My Electric Bicycle Project
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I have added an
electric power
assist onto a GT mountain bike. It is much more powerful than typical
electric bikes. The basic motivation for this was
to
have a fun and efficient and environmentally friendly way to stay out
my
car for short trips around town, and also as a silent trail bike for
exploring
the hills. It can still be pedaled. When I first put this together it
was even more of a blast
to
ride than I had envisioned. I have seen 70 year old people get on this
bike
and laugh like little kids - it's like a magic hand is silently
boosting
you along. This website will describe the background of this little
electric vehicle, offer some reflections on the electric and hybrid vehicle
scene, and detail most of the design and motor and batteries should you be
interested in making one yourself. Though it's a bit of a project, a
number of people have actually made very similar bikes based on this
design.
|
The Beauty of the
Electric Bike 
no gasoline - no oil - no tune-ups - no parking hassles - no car payments - no more exercycle (use the pedals) - no brainer |
Background:
Why build an electric bike? Why use such a big motor?
Since I live about 3
miles from my favorite grocery store and my house is at the top of a
fairly long and steep hill, biking and walking is just strenuous enough
to be discouraging to me on a spur-of-the-moment basis. However I love
biking. I have mountain bikes and folding bikes. In looking for a motor
to add to one of my bikes, I rode some ready-made bikes and noticed
that they are really really fun to ride. The silence and effortless
cruising along is just kind of magical. But the electric bikes and
motor kits for sale such as the Curry USProDrive, the eBike, etc. are
fairly
low power and really aren't that much faster up a steep hill than an
unassisted
bicycle. The Wavecrest was the first decently powerful ready-made bike,
though the company didn't last long defunct. (Electric vehicle companies
seem to have a curious habit of making a big splash - and then
vanishing beneath the waves.)
I love the idea of
electric vehicles in general and would love to have a pure electric
car. I have a Prius, currently the closest thing available. However, battery technology hasn't advanced enough to
make a pure electric car affordable yet. I do think, however, that light-weight, low-speed,
short-range
vehicles are wellt within the limits of current battery-electric
technology,
so I set out to see what could be done to make a more powerful and
longer-range motorized bike.
I am a great fan of all
electric vehicles and watch the new technologies closely. Hybrid cars
are a tremendous technology that in my opinion is the first step on the
road to all-electric vehicles. Hybrids will necessarily drive the
development of better and cheaper batteries, which is the only missing
link to the puzzle. Hybrids of today that are only charged by their
gasoline engines will lead to 'plug-in' hybrids that can be partially
recharged at home or at charging stations. This will lead to
astonishingly frugal use of fuel, but it will take larger battery packs, which
means better
and cheaper batteries will have to be produced. This will happen and is probably happening right now. Eventually
batteries will become so much better that the auto companies will start
to produce cars that simply leave out the combustion engine altogether
- the pure battery electric vehicle. Don't get me started on fuel cells
and the much hyped future 'hydrogen economy.' This is interesting
technology, but suffice it to say that there are so many technical
breakthroughs and logistic problems yet to be solved, that I simply
don't see it happening within the next ten or fifteen years.
This is
not really a build manual, however. . .
Experienced tinkerers
may glean a lot of ideas and solutions from my pages if they want to
use them and adapt them. There are parts lists and plenty of photos.
Over the course of this
project I've made many interesting discoveries, encountered problems
which all EV builders must face, and finally figured out
some fairly simple solutions. I've fitted a number of different
driveline combinations which range from simple single vee-belts to the
present double reduction using a heavy-duty timing belt. Earlier
versions are
shown on following pages. This
bike performs well, is reliable and nearly silent, and all the major
components
are available online or at power transmission supply houses.
Please note that
I had to weld and machine key components like the jackshaft mount using
ball
bearings, and the chain sprocket that clamps on to the spokes at the
wheel
axle. The right components and design solutions to this project
were
hard to come by, and I just want to share what I've learned to promote
what
I consider a pretty amazing form of transportation. More below.
Please excuse the
following brazen advertisement. I am not selling motors or plans or
components for this bike motorization project. I am providing some open
and free detailed information about the parts and designs simply to
promote the concept. This page gets thousands of visitors a
month and the bandwidth is starting to cost money. Rather than take
down the page I thought, why not keep it up and advertise my product,
which I designed, patented, and manufacture -
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The current version is what I am calling v1.5, which is the same bike as v1.4 but with some basic changes: a new timing belt drive system with a chain final drive. Basically, the motor drives a belt that spins a secondary shaft or 'jackshaft' mounted close on the motor and spinning on ball bearings. This shaft then drives a chain going down to the rear bike wheel. Same motor, and all other components. (Earlier versions are documented on following pages.)
Motor. The
Scott motor is 24 V DC, brushed, and is rated at 1 hp (746 watts)
continuous
power and draws 41 amps. It costs about $250. This model no. 4BB-02488.
I
bought
it from Wilde Evolutions.
You
cannot just use any old motor to drive a bike. People often ask me, can
I
use a motor from an electric drill, a starter motor from a motorcycle
or a
car. The short answer is no, these motors are generally not suitable.
Either the efficiency is poor, they will overheat, they are not
powerful enough, the speed is too high, or any number of other reasons.
The Scott is a very high quality motor with high efficiency, good
cooling,
and ball bearings. At 16 pounds it is pretty heavy but motors in this
power range were hard to find five years ago. If I were doing this
today I would
undoubtedly use a lighter motor of the type that has subsequently
become available. New lighter motors that I think would work well
include the Transmagnetic 600 watt brushless motor, some newer
600 watt motors that are meant to fit on the Currie USProDrive. There
has
been an influx of scooter motors. There are also a number of motors
used by
people into combat robots or battelbots. These are generally high power
and
and quite tough. However battlebots don't really need efficiency so
this is
something to look out for. Efficiency of greater that 80% should be
looked for at a wide range of speeds and currents.
In my opinion, a bike needs a motor of at least 400 watts and probably not more than 1000 watts. Any less and you might as well not bother, and any more and the motor will probably be large, heavy, expensive, a current hog, or all of the above.
The reason for this powerful a motor is simple: hills. I live on a steep hill. While a Zap or a US ProDrive rated at 400 watts goes a decent 17 mph on flat land, they slow to 3 to 5 mph on a decent hill. I can pedal that fast. I have ridden a Schwinn with the Currie US Pro drive. It was actually great, I would recommend it to anyone as a very good turn-key kit solution. However I wanted to go faster and be able to get up a hill. The simple fact is that you need from 5 to 10 times as much power to go a given speed on a decent grade. This is why a car that only needs 12 hp to go 60 mph on flat roads has an engine that can put out 100 to 200 hp.
The Scott motor is used
in
many go-carts and Electrathon vehicles. (Electrathons are efficiency
competitions using ultralight closed-course
battery electric vehicles carrying one person. The point is to go the
farthest distance
in a given amount of time.) So the efficiency is obviously pretty good.
The
motor cost $269 total shipped, and weighs about 16 pounds. It has ball
bearings and massive
cooling fins
and is built to last. Brushes do not wear out very fast at all, this is
not really a concern. It draws around 41 amps while producing a
continuous 1 hp. Put a greater load on it and will draw well over 100
amps
and produce up
to 3 hp. This is why it needs a 180 amp controller.
Today I would probably
use a different motor. As I say, there are brushless motors in the 7-8
pound range that offer decent power and efficiency at a similar cost.
However, the Scott remains a viable choice even today due to its
incredible power if you can live with the extra weight.
Big problem
solved - New Speed
Reduction.
I will spend quite a bit of time here describing the
speed reduction scheme, as this is the most difficult problem to solve
in fitting a motor to a bike. The big issue
with all motorization schemes on bikes is this: how to reduce the speed
of
a typical motor, which turns at upwards of 3000 rpm, to the necessary
speed
of a bicycle rear wheel, which for a 26" mountain bike wheel at 20 mph
is turning only about 265
rpm.
The overall reduction required is between roughly 8:1 and 11:1 depending on
what top speed you wish to achieve. Now it is certainly possible to
simply bolt, say, a 130-tooth chain
sprocket
to the rear wheel and drive it with a 13-tooth drive sprocket off the
motor.
But this is a very large wheel sprocket (about 15" in diameter!) and,
critically,
the
chain would be unbearably noisy. My basic observation about chain drive is that
if any chain sprocket turns faster than about 750 rpm, it will start to
make a racket. If only there were a good quiet
3:1
gearbox you could bolt on to the Scott, the final 3:1 or 4:1 ratio
could
be easily achieved with a chain and sprockets to the rear wheel.
However
I don't know of such a gearbox ready made, and a gear box is very
difficult
to machine from scratch. Straight cut gears also tend to make noise.
I have come up with
this belt drive to
achieve the primary speed reduction of 3:1. My new
dual-reduction drive system is much more solid than anything I have
tried
before.
It's a little more complicated but slippage is absolutely eliminated
and
I
feel confident that this system would be able to take even more power.
This
is important because the Scott motor is often bumped up to 36 volts
rather
than 24 and I may eventually switch over.
Why belt primary drive? Wouldn't two chains be easier? Basically, as I say, belts are much quieter at high RPM. At 3000 rpm a chain would be about as loud as many small gas engines. An electric bike should be whisper quiet, and this belt drive is. Many electric go-carts use a single straight chain drive; however, they go faster while having smaller wheels and thus need less speed reduction, and the wind and tire noise and speed tend to mask the racket the chain makes. Plus, carters don't really care much about noise.
I used a Gates PowerGrip GT2 belt. This timing belt has rounded teeth so it's quieter than other belts. The teeth are closely spaced for better grip on the sprocket wheels. This is the 5mm pitch type. At 25mm wide, the short belt is much less likely to stretch and is easily tensioned by pivoting the jackshaft mount slightly - three of the mounting holes are slotted, so the mount can pivot around the un-slotted fourth hole. Does this wide belt seem like overkill? The Gates Co. has a very nice simple little program they give away on their website that will tell you what size and type of belt you will need. You plug in the power, gear reduction, speed, and a couple of other things and the program will give you some recommendations. I used the program and this was one of the recommendations and what do you know, it has been great. Has never been adjusted and and has never slipped or worn. The jackshaft itself is a 1/2" steel shaft running on two R8 sealed ball bearings. The mounting plate and bearing housing is welded aluminum. Sorry that I don't have a diagram of this but hopefully you get the general layout from the photos.
If belts are so great, then why chain final drive? Since the jackshaft is now speed-reduced to 1000 rpm max, running a chain down to the rear wheel now makes sense. There is a reason so many bikes and motorcycles still use steel chain final drive. It makes sense here because the run is longer and the available space narrower, and chain is stronger and less stretchy than a long narrow belt. At this speed the chain may be slightly audible at top speed, but not noisy, and the wind and tire noise while riding will actually be louder. A chain doesn't need to be tensioned nearly as tightly as a belt. Finally, chains have a master link which makes assembly and removal much easier, like when changing tires and repairing flats. (Belts are endless.)
The #35 chain is a very common American size. It has a shorter pitch than bicycle chain but is much stronger. (Bicycle chain is made flexible to allow for the side to side misalignment resulting from different derailleur gear combinations.)
You will notice the extra bar running in between the chain, from the jackshaft to the axle. This is a reinforcement and has an adjuster nut at the lower end to establish proper chain tension. The bar acts to maintain proper driveline rigidity - the Scott motor can put out a tremendous amount of torque and the motor mount and bicycle frame will twist and compress under heavy power loads without this bar. Actually it attaches to the bike frame near the axle using the existing rack mount on the dropout.
The driveline parts are as follows. They were ordered from Bearing Belt Chain in Las Vegas at www.bearing.com.
A
number of machining fabrications and
modifications I
had
to do myself. The
jackshaft mount
is the most elaborate part to make and took some
time. The shaft and motor need to be very rigidly connected and
aligned, yet
adjustable to set the belt tension. I used a 1/4" aluminum plate and
welded
a 1-1/4" alum. tube to this. There are two diagonal braces welded to
these
- you can only see the upper one in the photos. The braces just clear
the
motor. Then the tube was bored in the lathe for the two ball bearings.
It also has ring grooves for retaining rings to hold the bearing in
place.
The
1/2" shaft is about 5" long. There are a total of 4 stainless steel
1/4-20
allen bolts holding the whole thing to the face of the Scott motor
using
the
existing tapped holes. As I mentioned, three of the holes are slotted
to
allow for adjustment to tension the belt, the whole thing pivoting a
little around the fourth bolt.
The 21-tooth motor sprocket was bored to 5/8" on my lathe. It is fixed to the motor shaft with a 1/8" steel split pin or 'roll pin'. I had to drill a hole for this pin through the sprocket and motor shaft. Roll pins are much stronger than set screws, and easier to machine than Woodruff key-ways. The large 64 tooth belt sprocket arrived in solid steel and was ridiculously heavy. Aluminum was not available. So I drilled it out extensively to lighten it as you can see. The chain sprocket bolts to the hub of the belt sprocket with two 10-32 allen screws, the holes for which I drilled and tapped.
The motor is mounted on a specially made 1/8" thick stainless steel bracket. The whole bracket bolts to tabs welded to the bike frame with three allen bolts. The chain, drive belt and motor can be removed from the bike in about 3 minutes.
Finally,
I had to attach a sprocket to the rear wheel.
This was done basically by boring out a standard type-B 36-tooth
sprocket
(the kind with an integral hub) to where the hole would just fit around
the
wheel hub for centering purposes. Then I basically faced the hub down
to nearly nothing on the lathe, just
enough
to space the sprocket out away from the spokes. Then I drilled 9 holes
for
10-32 screws. The screws go through the spaces where the spokes cross.
I
made three curved clamp plates with three holes each for the inside of
the
wheel. Thus the sprocket is basically clamped to the spokes. This
doesn't seem to stress or deflect the spokes that much since it's very
close to the hub. For those
of
you familiar with the Currie US ProDrive, a very popular electric drive
kit
for bikes, this is basically the same way they bolt their motor plate
to
the bike wheel. Looks somewhat unsophisticated? Fine, but it works,
bolts quickly to a completely unmodified bike wheel, and has
been trouble free.
For off-road use I intend to get a different wheel with a knobby tire and a larger sprocket to lower the gearing for better hill climbing. A couple of extra chain links will make it easy to switch out.
The
net
of all this is that finally I have got it really right.
I can whack the throttle open and closed at any speed and there
will
be no belt slippage of any kind. The first thing I did was to go down
the
street and climb this steep rocky trail that gave the old bike
problems.
Even with the relatively high gearing, the new driveline shot me right
up
the thing. I even popped a little wheelie over the top of it. That sure
never
happened before. Now I can finally use all of the torque this Scott
motor
can put out.
> next: more design
details,
components, electric bike issues and more EV thoughts
