Introduction: How to Build an Electric Bike for Less Than $100

About: A mechanical engineering graduate from JBU, I enjoy putting motors on things that don't already have them, tinkering with small gas engines, airsoft, paintball, and pyrotechnics. I am no longer active on this …

Yes, it is actually possible to build an electric bike for under $100. The secret to doing this is... get most of your materials for free! Now I am not just going to turn you lose and say go find this stuff either. There are a few tricks and tips that I will give you and places to look. In addition, you will need to have problem solving skills of your own, since everything you get will probably be a little different from what I have. Undertaking this project is going to be challenging, and if you do not have substantial knowledge of machining tools, you might as well back out now. However, if you know your way around a lathe and are handy with only a few simple tools, this project is something you can complete in a few months working in only your spare time. This is also my entry into the Epilog Laser Cutter challenge, so please do not forget to rate and vote! Also, if you have any suggestions on things I can add to make this better, PLEASE comment, as I will be handing this in for a very important grade (basically my whole 4th quarter grade) so any criticism and help would be greatly appreciated! Thanks!

Step 1: Background and Theory

Before we dive into the instructions, I will need to give you a little background on this project. As a senior in High school, we are required to do a "senior project" that includes writing and presenting a research paper over a topic of your choosing. Included in this research paper must be an observation, or an essay about a hands-on experience you had regarding your topic. The requirements are simple: the topic must be school appropriate and you must show both foreknowledge and a significant learning stretch. Electric bike conversion was the perfect topic for me, because I have already successfully built a friction drive electric bike, but my previous attempts with chain drives have failed, so obviously I had to come up with a plan to successfully build this thing, so first I took a look at where my first attempt was unsuccessful, and it was pretty obvious. My first attempt at building a motorbike found me not paying attention to tolerances. I was just guessing when sprockets aligned and welding them onto what looked like the center of the shaft! Ouch! There was no way that was going to work. In addition, the shaft on my motor was very small, and trying to attach a sprocket to that would not have worked anyway. Therefore, I needed a way to drive the rear wheel (using the standard rear cassette) from the motor. My solution was a belt drive. So then, I wondered how to convert the belt drive to a chain drive to drive the rear wheel. The answer to that was a (not so simple) jackshaft that will mount in the bottom bracket perfectly aligning the drive sprocket and the driven sprockets. To make this project work I also knew that there would be no more welding on of sprockets, so instead I opted for a much more accurate (and better anyway) pinning method. In addition, my first bike, with a measly top speed of 20 MPH, left quite a bit to be desired. Therefore, I wrote a formula to calculate gear ratios, and decided to gear my bike for a top speed of 40 MPH! Finally, I had to find a way to get all of these parts with very tight tolerances. To answer this question: I simply had to machine them, and machine them very accurately. Accuracy is the key to being able to make this project work. Without a metal lathe, this project would be impossible to pull off. Now, with enough background information, it is time to continue to my senior project: convert a normal bike to a powerful electric motorcycle! (For under $100)

Step 2: Tools/Materials

This step is very critical. If you do not have the tools or materials listed, I strongly advise against undergoing this project. And I speak from experience.
Lathe (this is mandatory)
basic hand tools (hacksaw, pliers, etc.)
dial caliper (if you don't have one, buy it. and don't skimp, get one with a depth gauge)
drill press with many bits
chain breaker
freewheel remover
metal cutting tools (piranha is very nice, but a plasma cutter or oxy torch will also work)
basic bike tools
optional tools that you might need:
surface grinder
tap and die

Materials: (other than the obvious)
angle iron
*9-tooth, ANSI number 40 machinable hub sprocket from McMaster, part # 6793k208
2 bearings from McMaster, size to be determined later
steel round stock (depends, between .5 and 1 in D)
*4 inch diameter v-belt pulley from Chicago die cast pulleys
*1 inch diameter v-belt pulley (I machined this one, but it would be much easier to just buy it.)
*note that these sizes may change depending on your bike and top speed requirements.

Step 3: Obtain Materials

This is probably going to be the most challenging step of all. There are three main components that you will need to find for free in order to do this thing for under $100 like me. You will need to find a motor, batteries and a bike. Let us start with the bike. It is very easy to find a bike for either free or very cheap. Just make sure that when you are looking, you try to find a bike with as many gears as possible. This will give you either a higher top speed or better acceleration. (Yes, gears are mandatory for this project, as it gives you more tolerance when it comes to the chain drive) Try Craigslist, or if you have family that lives on a farm like me, you might just be able to ask them if they have a spare one lying around. You could also just use the bike you already have, or if all else fails go to the dump or a scrap yard (just make sure it is legal first). However, be careful. If the bike has been setting outside for a while, you will probably need to do some maintenance and a general tune up . Finding a free bike is probably the easiest part. Now in the next stop you can kill two birds with one stone. Odds are your dad probably does not repair power chairs like mine does, but I would guess you will have some sort of medical supply store around where you live (this is not a hospital, but they are often affiliated). Once you find a medical supply store around were you live, go and ask to talk to a service technician. Explain to them what you are doing, and odds are that they will either give you a motor and possibly batteries or contact you when they get one in. My dad is a service technician and they get in old batteries and motors in all the time that they just throw out, and I have quite the stash of free electric motors and seven 12v power chair batteries, all free. This is by far the best place to go, but if you are turned down maybe try a scrap yard or dump for a motor you could scrounge. (Again, make sure it is legal) If you have family that lives in the country, talk to them. They usually have lots of junk lying around that you could get a motor out of.

Step 4: Machine Your Bearing Cups

The bike I picked out already had threaded bearing cups, so I got lucky. However, if you did not get lucky, you will have to machine your cups. I would recommend machining something that looks a lot like what I have, accept not threaded, and held into the Bottom bracket using set screws.

Step 5: Machine the Jackshaft

Since every bike is different, you will have to design and machine your own parts, but the jackshaft should be very generic. Assuming you bought a large pulley, bearings and sprocket with 1/2 diameter center holes, you will need to start with a 5/8 inch D piece of steel round stock. Machine about an inch of one end to 1/2 diameter. Then you will need to measure how far it is between your 2 bearings in the bearing cup, and leave that to 5/8 D. then turn the last few inches of your piece to 1/2 D. leaving the 5/8 in the center will keep the jackshaft from sliding back and forth. Next, you will need to drill your own holes for pins. I would recommend using v-blocks to hold the jackshaft while you drill the holes. It is very important that these holes line up perfectly. The size of pin you use is up to you, and depends on the size of shaft, etc.

Step 6: Modify Your Sprocket

If you ordered the same sprocket as I did, then the sprocket will be way too wide for your bike chain. This will require you to machine the sprocket. Chuck it up in your lathe with a facing tool and face away until the sprocket is .10 inches wide. Then set your compound rest to 10 degrees and machine the angle on the tooth so that it matches the other side.

Step 7: Main Drive Pulley

Since the odds of you obtaining a motor exactly like mine are very slim, so this will be just a guide as to how I machined mine. Since my motor already had a center pinhole, I bored the inside of a piece of 1 inch D aluminum round stock to the exact size needed to fit over the shaft. This is VERY VERY important that this hole is ABSOULUTLEY NOT oversized. If it is you will have to re-do the part. Then I drilled the pinhole, and machined one end down to .5 inch D to fit the pulley that I had to Machine, but you could probably just buy one. (I realized afterward that the size of the pulley that it said was the outside diameter, not the inside, which is what I thought it was)

Step 8: Begin Assembly: Jackshaft

This is the most exciting part! The bike finally gets to start coming together. Go to the hardware store and buy your roll pins and set screws, and, well, start assembling! This will take a little troubleshooting on your part, but if you machined everything correctly then it should all fit together.

Step 9: Assemble Chain Drive

This is where you will need your chain breaker tool. You should have already broken the chain to take it off of the bike. Now you will set the chain up like normal, going correctly through the rear derailleur and meshing on to the middle sprocket on your rear cassette. Make sure that the rear derailleur is in the right position to ride, and not all bunched up or on the wrong gear. Next, lay your two chain ends side by side so that you can get close to the correct length of the chain. This is the hardest part. Next, you will break the chain in that link, making sure that the links you break will mesh. Note: when breaking the chain, be sure to leave the pin STILL ATACHED TO THE SIDE OF THE CHAIN! If you do not do this, it will be very difficult if not impossible to get the chain back together.

Step 10: First No Load Test

This is where you will need to test your craftsmanship. Nothing would be more disheartening than finishing you brand new electric bike, going for your first test run when... It throws the chain. This test is very important. The bike should be upside down to allow the rear wheel to spin freely, and the bike may be in whatever gear you want, but I would suggest the lowest. Now is the tricky part. With one hand, hold the motor firmly down against the v-belt to provide tension. With the other hand, connect the motor wires to the battery. If everything was machined correctly and accurately, then this test should go fine. If it did not and threw the chain, there could be a number of problems. One that I ran into was my jackshaft sprocket was too wide, so I had to file it down a little bit. If your belt is slipping, either you have a way to high gear ratio or you are not putting enough tension on the v-belt. If it keeps throwing the chain, your sprockets are probably not lined up perfect and will have to re-machine some parts.

Step 11: Mock Up Your Motor Mount

Next, you will need to make a cardboard mock-up of your motor mount, for a few reasons: cardboard is cheaper than metal, you can cut it with a knife, and you can shape it much easier than metal. If you bike will allow it, I would suggest mounting the motor behind the seat post like me, so that it will give you more room for batteries and keep the motor (and most of the spinning parts) out of the way of your legs.

Step 12: Motor Mount- Rough Cut

Next, you will need to use your cardboard mock-ups to lay out the design on to a piece of sheet metal. Lay the cardboard on the sheet metal, and trace it as accurately as you can with a piece of sharpened soapstone. Then you will need to cut out the mount. This is where it is VERY VERY nice to have a piranha. If you do not know what that is, it is just a giant pair of hydraulic scissors that cut metal. It makes very clean, precise cuts and is excellent for cutting the outline of your mount. However, if you don't have one of these, then a plasma cutter is the next best thing, however when cutting through sheet metal this thick, there is going to be quite a bit of slag, and if your not very good at it there will be quite a bit of grinding to do at the end to clean it up. Of course, you could also use an oxy-acetylene torch or a hacksaw, but both of those are not very good options.

Step 13: Motor Mount- Bolt Races Step A

This is the most crucial part of your motor mount. These are the races for the bolts of the L-bracket (if you have one) and U bolts so that they can slide up and down on the main plate. Since you already have you cardboard mock-ups, the layout of these are very easy. Just lay the mockup out on the plate and center punch the two ends of each race. Then you will need to drill out holes of each end, so you will have four holes. Make sure that the holes are just the right size, not so big that the nut will not seat correctly, but not so small that the bolt cannot fit through. Since I used 3/8 inch bolts, I dug through the drill bit box until I found one that was as close to .4 that I could find.

Step 14: Motor Mount- Bolt Races Step B

Now you will need to cut the races. For this step, I considered using the mill, but decided against it for a lot of reasons. However, I would strongly suggest milling out the races if you have the proper sized end mill and the correct sized vise. However, I chose to cut out the races with a plasma cutter. Using a piece of angle iron as a guide to make a straight cut, plasma cut out the races for your bolts. Mine did not look to pretty after this, and yours probably will not either, so there will be A LOT of grinding in order. It is very important that these races are as clean as possible so that the bolts will be able to slide easily and lock down tight.

Step 15: Motor Mount- L Bracket

Note: This step is optional, and depends on your motor. I made an L bracket for my motor mount, but did not have enough clearance from the rear tire to use it. If possible, I recomend an L bracket to give your motor more support, but if it is not possible, then simply using U bolts will do.

Next you will need sort of an adapter bracket that will attach to your motor and slide up and down the main mounting plate to provide belt tension. Make a plate that will bolt on to the front of your motor and hang off to the side a little. Then take a small rectangle that will run parallel with your motor and bolt on to the main mounting plate.

Step 16: Weld on the Motor Mount

Now, after extensive sandblasting and a little time with a wire brush we are ready to weld! Make sure that your joints are clean and free of rust, paint, dirt, etc. Now since you are welding two different thicknesses of metal, this long weld is going to be particularly challenging. However, if you burn through it is not the end of the world. When you are running this bead, try not to weld the whole thing at once. Weld on one side, and then move to the other to allow the metal to cool. Also, try to direct most of your heat onto the mounting plate, and use the lowest amount of heat possible and still get good penetration. If necessary, you could even run a very small bead accompanied by a much larger, hotter one to attach the 2 pieces of metal. Finally, I would use a MIG welder, as stick would burn through way to easily, and I am not very good at TIG welding.

Step 17: Assemble Belt Drive

This step is self-explanatory. Slip the v belt over both pulleys, pull it as tight as you possibly can, and then tighten down all of your bolts. One thing that you may notice is that your belt will stretch with use. This is the main reason why we did an adjustable mount. You will need to regularly check your belt tension during use and adjust as necessary.

Step 18: Second No-load Test

Now is the time to do a second no-load test, which will test both your motor mount and gears. Put the bike in lowest gear, and start running the motor at full rpm. If the mount holds (and it should) then gradually start shifting up to the highest gear. If you added a speedometer, note the speeds that it is reading (unless you mounted it on the front wheel) and note any belt slippage. This could indicate either a loose belt or a gear ratio WAY to high.

Step 19: Battery Mount

Next is the battery mount. I hope that you got a good set of batteries from the medical supply store, but if you did not then you will just have to buy some. Make sure to get a matched pair and a good charger for them. Next, make some cardboard mock-ups of the batteries. It is much easier to move around a hollow cardboard box than it is two 30-pound batteries. Once you have made your mock-ups, find a good place to mount them. You will want them mounted as far back and as low as possible, to give your rear wheel more traction and lower your center of gravity. After you have found a good place to mount them, construct a "tray" of sorts out of angle iron, so that the batteries can be set in and held securely with zip ties or bungee cords. Then just simply weld the trays to the frame of the bike. These welds need to be strong, as they will be supporting a lot of weight, so the more support you can give them the better.

Step 20: Wire Up the Motor

By now, some of you are probably wondering, "Where is the motor control?" Well, there is not one, at least not a PWM controller anyway. Since you picked out a bike with gears, all you need for your motor control is a simple switch. I got a 10-amp single pole triple throw switch from radio shack. It has three different positions: on1 off and on2. This will allow me to have on1 be 12v as in the schematic, and on2 be 24, also shown in the schematic. The two different speeds will allow me to run the motor at either full or half RPM. These two different motor speeds and the different gears should give you a wide range of cruising speeds, thus eliminating the need for a very expensive PWM controller.
Note: there are two options for wiring the motor: The 3 battery and the two battery. Each has there own advantages and disadvantages. Picture 1 is wiring option 1: the three battery. Picture 2 is wiring option 2: 2 battery. The 2-battery option is the one I am using and the one I would recommend.

Step 21: Maiden Voyage and Troubleshooting

This is by far the best step of all! Now that you have finally finished your brand new electric bike, it is time to show it off. Call over all of your friends, throw a party, and take the maiden voyage of your new green vehicle! Do not forget safety gear, especially on the first test! If something goes wrong (and it probably will), you do not want to get your brains spilled out all over the asphalt. In addition, you should be prepared for a failure. It is highly probable that your bike may not work. Lots of things can happen, from a minor wire disconnection to a major gear ratio miscalculation. It is best to do this test where you will have access to tools to fix any problems when they arise. Now when you run into a problem, there are a few simple ways to detect it, provided in the table below:

bike is not moving:
Disconnected wire
Gear ratio to high
Dead batteries
to diagnose this problem, pick up the rear wheel and turn it on again. If the tire spins, then your gear ratio is way too high. Try making the jackshaft pulley larger, or the motor pulley smaller. Both of these will decrease your gear ratio and give you more torque so you can move. If the tire does not spin, this could mean either disconnected wires or dead batteries. Charge your batteries and check them with the voltmeter. They should be outputting around 26 to 27 volts at a full charge. Also, check your continuity with a voltmeter. Disconnect the wire leads going to the motor and reconnect them to the leads of your multimeter. Turn the switch on, and if you get a reading of zero then you have a complete circuit and the problem is with your batteries.

Bike goes to slow:
Wrong gear ratio
to diagnose this problem, pick up your rear wheel again. If it spins much faster than what you were going, then your gear ratio is too high, and will need to be fixed by either increasing the size of the jackshaft pulley or decreasing the size of your motor pulley. If the tire spins just as fast as you were going without load, then you can increase your gear ratio by either decreasing the size of your jackshaft pulley or increasing the size of your motor pulley.

Step 22: Extras

This step is for those of you that are wanting a little more, but are also willing to go over the $100 dollar budget. You may have noticed that, in order to keep this project in budget, I have eliminated a speed controller. This is not necessary for my project, because all of the gears will give you a wide range of speeds. However, a speed controller would defiantly be beneficial. I have heard that alltrax makes a good one.

You could also add lights and turn signals to the bike, but that is a whole instructable in itself.
a little more complex
Just amazing!
turn signals

You could also give the bike a custom paint job

You could also use an epilog laser cutter to cut out decals and stickers for your bike.
An Epilog laser cutter would be totally awesome. If I won one of them, There are numerous things I could do, from cutting out accurate templates to custom engraving all of my projects. If I won an Epilog zing 16 laser cutter, The instructables community can expect many more amazing instructables from me!

Step 23: Math

Yes, there is a lot of math involved in the construction of an electric bike, so here is what I used and a few formulas that should help you out.
First off, gear ratio calculator. This formula will give you top speed under no load, so be sure to compensate a little bit for load.
((R((pi*A)/ (pi*B)))(C/D)(pi*E))*.000946969697 where R is rpm of motor, A is diameter of motor pulley, B is diameter of Jackshaft pulley, C is teeth on jackshaft sprocket, and D is teeth on the rear sprocket (if your bike has gears, use the smallest for top speed, largest for slowest speed.) and E is the diameter of your rear wheel.

Next is figuring out the 5/8 inch length of jackshaft. Assuming the outer face of your bearing cups are the largest dimension of your bottom bracket, insert them and take a measurement with your caliper. Mine measured 2.817 in. Then remove the bearing cups, and insert the bearings with the cups farthest edge laying flat on a table or other hard surface. Then use the depth gauge on the caliper to measure from the inner edge of the bearing down to the table. Do this with both cups. I got the measurements of .591 and .595.
Then add those 2 together, and subtract them from your largest dimension to get your 5/8 inch length of jackshaft. I got 1.631 for mine.

Figuring out the size of your bearings is very easy, and does not really require math at all. Just take an ID measurement of your cups and purchase a bearing as close to that outer size as possible, and as wide as possible, and preferable with a .5 in center hole. To be more Accurate if you are machining your cups anyway, you might as well purchase the bearings and then machine the cups to fit them.

3rd Epilog Challenge

Finalist in the
3rd Epilog Challenge