Introduction: Electric BMX Bicycle

About: I'm a recent college graduate in the fiend of mechanical engineering that is is still in search of employment. In my free time I enjoy building and designing new vehicles (electric usually). I also race shifte…

Step 1: Frame

The first thing to decide when making an e-bike is which frame to use. When choosing a frame, keep in mind that the size, material, and rim options. If you are going to weld mounts to it, look for frame that are steel and not aluminum. Also keep in mind that usually only 20in BMX rims and road bike rims have threads on both sides to mount an additional sprocket to. I chose a BMX frame because it it's steel, has rims I can mount to, is light weight, and compact so it can fit into a car.

Step 2: Electronics

The next thing to consider is what electronics to use. Keep in mind price, size, speed and distance you want to travel on one charge. I was given my electronics for free so I did not have any options, but my bike has:
Motor- 500W 30A
Battery- 26Ah sealed lead acid
Speed Control- Alltrax 300A 24-48V
Throttle- Currie 0-5K ohm potentiometer thumb

Step 3: Mounting

There is a few ways to mount electronics, you can bolt things on or weld usually. I used 1/8in thick L-bar from Ace Hardware. I measure out the batteries and welded 2 mounts that the batteries fit snugly into. I then made a similar mount for the motor with slotted holes drilled for the motor to change chain tension. Once the mounted were made, I measured and sanded the frame where they will be fitted. The final step I did was masking the frame off so I could paint over the unfinished metal components.

Step 4: Drive Train

This can de difficult, but with a little research, you can usually find a hub that has threads on both sides allowing for a sprocket on the left and right. Because the thread is both right hand, you need to make a key on the left side of the rim so it doesn't unscrew. I filed a small notch in the freewheel, threaded it on, and and drilled a c shape in the hub where notch sat. I then cut the solid end of the drill bit off and used it as the key as you can see it sticking out on the picture. The sprocket, chain and freewheel were all from tncscooters and cost about 30$.

Step 5: Chain Tensioner

On this bike I had to make a chain tensioner to clear the frame. To make it, I cut the end off of an old scooter motor I found in the garage (sprocket side) and removed the 2 bearings out of the motor to side on the shaft I cut of. I peened off the cut end so the bearings wouldn't fall off, and finally secured it to the frame by using 2, 1inch hose clamps. (Make a chain out of the 2 and tighten one around the 2 bearings and the other around the frame

Step 6: Mounting Batteries

Once the frame is welded up, you can secure the batteries a few was, I used stainless steel hose clamps (2foot long) and wrapped 3 around each battery and the mounts I made. Large heavy duty zip ties is a slightly cheaper alternative, but not quite as strong in my opinion.

Step 7: Speed Control Mount

Because I will soon take of my overkill of a speedo, I decided to bolt on a mount instead of weld a mount on. I cut some 1x1/8in aluminum bar to the same width as the speedo and countersunk a hole in the middle for the mount screw. Then I drilled 2 holes in the top of the frame between the batteries to mount the bracket. Once it was on I bolted up the speedo.

Step 8: Wiring

When wiring take into consideration the amp draw your project will take. For mine I used 10awg wire all around and soldered at all points. I also shrunk wrap any exposed points to eliminate and arcing that may be caused. I also wired each battery to deans plugs. That way you can unplug it when you're not using it, and it makes charging easy. I'm using a harbor freight car charger, so I wired it to plug right in and charge the 2 batteries in parallel.

Step 9: Test and Tune

Once the project was done, I fine tuned the motor by cleaning the com and playing with timing to optimize power. I also aligned the chain better and played with the brakes to cope with the added weight. Finally I added a light and speed gauge to help with testing.

Step 10: Results

The bike now does 29-30mph on flat ground and has a range of 13-20 miles depending on how hard you ride it.

I've never posted on here, but if you have any questions feel free to ask if it's possible.


Step 11: Resources

Bike- $40
Statru Rim- $50
Steel- $32 ace hardware
Motor- $60
Batteries- $40-60
Thumb throttle- $50
Alltrax speed control- $300

I got all the electronics from a friend. A 100A speedo is more around $40 and would still be plenty of power for such a small motor.