I did a bit of research into the Rohloff SpeedHub and a few different gear shifting solutions that are already on the market. The SpeedHub looks like a fantastic internal gear drive! I want one just to play with and look at!! Classic German precision engineering! Despite its internal complexity, the gear indexing is accomplished with a simple mechanical pull-pull cable system. It’s a tried-and-true 1:1 mechanical linkage0
There are a few different possibilities that would appear to be most likely candidates, depending on the availability of commercial off-the-shelf (COTS) parts. The SpeedHub makes use of a mechanically simple pull-pull cable system. It should be possible to basically do away with the shiftgrip and integrate a linear actuator to work the cables. In fact, this application all but cries out for a liner actuator!
The reasoning behind using a linear actuator, rather than any rotary system, is the pull-pull cable system. If you attach each cable to the end of a linear actuator from opposite directions, you replicate the pull-pull motion that the SpeedHub is expecting by replacing the user’s hand rotation. If memory serves, each shift would require the actuator to move 7.2mm. That’s easily accomplished with a control board and coding. Also, it’s much easier to achieve the forces required in a linear actuator, since it has a built in reduction gear and produces enormous linear force from the screw drive system. The trade off is likely to be weight, but that’s just a guess, and probably not a big issue to begin with.
With that design in mind, I would be looking at the following components for a functional system.
Power:
LiPo battery pack. Size, configuration, and capacity, are all fairly negotiable, but the voltage will be based on the needs of the actuator.
Drive:
1 x linear actuator
1 x motor controller for linear actuator
1 x custom bracket to attach the actuator and the SpeedHub control cables to a fixed point on the bike frame.
1 x custom bracket to attach the cable ends to the moving end of the actuator.
Control:
1 x pretty much any Arduino or similar embedded module on a chip or board would do the trick. This could actually be tiny and physically be put anywhere in the overall solution.
1 x set of control buttons, switches, lever(s), or other form of rider input that fits the user’s abilities and desires.
1 x housing/mount for the user controls
Misc:
Wires, connectors, and similar
Now, having said all of that, this is sort of reinventing the wheel. There are already a few electronic shift systems out there that are available now. In particular, the two units from ProShift Racing look especially appealing. They automatically shift to keep you in your cadence and are especially nice when bottoming out in a valley. One handicyclist says it’s keeping him from rowing up the next rise because he/she no longer has to both drive the bike forward and shift down into the lower gears before his/her momentum is lost. Cost is $600-900, depending on which model, shipping, and install. There are a few other systems out there, as well.
Hope all of this has helped! Please don’t hesitate to ask further questions or contact me directly for info and/or additional research and/or advice.
Be safe and well!
Dan
