it for having my own crankset on the bike, because I don't care much either way for pedaling without the motor on.

Which of the above if any are correct?

 

[amberwolf]

Some thoughts:

If you have a cadence-type BB-mounted PAS sensor, or any torque sensor that doesn't detect crank torque (instead detects chainring torque), and the cranks/crank chainring is backdriven by the motor (no freewheel from motor to cranks), then once the motor is started, the PAS cannot be deactivated and the motor will be self-continuing; you'd have to use brake levers (or some other means) to disengage the motor. (because the motor will drive the PAS sensor and the PAS sensor will drive the motor and the motor will drive the PAS sensor and the PAS sensor will drive the motor and the motor will drive the PAS sensor and the PAS sensor will drive the motor and...).

If you can setup the PAS sensor so it is not backdriven by the motor, but is driven by the cranks, then you can use PAS, otherwise you're stuck with some other control method (like throttle).

There may also be more complicated ways to setup a PAS so that being backdriven by the motor doesn't affect it, but I can't think of them right now.


Regarding pedalling cadence vs motor speed...you shouldn't have any problems with that if the motor system is geared correctly, since it should be geared to drive thru the pedal drivetrain the same way the pedals are, so they should produce the same "crank RPM". If the motor is geared to produce a faster crank RPM than the pedals, then the gearing of the entire bike has to be moved up to match that RPM to make the motor do what you want, and then it is all wrong for pedalling (you'd have to shift to very different gears to pedal with than using the motor).


If there is no freewheel from cranks to motor (opposite of the first problem above) then the motor will create drag on the cranks as long as the motor is not running and the cranks are being pedalled. This one is very simple to correct, by using some method (controller settings, throttle modification, etc) to cause the motor to always have at least a tiny amount of current to overcome the drag--it doesn't have to be enough for the motor to actually drive the cranks at all, just enough to remove the drag of pedalling it. This saves you one freewheel/clutch mechanism at the cost of a small amount of power and battery capacity when not using the motor.

 

[richj8990]

Thank you Amber that is very helpful.

In reading more about mid-drives I think the CYC Stealth is probably the best bet since I'll be putting it on a carbon frame. They say the Bafang BBSxx often cannot fit properly in front of carbon frames. The only drawback for the CYC is that it uses an ISIS bottom bracket / crank spindles which means I can't use my own crankset, but oh well.

 

[richj8990]

OK this is funny but I just saw another Chinese mid-drive that has the motor disconnected from the cranks lol.

$450 with shipping. Yes it's probably a complete waste of money but...9 PAS levels, and supposedly peak torque at 750 RPM is 140 Nm. So it's similar to a BBS02 but the cranks are not directly attached to the motor housing. Sucks that the controller is external but I can deal with that if, and I emphasize if it works. PAS hole looks wide enough for a Hollowtech II 24mm spindle. And no you are not drunk, it really does read 'senor'.

This comes with a square taper BB and cranks but look at the crankset below:

Four chainrings. It's hard to count the teeth since it's black on black but I think the smallest ring on the inside, which has a 2nd chain attached to motor, is 28T. That's just enough to have it on the small ring for a triple hollowtech crankset! So I'd use my own crankset, get rid of one ring, run the inside chain on my own 28T narrow-wide ring, and then probably do 30/42T for the middle and larger rings. I'd have a double-chainring mid-drive, and it would for sure be lighter than this setup, which is a whopping 21 lbs for the whole kit. I would simply have to be very careful to turn off the motor while manually moving the outside chain over between rings. I think I just may try this out. Not light, but my carbon bike as is with the hollowtech crankset weighs 27 lbs with heavy-duty DH tires on; including the hollowtech & my own rings knocking off a few lbs it would wind up 55 lbs total including 17.5 Ah Hailong II battery. Not the end of the world for weight, and if it works it's worth it.

 

[amberwolf]

Reminds me of the early Cyclone or GNG.

clones of clones of clones of clones of clones of clones of clones of clones of .....

Quality usually gets lower and lower for each "generation"...

If you haven't already, you might want to look at the many kinds of middrives (and their potential issues) in the non-hubmotor drive section; it goes back a long long ways.