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Bafang BBS02 gearing for 20" wheel

Posted: Apr 06 2021 10:25pm
by Reflector
I want to reduce my 24 gears and also dispense with the rear derailleur and to fit a modified Hebie chain glide to the rear wheel of my Alleweder A4 velomobile.
My currently installed setup is;
Bafang BBS02 500W, 42T Lekkie, SRAM DualDrive 8 spd cassette (11 - 30), 20" wheels.

After much comparing of reports from the Sheldon Brown Gear Indicator, I note that my DD exhibits better developments in both low and high range gears than does the Alfine 8. The charts also reinforced how many 'double-ups' there are with my (too many since adding the BBS02) 24 gears.

One of the Bafang Higo e-brake connectors is fitted to a Wuxing 'Big Green Button' which I use as an electric clutch.

There is a front derailleur mast on the asymmetrical aluminium BB for the BBS02 and although the Precialps multi chainring adapter has been mentioned, what I consider stumbling blocks for multiple chainrings are;
  • Lekkie 42T chainring is a 'wide / narrow'
  • Larger chainrings not recommended for the BBS02 (Mk i controller FETs)
  • Keeping the motor revs up while in a larger chainring
  • Retaining my chain line
  • Would the BBS02 Mk1 500W compensate for an adequate gearing using just the 3 hub gears of the DD3 (thereby allowing a stable chain line for an enclosed chain guard)?
The alternative is an Alfine 8.
  • Any ideas on a final drive cog size for DD3 hub / Alfine 8?
What would be ideal is something like Wim Schermer's shifting mechanism which retains the chain line.

Or, a small 2 speed overdrive unit sited between BBS and rear wheel.

Re: Bafang BBS02 gearing for 20" wheel

Posted: Apr 09 2021 6:34pm
by Reflector
Plenty of lookers...but no feedback from anyone?

Re: Bafang BBS02 gearing for 20" wheel

Posted: Apr 09 2021 7:16pm
by Chalo
You have overconstrained your problem. Give up on either the chain case or the Dual Drive hub. I think you have a good application for Rohloff Speedhub ($$$$), Nuvinci/Enviolo ($$$), or Sturmey Archer RX series 5-speed hub (comparatively cheaper). These are the strongest internal gear hubs you can get right now. Using a gearhub designed for normal pedal power, and adding motor power to that, is likely to wreck the gearbox. You should get something that's demonstrated to hold up to heavy duty use.

Ignore the recommendation against large chainrings with BBS02. That's issued with the assumption that users will have a rear cassette with 11t high gear (and won't shift into appropriate gears). If you use for instance a Sturmey Archer RX-RK5 hub with a 52/22 primary chain drive, you'll have a 2.36:1 gear ratio, about the same as if you use 42/18 or 30/13. Also the reaction torque in the gearbox will be reduced by the reduction in wheel diameter versus a full sized wheel.

I'm not sure why you want a chain case inside a velomobile that amounts to an overgrown chain case by itself.

Re: Bafang BBS02 gearing for 20" wheel

Posted: Apr 10 2021 10:44pm
by Reflector
Thanks for the response Chalo. The rx-rk5 looks like a viable and relatively (compared to Rohloff et al.) inexpensive option.
Maybe a Sturmey Archer S2 inline could give an additional 38% gain?

The chain case is to cover the only exposed area of chain from the rear axle to half way along the swing arm. Our local drought / flood / dust and country roads combination out here ensure an early demise for an unprotected chain. Elsewhere inside the velomobile the chain is safe and dry. I only use the PAS in the low settings for inclines, have 2 dolphin batteries on board and the gearing is to compensate for my health which is not what it once was.

Thanks for your input.