Holy cow, what a difference!
I should have done this years ago - I might actually get something posted once in a while!
The formatting controls and simple user interface are awesome! Let's try some stuff out...
Frankenbike (Frank)
Background: I am an accomplished bike rider with many
thousands of miles experience with commuting and touring on many different
styles and configurations of bicycles. I am also a life-long mechanic and have
a keen interest in electromotive technology. I had been kicking around the idea
of adding an electric assist to my commuter bike, and by chance I came across
an industrial motor which, though not ideal for the application, was available
for the cost of a simple welding job (I own a 250 Amp MIG welder). Once I had
the motor, I set about tracking down the other parts needed, and figuring out
the drive train.
Humble Beginnings: Frank started out as a 1995 GT Nomad
“Cross” bike (Rigid MTB style frame with 700C wheels). Of course it’s been
completely rebuilt several times over the years and has over 10,000 miles on
it. The only original parts left are the frame, fork, derailleurs, and hubs.
The paint is all Martin Senour spray can work; one coat primer, two coats “Leaf
Green”, and one coat of clear.
Electric Assist Inspiration: The Stokemonkey electric assist
from Clever Cycles, which is no longer produced. A web search should bring up a bunch of info, as it was around for quite a while. Edit: It's back! Now produced by the good folks at Grin Tech, who can be reached at
ebikes.ca - check them out!
Philosophy: I ride primarily for fitness, and I wanted
electric assist for increased speed, fun, and to take some of the load off
after a long day. I did not want an electric motorcycle in this application
(though I think the Enertia is cool
http://www.brammo.com/
). The motor assists with pedaling of the bike, but the rider must pedal while
the motor is running; running the power through the bike’s drive train means
the rider can’t simply sit there and let the motor do all the work, but it also
means that the additional torque is multiplied through the drive train, which
enables cruising two gears higher (18 vs. 16 MPH with the same effort), and near
vertical hill climbing with moderate effort in the lowest gears.
On the motor output shaft an adapter bushing is pressed into
a one-way clutch bearing, and this assembly is then press-fit into a custom
chain drive sprocket. The one-way clutch bearing allows the bike to be pedaled
freely without motor assist, and then the clutch locks the sprocket to the
motor when the motor starts.
The motor drives the timing (left) chain ring of a tandem
bike “stoker” (hence, “Stokemonkey” above) crank set, which I purchased off
EBay for about $50.
Power is supplied by two 12V 21AH Absorbed Glass Mat (AGM)
batteries which ride in the trunk bag. The batteries are wired in series to
provide 24V to the control and power electronics.
Control is via a Razor scooter trigger on/off control (another
EBay purchase - $12) to a 24V power relay, which provides power to the motor.
The system is on/off – no variable speed control is needed with this low power system
which is geared to a comfortable 85 RPM cadence.
Technical Information: The motor is a permanent magnet
brush-type rated 24VDC, ¼ HP, 8.8Amps, with a 20:1 worm gear reduction to 125
no-load RPM. Torque is rated at 74 lb-in. I have not checked the amp draw under
load, but the rated draw (24V X 8.8A) is 211 Watts – a little over ¼ HP. The
motor weighs 16.8 lbs. The motor is not wash-down rated, so I sealed the brush
caps and wire lead with clear silicone sealant. Frank is a fair-weather bike,
but living in Florida
means a rain shower is always possible. I would not have selected this motor
had it not been available so cheap; it is heavy, not nearly as efficient as a
brushless motor, and the worm gear reduction loses more efficiency to friction.
The optimum motor would be brushless direct drive like the Stokemonkey. The
motor is, however, virtually silent in operation, and provides just about the
perfect amount of assist I was looking for.
The motor mount is fabricated from a piece of 1/8” wall tubing
with an ID matching the OD of the down tube. The tubing was cut in half along
its axis and 3/16” flat stock flanges were welded on each edge of each half, and
then holes were drilled in the flanges for ¼” clamp bolts. A piece of 3/16” flat
stock was cut to match the size of the motor base, then plug welded to the
upper half of the mount tube so that the drive sprocket is in line with the
driven chain ring. The motor axis is offset of the bike center line by about ¼”
to achieve chain alignment. The motor mount is clamped high enough on the down
tube to provide toe clearance on the drive sprocket. I considered mounting the
motor below the down tube to keep the weight down low, but there is not enough
clearance behind the front tire.
The crank set is a Truvativ GXP Tandem Stoker with triple
chain rings of 52T-39T-30T on the drive side and a 38T timing chain ring. I
figured the motor drive ratio after I bought the crank set and calculated the
size motor drive sprocket based on 85 crank RPM and 125 motor RPM. This gave a
result of 26T on the motor drive.
The rear cassette is seven-speed (common in 1995) and was
changed to better match the new, larger chain rings on the new crank. The cogs
are now 13-15-17-19-21-23-26T, which provide close ratios and a good overall
range. Top speed (52-13 @ 85 RPM) is 26 MPH, which I achieve regularly on mild
downhill runs. Low gear (30-26) is a stump puller or stair climber, take your
pick.
The motor drive sprocket is a 26T #40 (½” pitch) chain
sprocket which I had machined to fit the clutch bearing and to thin the teeth
to fit standard bicycle chain (also ½” pitch, but much thinner at 3/32”). I
ordered the sprocket for a large shaft, so that it was supplied with a large
amount of material in the hub to support the bearing.
Batteries are Power Sonic 12V 21AH AGM Lead Acid type.
http://www.power-sonic.com/images/powersonic/sla_batteries/psh_series/PSH-12180FR_11_Jan_12.pdf I chose these for their high rate discharge
capabilities and physical size (they fit side-by-side in my Topeak MTX trunk
bag). The batteries are heavy (26 lbs.) and do affect handling somewhat being
mounted high on the rear rack. Of course, I would much rather have the smaller,
lighter Lithium Iron Phosphate (LiFePO4) batteries, but I could get the Power
Sonics for 1/3 the cost. I could get away with smaller 10AH batteries for my
normal commute distance, but they would be drained almost completely each day
and that would shorten their life.
Batteries are charged with a Battery Minder 24V charger I
set to a 2A rate.
Performance/Riding Characteristics: The system works, and
works well. There are a couple of design compromises which only come in to play
in extreme circumstances.
Frank is heavy. Really heavy. Frank weighs about 75lbs as
pictured. Keep in mind that Frank is a fully dressed, older steel bike to start
with, and then I added about 44lbs of batteries, motor and control electrics. I
also carry my laptop, lunch, raingear, and a change of clothes to work, so
fully loaded the weight could be almost 90lbs. The only heavier bike I’ve
ridden was my fully loaded cross-country tourer. I do choose my route carefully
to avoid large bumps and potholes.
Much of the added weight is up high, so I keep the bike as
vertical as possible when out of the saddle. The Topeak rack has held up well
to the added weight of the batteries. I had considered mounting the batteries
down low, but the MTX bag is just so convenient; I can slide the bag off the
rack to install my son’s Schwinn Runabout trailer bike.
Because of the off-on nature of the power assist, I only
turn it on after I have started rolling and have the cranks moving to reduce
the amount of amp draw through the motor brushes. The power relay is rated at
50 amps, and I have a 20 amp circuit breaker installed on the motor supply. The
motor would trip the 15 amp breaker I had installed previously if I let the
motor stall.
As previously mentioned, the motor is smooth and quiet. It
is a lot of fun to see peoples’ reactions as I cruise by at almost 20 MPH, and
they have no idea the bike is assisted because even though the motor and chain
are obvious if you are looking at just the bike, while I am riding it there is
no extra sound to draw attention to it.
Cost: The motor can be had on EBay for about $250. Add the
crank set for $50, the batteries for $120, relay/trigger/misc. electrics for
$50, one-way clutch bearing for $35, sprocket for $20, and the machine work for
another $25. The motor mount is dependent on skills, but if you had one made, I
would guess $75-$100 tops.
So, $625 or so total to duplicate, but my cost was around
$275 because I fabricated my own mount and bartered for the motor and custom
machine work. I got about 1/3 the performance of a Stokemonkey for 1/5 the
cost…
The story above was written for
Popular Mechanics magazine in 2012, but never published. The Frank experiment ran its course, and was disassembled last year.
JP
