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I have added an electric power assist onto a GT mountain bike. With a 3 HP peak motor, it is much more powerful than typical electric bicycles. The basic motivation to build an electric bike 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. I wanted to get some exercise - this bike 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 my basic design.Attention: have you built an electric bike or vehicle based on this design? Send me some good photos and I'll put them on a projects page!
Beauty of the
no gasoline - no oil - no tune-ups - no parking hassles - no car payments - no more exercycle (use the pedals) - no brainer
Why build an electric bike? Why use such a big motor?
love the idea of
electric vehicles in general and would love to have a pure electric
car. I have a Prius, currently just about the closest thing available. However,
battery technology hasn't advanced enough to
make a pure electric car affordable yet. I do think, however, that
vehicles are wellt within the limits of current battery-electric
so I set out to see what could be done to make a more powerful and
longer-range motorized bike.
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 magical. But the electric bikes and
motor kits for sale at the time, such as the Curry USProDrive, the eBike, etc. were
low power and really aren't that much faster up a steep hill than an
bicycle. The Wavecrest was the first decently powerful ready-made bike,
though the company didn't last long. (Electric
seem to have a curious habit of making a big splash - and then
vanishing beneath the waves.) Now
you can buy decent electric motor kits from Wilderness Energy and
Crystalyte, among others, but I was curious about the nuts and bolts of things, and
wanted to learn more about exactly what it takes to drive a vehicle
with pure electric power.
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 these cars will need much larger
battery packs, which
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. In other
parts of this website and in the Electric Bicycle FAQ, I elaborate a
little more on the near and far term technology, where I see it going,
and what the most promising developments are.
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. This driveline is now extremely robust and probably suitable for larger electric vehicles, not just bikes, up to three to five horsepower. Over the course of this project I've made many interesting discoveries, encountered issues and problems which all electric vehicle builders must face, and finally figured out some fairly simple solutions. I've fitted a number of different driveline systems which range from simple single vee-belts to the present double reduction using a heavy-duty toothed belt and chain. Earlier versions are shown on following pages. This bike performs well, is stone-reliable and nearly silent, and all the major components are available online or at power transmission supply houses. 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 hub. 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.
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 an intermediate shaft or 'jackshaft' mounted next to 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 cost about $250, however in 2008 I see the price is closer to $350. This model no. 4BB-02488. This motor is available still from cloudelectric.com. Don't let the rating fool you, this motor will put out 2.5HP peak and can even be over-volted (judiciously) to 36V where it will put out 1.5 HP continuous and around 4HP peak. If you're crazy enough to gear it up, it would propel a bike to around 50mph (please don't do this.) So the Scott is pretty heavy and suitable for slightly larger vehicles; for a bicycle, a better alternative might be one of the motors at evdeals.com such as the 500 watt 36V motor for only $60. Cloudelectric also has a selection of motors from $80 to $170 that would be very suitable according to your budget and power desired. A very interesting choice is their 'Motor 36 Volt 1000 Watt' model that has an integral freewheeling clutch. This can also be run at 24V to get 535 watts of power. (I may have to switch over, this sounds like an ideal motor for a project like this.)
cannot just use any old motor to drive a bike. People often ask me, can
use a motor from an electric drill, or a starter motor from a motorcycle
car. The short answer is no, these motors are generally not suitable at
Either the efficiency is poor, they will overheat, they are not
powerful enough, the speed is too high, they're too noisy, or any
number of other reasons.
The Scott is a very high quality motor with high efficiency, good
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. There
been a tremendous influx of scooters and smaller electric vehicles, and
many of the motors on these vehicles would be suitable for a bike.
There are also a number of motors
people making combat robots or battelbots. These are generally high
and quite tough. However battlebots don't really need
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, deplete your batteries in a flash, or all of the above.
The reason for a motor this powerful is simple: hills. I live on a steep hill. While a Zap or a US ProDrive bike 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 was 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. More about controllers later.
Speed Reduction and Gearing: a big issue. 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 or other small EV. The big issue with all motorization schemes on bikes is this: how to reduce the speed of a typical motor, which may turn at upwards of 3000 rpm, to the necessary speed of a bicycle rear wheel. A 26" mountain bike wheel at 20 mph is turning only about 265 rpm. The overall reduction required is therefore 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 a whining noise.
big problem of gearing down the motor has lead to the development of
the hub motor. This is simply a bicycle wheel hub with a motor built
in, which can then be spoked to a rim. Such hub motors either have gears built in, or they are very
particularly designed to just turn at a very low speed while still
having good power and torque. This is a difficult trick. It's much
easier to make a motor that derives its power from spinning at higher
rpm. At any rate, at the time I could not find many hub motors, and the
few that were available were expensive, made a gear whine, and were
underpowered. Also, a hub motor has one set gearing by design, and you
can't alter it. I wanted something where I could experiment with
different gear ratios, top speeds, and hill climbing ability.
have come up with
this belt drive to
achieve the primary speed reduction of 3:1. The
dual-reduction drive system is much more solid than anything I have
It's a little more complicated but slippage is absolutely eliminated
feel confident that this system would be able to take even more power.
is important because the Scott motor is often bumped up to 36 volts
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, so they 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 small 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 very nice simple little engineering software 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 much 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 stock 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.
number of machining, fabrications, and
to do myself. The
is the most elaborate part to make and took some
time. The jackshaft and motor need to be very rigidly connected and
adjustable to set the belt tension. I used a 1/4" aluminum plate and
a 1-1/4" alum. tube to this. There are two diagonal braces welded to
- you can only see the upper one in the photos. The braces just clear
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
1/2" shaft is about 5" long, also with ring grooves. There are a total of 4 stainless steel
allen bolts holding the whole thing to the face of the Scott motor
existing tapped holes. As I mentioned, three of the holes in the
jackshaft plate are actually curved slots
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 they are pressed or driven in, so they don't come loose. They're also 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 to the bicycle frame 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. I finally decided to just try clamping a sprocket to the spokes. This may seem like a clumsy solution but I have seen it done on a number of other applications. It has been reliable and has not damaged the spokes at all. Sometimes simple and direct wins the day. 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.I started by boring out a standard type-B 36-tooth sprocket (the kind with an integral offset hub) to where the hole would just fit around the protruding wheel hub. This locates the sprocket nicely on the hub center. 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.
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.
of all this is that finally I have got it really right.
I can whack the throttle open and closed at any speed and
be no belt slippage of any kind. The first thing I did was to go down
street and climb this steep rocky trail that gave the old bike
Even with the relatively high gearing, the new driveline shot me right
the thing. I even popped a little wheelie over the top of it. That sure
happened before. Now I can finally use all of the torque this Scott
can put out.
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