Alternatively, I'm thinking of getting the Bionx unit for a 20" or 24" wheel and building it into a 29er rim. Anyone try this? Wisdom to share?
That way will definitely work. When the system is sold with the motor laced into a wheel, there is a max-rpm programmed into the system. The max rpm is based on the wheel size so a "20 inch" motor can be removed from the wheel and rebuilt into a larger wheel and the "speed limit" changes just as you would expect. I know this having bought a used 26" system and having the motor rebuilt into a 20 inch wheel. I can enter the correct wheel circumference in the controller (codes 2005 and 3771). My mileage and speed are dead accurate but the assist is rolled over to zero at 17mph from the max rpm cutoff.
Going the other way should work just fine with a couple of possible pitfalls.
First and most important, on level ground with the 48V system, back EMF will likely limit you to 700W or so and that is going to translate to 25mph or so in an upright position. Tuck and get up to 27 or so. Go with a full recumbent (again with the 700c wheel) and you'll just make it to 30 but since the back EMF is higher, you will only be using ~430W or so. For just that reason, the recumbent will also get you a LOT better range at higher speeds.
The second pitfall is that if you enter your wheel circumference properly (to get accurate odometer and speed) I think the system will roll the assistance to zero just a little above 20mph. I'm not sure this is true (I have never built a 20" motor into a larger wheel) and you should certainly try being honest about the wheel circumference. But if the system does limit at 20mph, just enter a wheel circumference half what it actually is and multiply the display by 2 to get your mph and mileage.
There aren't too many folks on ES who are interested in Bionx systems and it is certainly true that you can get a LOT more horsepower for a LOT less money with other components.
But...the torque sensor in the Bionx motor and the effort-based assistance provided by the Bionx system give the bike a nearly magical feel. I bought all four of my Bionx systems used and at VERY good prices and I am quite happy with the performance (even with the 17mph assistance limit on my tadpole). I find I don't often need more power than the 48V PL350 (canbus G2 motor) provides. There is this one hill (4% and 6 miles long) that does drive the 700c (TREK Valencia+) Bionx system into the thermal protection mode (which roughly halves the power). The 20" tadpole with pretty much the same Bionx system goes right up that hill without even a flicker of hesitation (smaller wheel, higher rpm, greater efficiency at the motor, faster too).
In any case, I love mine and am riding them about 2500-3000 miles per year between the two.
All that said, if I were starting from scratch I would take a LONG HARD look at systems like this one from ebikes.ca:
http://www.ebikes.ca/shop/conversion-kits/crystalyte-h-series/crystalyte-rear-hs-kit-advanced-pas.html
to that I would add a torque sensing bottom bracket (a possible selection in the kit above):
http://www.ebikes.ca/shop/ebike-parts/torque-sensors/isis-128.html or http://www.ebikes.ca/shop/ebike-parts/torque-sensors/tdcm-128.html
and a really nice big battery:
http://www.ebikes.ca/shop/ebike-parts/batteries/b4820lim-kit.html
Please understand that I DO NOT have any experience with these components. I just looked once to see how I could replicate the Bionx effort-based assistance in a less "closed" system. The component system above will do effort-based assistance through the Cycle Analyst (CAv3 included in the motor kit) will go MUCH faster, and MUCH further than ANY Bionx system and will run around $2400, compared to $2000-$2500 for the Bionx (depending on model).
In any case, enjoy the adventure.
