Re: Any success with add on electric motors to Conventional Bikes?

Jenny Brien wrote:

A direct-drive hub is quiet and simple, but very heavy for its power.
Would it be good to use a smaller, high-revving motor and gear it down?

Gear reduction imposes wear, noise, efficiency losses, and points of
failure. If you can live with the implications, you can
(potentially) save some weight by using a gearmotor or reduction
gearbox. It's likely to be lighter and more efficient if you use
chain or toothed-belt reduction, but the physical size of the
components and packaging can exceed available space, and often that
means more maintenance, grime, and things that must be shrouded for
safety.

There are hub motors with gears and freewheeling clutches, but
obviously none of them would be suitable for running in reverse to
power a left-side crank.

All of the following geared hub motors are highly regarded:

http://ebikes.ca/ezee/
http://www.thesuperkids.com/500wabmcelbi.html
http://estelle.de/e/motoren.asp

I'm thinking of a primary chain from the pedals to a powered jackshaft
running about 200 rpm.

Small, non-hub bike and scooter motors will typically deliver their
rated power and efficiency at close to 3000rpm. For radio-control
vehicle motors in the same power range, the typical speed would be at
least 10,000rpm. Reducing these speeds to 200rpm at the output side
of a jackshaft is not trivial, and it implies either a gearbox at the
motor shaft or a very large input sprocket at the jackshaft.

Some Chinese brushed scooter motors use built-in gear reductions,
which could greatly simplify installation:

http://tncscooters.com/product.php?sku=106124
http://tncscooters.com/product.php?sku=106155

Ebikes seem to require fewer gears. Would an Alfine hub be a good match
for this application?

An Alfine or Nexus hub would be OK for a modestly-powered e-bike and a
normal sized or smaller rider. It would be prudent to raise the
primary gearing to minimize the torque applied to the gearbox. This
would make sense anyway, if one objective of the power assist is to
raise the top speed of the bike.

What would be the effect on efficiency and
reliability of running it inboard too, at higher revs and lower torque?

That would be easier on the hub mechanism, but it would be a bad idea
for a crank assist. Crank assist should be set up to power the cranks
at a speed that feels normal to the rider. Electric traction motors
usually deliver their highest power at about 50% of their free speed,
and their highest efficiency at about 80% of their free speed. So if
you feel best pedaling at 90rpm, then 90 crank rpm should correspond
to 80% of the motor's free speed. Variable gearing at the jackshaft
interferes with that relationship, and at worst could drive the cranks
at an untenably high speed with high power and cause you to beat up
your legs when you lose the pedals.

One of the elegant characteristics of crank assist is that, properly
implemented, shifting for the rider's comfort and performance will
tend to keep the motor in its most efficient speed range. And then
there need only be one set of changeable gears.

Using an Alfine hub fitted with a sprotor as a separate wheel drive
would be workable, but then there would be two separate shifting
mechanisms with different operating strategies. Not bad as a mental
exercise, but probably bad for efficiency and/or performance in the
long run.

Chalo
.

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