Quoted post said:Originally posted by Richard Schumac
My searches have not turned up anyone selling a bicycle with a continuously variable electric
transmission. I don't mean an electric scooter: I mean one in which the crank turns a
generator/alternator, whose output through an electronic controller then drives an electric motor on
the wheel rim(s) or in the hub(s). There would be no chain, derailleur or gears, except perhaps for
some fixed-ratio gearing inside the generator/alternator or motor assemblies. A control equivalent
to a shift lever sets the ratio between crank torque and RPMs, which can be varied over some wide
range. The rider sets the speed by changing this ratio and/or the crank RPMs as they find
comfortable. Scooter-type assist would be possible through use of an optional battery. (Earlier
threads on the subject addressed mechanical CVTs and auto-shift mechanisms, not an all-electric
transmission.)
What's the point? Mostly geekiness, I suppose, but the effectively infinite number of "gears" while
reducing the number of moving parts is appealing. Also appealing is the notion that a battery in the
system would provide load-levelling (hill assist) but with the rider still ultimately providing all
of the energy, unless of course the rider cheats by re-charging the battery from an external source.
For non-cheaters :_> the battery need only be large enough to help climb one hill. Efficiency would
not be as good as with a conventional drive, but that's not really a concern for commuters or
pleasure riders.
Surely by now someone must make such a thing?
Nobody manufactures such a system. The losses on the conversion and back would exceed any benefit of the continuous transmission in a commuting or touring applications.
There might be some off road or winter driving applications in which multiple wheel drive benefits might overcome the losses and weight penalty.
The weight of the electric drive would exceed the savings on the chain/derailleur/internal gear hub of conventional mechanical drivetrains.
A couple of electric bike systems on the market use regenerative braking to store energy but still rely on pedal and chain to provide the pedal propulsion to the wheels. Those that use regenerative braking get power off the wheel.
ZAP’s DX model has regenerative braking on a roller that engages/disengages the rear tire, twin 200W motors-only regerates at speeds above 15 mph, its intended to recover energy in hilly areas. Sanyo has 250/500W brushless front hub motor versions with regenerative braking charging a very small NiMH battery pack, but it’s not yet available in the US even though it was supposed to be available last March. There old website alleged they will have it available as complete bike (Enacle Gene 27) or kit. Birkestrand’s Rabbit Tool’s EX-Bike (not yet on the market either) also has regenerative braking on a 500W rear hub AC motor. Birkestrand will have a kit as well-they’re still working on getting the reliability of their AC controller up before selling to the public.
The major electric bike retailers in the US discourage such action asserting regeneration only extends range about 10% so they don’t think its worth the weight/expense. I’d like a light weight battery/motor myself for hills with a recumbent, so I’m watching the Sanyo system closely and I’m hoping the Sanyo 250W kit falls in the 5-10lb range. I have a 400W Currie upright (no regeneration), but I wouldn’t put that extra 25 lb electric system weight on a bent. Those retailers seem oblivious to the hill issue that is amplified on recumbents.
The continuous ratios have been a high point of the hydraulic drive systems, but the most optimistic 10% or so losses there take away their benefit over chain drive. You’re going to get higher losses with the electric system than the hydraulic in most instances.
If doing so for an engineering exercise or one of those niches I described, I’d look at a separate high efficiency generator rather than a motor conversion as the generator.
Among the non-Japanese, Heinzmann is usually the development leader in hub motors, I’d keep an eye on what they come out with to see if they have any hub motor systems with regenerative capacity.
Yahoo groups power assist is an electric vehicle group, bicycle majority, there tends to be very little discussion toward onboard generation:
http://groups.yahoo.com/group/power-assist/files/
The losses transmitting over the cables from generator to motor should be trivial compared to the generator and motor losses. The 50-60% efficiency Dave mentioned sounds low to me based upon my school experiences with electric motors, but Dave’s figures are higher than what those previously mentioned retailers allege.
