Ideas and advice for a DIY electric bike on the cheap.
October 29, 2013 11:03 AM   Subscribe

For a long time I've thought that it would be a fun project to put together an electric bicycle. However I've been stymied by the cost and questionable quality of the existing kits, and the complications and compromises of the various DIY methods that are out there online. I have some ideas for how to go about making a cheap and simple electric bike, but I'll be the first to admit that I'm nobody's idea of an engineer so I thought I'd run some things by y'all and see what insights you have.

My overall goal here is to have a bike that could take me to work and back, seven miles each way across mostly flat roads, at a reasonable pace and if necessary with a recharge between the trip out and the trip back. I'd like to be able to pedal the bike as well. It needs to be reliable and to be able to transport about 225 lbs of bike, rider, and gear at say 15mph over flat ground. I also want to keep costs, weight, and complexity to a minimum.

I have already ruled out the idea of buying a purpose-built electric bike. They are very pricey for what you get (my understanding is that this is due mostly to economies of scale, plus the fact that a decent new bike is already at least several hundred dollars even without electric running gear) and somewhat counter-intuitively tend to be built rather cheaply in order to keep costs down. Also they are usually not the best as far as pedaling; aside from the inevitable extra weight of the electric drive system, they usually seem to use heavy high-tensile steel frames and to omit the kind of transmissions that allow traditional bikes to be fast and efficient. Most importantly though, I want to be able to enjoy the fun of building something useful on my own.

There are kits out there that purport to allow one to convert a standard bike into the kind of electric bike I am interested in (Here's an example, note that it does not include batteries) but they tend to be expensive -- around $1,000 once you factor in a set of good batteries -- and my understanding from reading reviews is that they are built rather cheaply in China and tend to have pretty inconsistent build quality and reliability. That's out of my price range, especially for something that is possibly not going to last very well, so I've been thinking a lot about hacking something together out of repurposed parts.

The plan is to start with a used but good-condition hybrid or mountain bike, something like what I was asking about in my Ask from last week. (I haven't bought anything yet as a result of that question, but I got some great ideas and anticipate getting a new bike soon, thanks!) This would give me a nice sturdy frame to work with, something well built from aluminum or chromoly steel, with street-appropriate tires and a transmission that would allow for decent pedaling. I think something like this would be an ideal platform to build an electric mod around.

Currently my favorite idea in terms of the electric "upgrade" is to build it around a powerful cordless drill with a brushless motor. The model I've been looking at most is the Milwaukee 2604-20, an 18v cordless drill with a brushless motor (for quieter running and longer life -- most cordless drills use brushed motors which inevitably wear out, especially under sustained heavy use) lithium-ion batteries, and an all-metal gearbox that can be had for about $250USD on eBay complete with two batteries and a charger in new or lightly-used condition. It's gotten excellent reviews in the contractor community for being one of the more powerful drills in its class, and for being able to run for a long time on each battery. Milwaukee tools also seem to have a good reputation for durability, reliability, and overall build quality. There are similar drills from other brands as well, but that's the sort of drill I'm looking at. (All these drills seem to be built in China also, but built well rather than cheaply.)

The advantages to building my system around a cordless drill mostly come down to simplicity. Basically a cordless drill provides a motor, a chuck, a speed controller, a transmission, and a battery all in a single package. The only part that I'd need to add myself would be a throttle, which would be cable-actuated either by a hand brake lever or a twist handle. I would also need a similar system to disengage the drill, though depending on where I end up mounting it I may be able to simply put it into neutral with the button on the drill itself. I'm not remembering clearly whether cordless drills will spin freely when in neutral, but even if I need to use a freewheel hub or some kind of disengagement device like a derailleur or a movable cog I should be able manage.

I can think of a few ways to make a drill drive the front wheel. I could simply use a roller pressed against the tire or rim, though I am aware that this method tends to sap a lot of power from the system and I'm already worried about it being underpowered so I probably won't do that. I could mount the drill directly to the axle on the front wheel (after swapping to a suitable axle) but then I would end up with a rather large, heavy drill hanging off the side of the bike and making it unbalanced. The third option I have in mind is to use a chain drive to connect a cog on the drill to another cog on the front wheel (which would probably have to be switched out for a back wheel that was modified to work with the front fork). The last option sounds like it's probably the most practical (I could simply mount the drill on the back side of the front fork, assuming suitable clearance) albeit it's the most complicated as well.

So far so good, but I'm not convinced that the underlying plan is a sensible one. I'm concerned that a cordless drill won't be able to do what I want it to do, and the specifications available for drills don't really give me the kind of information I need in order to work out from first principles whether or not this is a real option. I'm concerned that a drill won't be powerful enough to drive the bike, that it won't be durable enough to hold up to sustained use, and that the batteries (even if I carry a spare in my bag) will not give me the kind of range that I need and/or that they'll wear out quickly and be unable to hold a charge. Drill batteries are a lot cheaper than big LiFePo4 bricks, but if I'm having to buy a new set every week or two then it becomes sort of a pointless endeavour. There's also the issue of theft and weatherization, but I'm convinced that I can deal with those problems.

What do you think about all this, hivemind? Have you ever tried something like this, or seen or heard about it being tried either successfully or not? I've seen a few instructables and such that involve building an electric bike with a cordless drill, but none of them have satisfactory end results. They all use very weak drills and crude drive systems however, and I think I can do a lot better there. I've seen some debate on online forums around this idea, but nothing that seems really conclusive -- people seem to be making a lot of uninformed assumptions that don't seem like they'd necessarily hold up in the real world if I "did it right".

If you do think that this is potentially a workable concept, I'd appreciate advice about the best way to set this system up so that it would work well or about any pitfalls or hurdles that I haven't thought of. If you think it's a poor idea overall then I'd love to know why you think so and would especially like to hear about any designs or concepts that you know of or have thought of that you think would be superior. My knowledge of electronics and mechanics is pretty rudimentary but I am fairly handy in general (and bikes are one of my stronger suits, albeit I recognize that I am no professional in that area either) and am looking at this project as a learning exercise so I'd be happy to expand my skills as long as whatever I have to construct comes with clear instructions. I think my absolute maximum budget for this project, not including the original bike and assuming that I have all the necessary tools to pull it off, would be something like $500USD. Less would be better of course, as long as I'm not sacrificing my performance minimums -- 15mph sustained top speed over flat ground, seven mile range, 225lb load, ability to pedal, and a reliable system that will stand up to years of frequent use.

All your advice is greatly appreciated. I'm very much looking forward to hearing what y'all come up with.
posted by Scientist to Travel & Transportation (12 answers total) 8 users marked this as a favorite
I'm not up to date on what the current trends are in this direction but I agree with your points concerning ready-made bikes, notably concerning an expensive pricetag while also being not exactly expensive components.

My other 2 cents, and this is my opinion without a practical basis beyond taking apart a few drills and using said drills in their designed role, is that the drill motor isn't going to be suitable for what you're wanting to do. I'd expect the drill motors to have pitiful longevity if they were used for primary propulsion of the bike. I know, I know, this falls under the category of pretty much uninformed assumptions but I have some decent experience with tools, EVs, and mechanical engineering.

The drill's battery packs, now those might be useful as part of a bike power supply. One of the cars we competed against in the Hybrid SAE event I was in used a ton of those as their battery stack. Totally respectable if your pockets aren't limitless. Be aware, one or two of them will not be enough. And don't forget to figure in for how you're going to charge the pack/batteries, that's a crucial part of the build and won't be free.

Also, and obviously, you want to avoid anything that's powering the bike via contact on the rubber or rim. You want something driving the rear wheel's axle itself, just like your feet would be otherwise.
posted by RolandOfEld at 11:19 AM on October 29, 2013

I've looked into this a lot recently myself. Not sure if you've seen these sites or not: - linked directly to the DIY section. The site in general has a good overview of the available commercial options, component reviews (including motors), etc. They also have a handy recommended vendors page. - electric bike forums. Plenty of help available. A much better place to ask this question than here, probably.

I haven't made mine yet, but I plan on doing so before 2014 rolls around. I won't be using a drill motor when I do. I will likely build a kit from these people.

Word of caution! My understanding is that these motors seriously stress the dropouts. You will want big, beefy steel ones and possibly a torque arm so you don't kill yourself.
posted by jsturgill at 11:26 AM on October 29, 2013

Not going to work well, sorry.

The batteries you link to provide 18V / 4Ah = 72Wh. Compare that to the batteries people are using for electric bikes (mine is 10x that). You could cobble together a battery from a bunch of those (and people did do that with DeWalt A123 batteries many years ago), but it will be more expensive than other options. You would drain that battery in minutes, with the power required for an ebike.

The drill motor is not going to be able to provide enough power to move your bike at any appreciable speed and it will burn out. Compare the size and mass of an electric drill motor to the motors that power electric bikes and the difference is clear (again, about an order of magnitude).

I recommend for a kit that will really work.
posted by ssg at 11:29 AM on October 29, 2013 [1 favorite]

And remember a recumbent, although most likely out of your budget unless you're building a frame from scratch, will get more bang for your buck/watt with respect to range/speed. Doubly so if you have a windshield.

jsturgill, good point about the torque arms. That's a really good thing to be aware of.
posted by RolandOfEld at 11:29 AM on October 29, 2013

I noticed an interesting electric wheel startup that may give you some ideas.

What jsturgill says, the idea should work fine but there are real engineering issues. Finding the right gearbox and various components may be a challenge if you're not in a serious tinkering mode.

I don't think the application would "kill" drill batteries, but you'd need more than one for any distance and that means a custom holder and a lot of chargers. Doable but inconvenient, the basic 12 volt wet cell could still give you the best value.
posted by sammyo at 11:35 AM on October 29, 2013

Head to golden motor, even if you take nothing but inspiration from it.
Hubmotors for under $100!
posted by the Real Dan at 11:42 AM on October 29, 2013

Ditto Golden Motors. Get a steel mountain bike with a standard fork, no shocks allowed. Get a Golden Motor Magic Pie with a torque arm adapter, front wheel mount. Get a lead acid battery, do your learning curve, probably ruin it but then you'll know how to run one properly. Then upgrade to a LIPO brick.
posted by diode at 12:23 PM on October 29, 2013

retail is getting close and will eventually get there, at prices primates can better afford.

self-link - was a lot of sweat and blood and cursing. (seriously you have no idea the amount of blood. or how many curse words.) but around your budget (assumption of course, I already had the bike cheaply my-ownself) it mostly worked out. it will start out fun to put together; it will end up very-not-fun to put together. it will work, if you persevere.

you will save a lot of money getting the parts directly from china. you will also gain about zero support once you pay for it, and have to be willing to repair/solder/figure-out-things/buy-the-correct-bits-and-bots yourself.

soldering power/signal wires ok, one can learn to do. if you get a rim with massive flat spot, probably due to shipping -- like I did, you either live with a bumpy bike, or pay to have a new wheel made (which price regardless, is harder than it sounds because goldenmotor hubs are built with ginormous motorcycle spokes and double wall heavy rims, unknown to many bike shops - but still, you may do better to buy simply the motor and try to get your local shop to build it up for you)

goldenmotor is pretty brilliant (that's what my monster-gone-wrong is based on) and you would do better to deal with their official Canadian branch. they will support you to the letter but you will also pay 2-3x overall.

you will need to figure out what voltage (speed) and amp-hours (distance) battery you need. and how much that costs (and if from china is unlikely to be replaced if there are problems). lithium iron phosphate cells are great, but there are different ones that interact differently with different motors and different chargers. you will need to know what charger you need at home/work. you will need to find out what spacer-washers and clamps you need to safely bolt the crazy thing to your frame. you will need to know what replacement-freewheel/cassette you need to correctly fit within your frame. (again, this is why buying direct from Shanghai will cost 1/3 of buying from the Canada distributor of same)

finally -- ok, if you go for a hub motor, prebuilt into a rim or not, whether front or rear wheel (there are dis/advantages to each), as ppl have said - put some dang torque arms on it or it will eat your dropouts (front or rear).
posted by dorian at 12:31 PM on October 29, 2013

Ambitious project, sir.

7 miles at 15 MPH is a half hour. 8 hours is your recharge time. these are critical items in your design.

i think you'll need less energy than you think if it really is flat. all you need to do is overcome friction losses and air resistance. self-acceleration with pedals would also be a good idea, versus with the motor. lowers the maximum current needed.

I like powered wheel chair motors and controllers. not particularly light, but you can find them and they start out ready for transport duty, are DC, the proper voltages and a variety of controller activators are out there. they have the connector issues dealt with. they are environmentally robust. they have some safety interlock stuff done. mostly, it's mounting. i think i could do a bike mount and make one work in a week, but i have three recent wheelchair projects under my belt (and 40 years of making complicated stuff, too.)

on power plant... honda insight battery sticks are available that will provide 12V or so at 100 Amps per stick, peak, 50 running. not amp hours, amps. they are in the 5-7 amp hour range. they are a stack of Nimh batteries in D size, welded together. they sell on ebay for $20/stick, used.

as someone who has worked on human cargo mobility stuff, i suggest you stay away from drill motors. you are outside your competency and a motor can take your hand off. i kid you not. moving a 200 pound cart/battery with a 400 pound payload in controlled fashion isn't small power territory. (that's what the mobility vehicle controllers i designed would do. fat guy up and down a ramp with no speed changes. 6 MPH, max, crawl speed min. )

again.... anything you do... needs a Big Red Switch to kill the unit if it needs to die. I'd make one with a tip over switch, too. if your hand is caught in a motor than can move your 225 pound ass, you'll be glad you have a stop method. No kidding... Big Red Switch.

i have some specific model suggestions if you want to take this approach. memail.
posted by FauxScot at 2:58 PM on October 29, 2013 [1 favorite]

Consider how you'll feel if you put money into things that burn up. Just Lemire-Elmore of (Grin Technology) has been buying, installing and modifying Chinese hub motors for years. He seems to have a real passion for this stuff and supports people putting together system piecemeal (unlike Bionix). He sells torque arms, motors, controllers (that don't burn up) and the Cycle Analyst that will work with batteries you get direct from China off eBay (e.g. Ping).

Video of him talking about experiments with rain-proofing ebikes. I'm not a customer yet, but when I get an edgerunner, I'll get the motor from him.

Edited to add: there's a simulator on that lets you plug in weight, speed and grade and see how much power you need for it. Then you can figure out your Watt*hours for the battery capacity.
posted by morganw at 5:09 PM on October 29, 2013 [1 favorite]

Ok, scientist... in response to your memail, i will post the info here for general enjoyment. i will have to move some pix to imgur and gather my info, but will do so and check with mods to make sure i am not self-promoting.
posted by FauxScot at 2:46 PM on October 30, 2013

Best answer: OK, here goes.

Caveat 1 - do not get killed. A bike stand beats the road for testing until everything works right, most especially the Big Red Switch.

First, a note on Big Red Switches (BRS). Powered equipment involves concentration of force. Just like inncocent little scissors, they can use leverage to concentrate forces strong enough to cut metal, bone, flesh. Humans are basically bags of water with sticks inside, and we tend to think of ourselves as strong and durable. Not a day goes by in this country that someone doesn't get a tie caught in an alternator, a pony tail in a farm machine, a finger in a shear. In the time it takes to realize something has happened, your days of playing guitar can be over. So when you make a power machine that can move YOU, you need to have a means of stopping it from doing so. The BRS is one means.

My favorite approach is called a crowbar. It is a normally open switch wired across the battery terminals on the safe side of a fuse. If you need to activate it, it blows the fuse. You can get dead-man versions that have strings attached or make you own, but when you do this, spend some time thinking about how you might want to activate it. Several of these can be wired in different places to make sure you can reach at least one. For a bike, it may be overkill, but hey.. fingers.

This idea of using wheel chair parts for a bike is suboptimal for a bike. It's a hack of a hack. Hub motors are more purpose built, and resolve some of the things you'll have to do to make a bike from a wheelchair. However, since I have a pile of parts from wheelchairs, it's the first place I would start. ( When you only have a hammer, etc...... )

I got my first two chairs from my client who bought them for a song. The batteries die and are several hundred dollars. Lots of folks get new chairs with insurance and the old ones are sold off for what they can get. With bad batteries, it's not much. You don't need big batteries for a lot of small project, though and the hardware is good. I'd buy a chair. Get it working. Take it apart. In that order. If you can't get it to run on the bench, you have no business running it on the road.

I have done three wheelchair 'robots' for an artist. One had a programmable logic controller (PLC) someone else had done. It was useless. The second was built from an Invacare chair and had been started (in a sense) by an engineer with no idea what he was going to do once he cut it all up. I made it work, using a small single-board computer I have used on a lot of projects. The third was another Invacare unit that moved the electronics from #2 to a better chair. That one shipped. I used a lot of different tricks to make it work, but in the end, it was just an open-loop, record and playback bot that repeated itself constantly. It had a little radio remote to kill it, and internal smarts that made that a decent enough safety. (Still dangerous.)

Unit 1: No picture. Trying my best to forget it. I hate completing someone else's work!
Unit 2.
Unit 3.

These are the motors:

They are badass mambo jambos. They can propel large humans quickly and precisely. They are DC motors, 24Volts, driven with Pulse Width Modulated DC (PWM) and wide-ass-open, they spin at about 138 RPM. If you could put one on a hub of a 26" diameter wheel (81" circumference) it would move you at 11 MPH. The motors themselves are a lot faster than 138 RPM, but are reduced and translated 90 degrees. The output shafts are keyed 5/8"-3/4" (more or less).

If I were doing it, first I'd power the rear wheel with a friction wheel/cylinder with a diameter and material that would drive the bike wheel and a mechanism that would connect the driver to the tire when I sat on the unit. That way, if I fell off, it would be more likely to stop. It would be awfully slow, but a lot easier to fab with limited tools than a direct hub drive. There are zillions of ways to connect the motor and wheels. That's where creativity comes in.

The controller itself looks like this. This one is an Invacare Mark 5 NX-75 controller.

It connects to the motor with these connectors.

The joystick that runs it looks like this. It's an SPJ+ joystick. The SPJ+ is a customized CAN bus device, and internally it uses inductive sensing for the joystick control. It took me several days to reverse engineer the hardware, and the software was not easy enough to warrant the effort I would need to simulate it with my own hardware. So I didn't.

Here is what is inside the joystick. The SPJ joystick (incompatible with this controller) is resistive. I was able to splice into the boundary of the joystick and the micro and insert my own sensing and playback computer and that's how I accomplished what I needed to do. Lame, but elegant. You would be safe with either the bulkier Mark 4 controller or this one. If you are taking apart a chair, you use whatever controller it has. The cool thing is that the charger, controller, motor, and joystick all already work together. You can ignore a lot of details.

The two green PC boards are all that's in there and you really don't need to know that or how it works. My pix is of my development/reverse engineering setup and shows the singleboard computer I used to make the bot, and the interface module for the wireless link to the single board computer. ( According to a very young child I know, I am good with wires. )

You'd want to hack the joystick some. The reason I like this thing is that it has some things like speed limits, battery checks, power-on interlocks, etc. that contribute to safety.

I use these completely inadequate batteries to get a few hours of use from the thing. They are cheap and easily available, like the girls I dated in high school and probably still should. They are not the batteries I'd recommend. However, they ARE compatible with this nifty thing that comes with the chair... It's the charger for Sealed Lead Acid (SLA) batteries. For starters, and bench work, it's the bomb.

In the extremely unlikely event you decide to go through with this, and you get far enough along to worry about actually riding it, I'd look into Honda Insight battery sticks.

Here is a recent ebay listing for a pile of them.

You would need three or four, but they are kinda bikey looking, don't you think? All round and stick like? Super power dense. If you got some good ones, you could have some charge at work and some charge at home and swap them out at either end. They charge fast enough though, with the right equipment. That's your problem.

So if you have questions, ask. I've been known to answer some. Have fun and be safe.
posted by FauxScot at 9:43 PM on November 2, 2013 [4 favorites]

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