Electric BMX Bicycle

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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 shif...

Intro: Electric BMX Bicycle

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.

Websites.
tncscooters.com
Craigslist.com
Electricmotorsports.com
Evdrives.com
Batteryshark.com

Step 11: Resources

Bike- $40 Craigslist.com
Statru Rim- $50 amazon.com
Steel- $32 ace hardware
Motor- $60 tncscooters.com
Batteries- $40-60 batteryshark.com
Thumb throttle- $50 amazon.com
Alltrax speed control- $300 evdrives.com

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.

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    54 Discussions

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    NickeyV

    9 months ago

    How is the wiring done? I'm building by scratch and I need help on wiring since I'm working with those 6v battery packs from family Dollar... I'm combining 2 of them to make a stronger one. Jus to wire or connect that to an electric weed eater???

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    Ethan1976

    1 year ago

    how to make a motorcycle like throttle OR, a hybird (idk), can be used like normal and electrically, i mean. when the battery runs out, you can just ride it like a normal bike

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    michaelmonaghan

    3 years ago

    Yeah you can always email me at michaelmonaghan01@gmail.com

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    Velvelkjell

    3 years ago

    Wooohoo man:) that's one NICE instructable:) thank you so so much for this you've just saved my year and probably a lot more;) I've been thinking about how to make my downhill bike into a ebike without breaking the bank :) is it possible to brainstorm and share some ideas on mail ? Best regards Kjetil

    how to make a motorcycle like throttle OR, a hybird (idk), can be used like normal and electrically, i mean. when the battery runs out, you can just ride it like a normal bike

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    michaelmonaghan

    3 years ago

    If you would like you can email at michaelmonaghan01@gmail.com and I'll send you everything in a word document.

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    birdsloper

    3 years ago on Introduction

    i was looking to reference it and start building my own. Needed for transportation and it is so small as well as battery powered, I can keep it inside.

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    birdsloper

    3 years ago

    What happened to the pitbike build? I know i saw it here? Maybe?

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    michaelmonaghan

    4 years ago

    https://www.instructables.com/id/Electric-Motorcycle

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    michaelmonaghan

    4 years ago

    It really depends on what you want from them and your price range. The sla batteries are big and heavy, but they're also bulletproof compared to lithium which sometimes catch fire randomly (had it happen to me once) also keep in mind that a 20ah sla has less capacity than a 20ah lithium at a high discharge rate because they loose efficiently with anything over around 1amp. To make a bike feel lighter with sla batteries you should also place them as close to the front wheel as possible (that way it won't feel heavy when turning). I also made a 15hp pit bike with sla batteries and it works great as still has plenty of range.

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    Qrys13

    4 years ago

    I have an electric bike hub kit and I'm torn between lithium and agm sla for the batteries?

    Wondering about your experience with sla? I'd rather do that than spen 500$+ on a lithium.

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    michaelmonaghan

    4 years ago

    I am currently working on an electric motorcycle that will be posted soon. Unlike the bike, it will have a purpose built machined out chromoly cradle frame for all the electronics. I'm still working out all the parts on AutoCad, but will start the fabrication here soon, so keep a look out for an electric dirt bike!

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    michaelmonaghan

    4 years ago

    How do you post videos on here? I can do it this weekend. And yes, there are better electronic designs, but I got all the electronics for free from a friend.

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    Hallofo

    4 years ago on Introduction

    Awesome build! Have you had a chance to post a YouTube video of it in action?

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    astral_mage

    5 years ago on Step 10

    keep up the great work. also if u could post wat it cost u and or linked to where u get all yr parts as well?

    2 replies
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    padbravoastral_mage

    Reply 4 years ago on Step 10

    BTW: You write something about the motor that is AC? so, U use an inverter? or is a typo?

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    jwall18158

    4 years ago on Step 11

    Just curious about something. Couldn't you use the existing sprockets and take advantage of the gears and increase your speed?