DIY Electric BMX

I am going to present my electric bike project here. Or more precisely my electric BMX.

Why BMX? Because it is more practical and more fun. Easier to put in a car and take with you on holidays, easier to store in a garage or park in your apartment. And riding a BMX is just more enjoyable than riding a normal big boring bike. And yes, it is even more enjoyable if it is electric and fast :)

I am riding this bike for years now. If I am not wrong I built it 4 years ago. Since then I made more than 4.000km or more than 2.500miles with it and it is still not 100% finished, because I am getting ideas for improvements every winter. So, this presentation will take you from the first prototype and how it was made, to the latest improvements and plans for the future.

First take a look at the video of my first prototype that I built 4 years ago and was able to make 22km or 13.5miles with one battery charge. At max speed of 35km/h or 22mph.

After many improvements this is how mi bike looks today. Having a new battery pack, it is now able to make 80km or 50miles with a single charge at the max speed of 38km/h or 23mph!

This video is much better than the first one, but also the difference in the bike's performance is a totally different story.

So let's follow the steps to see how it is made.

Nothing more to add here why it is a BMX. Having a tight budget still being a student when I started making this I wanted to save money for the electronics, so I bought online the cheapest used BMX that I could get with both front and rear brakes. So, this is just a cheap bike, you can of cohouse choose any bike you like to make it electric. As a lot of BMX bikes are without brakes or with just one brake, this was the most important detail when I was choosing it. And it happened to be the right choice since the bike still runs today and the brakes still work with only basic brake pads maintenance.

I choose to use a hub motor, so a motor integrated in the rear wheel. I did not like the front motor as having the front wheel heavy it is not good for riding. And I did not like other motors, because they were difficult to fit on the bike.

So I bought a set (motor + inverter + brake handles with end switches and a light) in ebay.

I choose an 800W 36V version. Of course, with a 20'' rear rim to fit on my bike.

Why 800W? Because I had a chance to try another 500W electric bike and it was not powerful enough for me, so I choose to make it with more power. 1000W or more was just too expensive at that time not just because or the motor price, but also because this would lead to higher currents and bigger batteries. So 800W was the best choice of price/performance for me at that time.

Mounting
the motor may seem like an easy job. Just unscrew the old rim and screw back the new one with the motor, but there are some tricks.

The first trick is, that the hub motor's mounting shaft is way wider than the normal one. In my case +7cm or +2.7inch. That's a lot. But I managed to stretch the bike's frame as shown on the photo and fit the motor inside. Luckily it all went well, and I did not need to cut the frame and weld it again. The next trick are the gears and the chain. As the BMX had no gear change system I just picked one of the gears to use and adjust the chain's length to fit on it. I left the others gears on. I did not lose time destroying the whole system to get rid of the ones that I did not plan to use.

So be careful on this detail when choosing the motor.

Next I mounted the inverter in between the frame with cable ties and some plastic clamps. Connecting the motor and inverter is easy if you buy them together as they have connectors and you can't mix them as they are different.

But I did not like those "cheap looking" connectors, so I cut all of them away and soldered some better ones instead.

Another thing when choosing the motor. I did have some problems with broken spikes during the years. Seems like they were not strong enough to hold a heavy electric bike + me on it with 800W high torque motor. 3 of them failed during 4 years, but I made spare ones form spare motorcycle spikes and changed them. So i suggest you to choose a solid rim with motor that has no spikes. It is possible to get it for 20'' bikes. Much harder to get for 24'' or bigger bikes.

Main power source... the battery must be strong enough to provide high currents and have enough capacity for a long ride. I choose Wina 10Ah LiFe cells and connected 12 of them in series (43V fully charged). I used copper plates for connections and made "main" and "balance" cables with connectors.

I also made a wooden enclosure with some metal plates with screws to mount it on the bike under the seat.

This battery was good when it was new. But the discharge current was on the limit by the specifications, so slowly in 2 years this battery pack was gaining the internal resistance of the cells and losing capacity and power.

Simply to say, I was not happy at all with the quality of this cells. They did are not even half that good as it is stated in the specifications.

I made a new battery pack with LiPo cells. They have higher voltage, so I used 10 of them in series (42V fully charged) and not 12 like LiFe. I also increased the capacity from 10 to 30Ah. This slightly increased the bike's top speed because of lower internal resistance and voltage drop. But it made it go from 22km to 80km with a single charge.

There is no BMS. I only balance the cells when I charge them. There is only a cutoff function when the cells are empty in the inverter. So I made a voltage display that displays the status of all the cells to monitor the battery. I will add photos of it.

I will add some more details about batteries soon.

To finish the project, I needed to connect the brake handles with safety switch to the controller. This handle "stops" the motor when you are braking. Next, I needed to connect the "thumb" throttle stick to enable power. This was also in the set with the motor and inverter, but I changed it for a "twist" stick as motorcycles have.

There was also a front light with key switch also in the motor set. You can see this light in the first video. It was poorly made, and it did not give enough light to ride in the dark. So I just took the key switch out of it and threw the light away. It also had a horn. I took this out too and mountedit near the inverter. I than installed a DC DC converter "turnigy 42V ubec" to make 5V from 42V of main battery. So, I could us a bright led for front light and connect it to this 5V line. I also added d back light made form led strips connected in series and a usb fast charger :) You can see all of this presented in the videos.

I will add more details and close up photos to this project soon! Feel free to comment and ask any detail you like to know.