BBSHD Internal Step-down Gearing Mod?

[jonsully]

Has anybody thought about or attempted to change the internal step-down gearing of a BBSHD? I'm not sure if it's even possible, but it's something I'm curious about. I know the BBSHD has a final step-down (total ratio of both steps?) of 21.9:1, and that yields a top output shaft speed of around 150rpm, I think? Most people reference the full speed of the motor around 150-170rpm (WAY beyond human cadence), though of course this varies with voltage.

Essentially I'm curious if it's possible to modify the internals of the BBSHD so that at 'full' / top electric motor speed the output shaft RPM is actually halved from OEM. E.g. instead of 150-170, 75-85 rpm.

Would that require fabricating a new set of gears to fit in the same physical space but have a different number of teeth or something? Has anybody considered doing this? I love the BBSHD through and through but would love it if its maximum efficiency point was at human cadence levels (plus it would mean high-torque moments are even softer on the nylon gearing)

 

[circuitsmith]

Not feasible to make the pinion (little gear) smaller and handle the torque, so you'd have to make the secondary (big) gear larger.
No room for that on a typical diamond frame bike.

 

[jonsully]

Could both be remade with the same physical size but with more teeth on the larger gear?

 

[amberwolf]

Youd' ahve to put more teeth on the smaller gear too, and the teeth would be smaller, so you get the same ratio out of it.

To put more teeth of the same size on the larger gear it must be larger diameter to fit them.

 

[scianiac]

I don't exactly think this is a good idea even if it could be done, if it's just efficiency you are after are you use that the motor is really that much more efficient at higher RPM? The optimal efficiency of a motor depends on many factors and faster RPM is not always better. And even if it was slightly more efficient you would lose some again to increased gear loss. There are probably much better ways to improve efficiency.

If instead you want more power at human cadence well that's easy you just up the phase current but just like lowering the gear ratio at some point you will run into the torque limits of the drivetrain. One of the reason higher power mid drives like the BBSHD run at super human cadences is because if they geared them down at that much power it would not be good for the bike drivetrain. With a large reduction between the chainring and cassette and high chain velocity you can put a lot more power through a bike chain than a large chainring and small side of the cassette that would be human cadence for anything other than very slow speeds.