Super Light Electric Fixie Bike

About: After a degree in micro-engineering in Switzerland, I moved to Russia to discover new landscapes

I have been testing different options to make an electric bicycle this last year and I came up with this pretty satisfying solution.

I was thinking of doing my own electric bike since a long time but when I saw the Tom Stanton video in April 2017 it was the trigger to start ordering some stuff on the internet.

I bought a fixed gear bike initially because it has more possibilities to add a pulley or a sprocket on each wheel side (flip-flop) but finally I am still riding this bike with the fixed side because I got used to the sensation. The motor is also directly connected due to the friction transmission.

Because it is a minimalist bike I find now that having a minimalist electric assistance makes sense. Indeed the motor weighs 400 grams and the battery 800 grams which makes the whole bike under 15 kg. It is also a good combination because I never go super fast with the single speed so I don't need a high power continuously. But the motor helps a lot for standing starts, slops and wind conditions. The motors allows me to have a slightly smaller sprocket.

Step 1: Tools and Material

Tools

To mount this system you just need a screw driver for the battery box and a wrench for the motor mounting. The box itself is just glued and nested. I developed a small circuit to interface the throttle with the electronic speed controller (ESC) so I used a soldering iron and a mini milling machine but the first ESC I bought arrived with connectors so you may just use butt connectors with a crimping tool. I made my own pivot to open the battery box so I made some holes with a drill but you can just buy a hinge instead. On the first version I made, the box was simply cut with a hacksaw but on the last version I used a laser cutter. I also developed a my own 3d printed throttle, for that I used a 3d printer but this is totally optional.

Material

main components

  • BLDC Motor 325Kv 1800W (https://www.horizonhobby.com/rewardclub/motors/power-90-brushless-outrunner-motor-325kv-eflm4090a)
  • LiPo battery 22,2V (6 cells) 5300mAh (https://products.teamorion.com/full-view/5/product/Softcase-LiPo-Batteries)
  • Electric Speed Controller 65A (http://www.dualsky.com/Xcontroller_ESC/Xcontroller_Lite.shtml) I wanted the VESC open source controller but couldn't find one quickly so it will be for the next bike version.

mounting and battery box

  • plywood 6mm (210*400 mm)
  • plywood 3mm (200*80)
  • 4 wood screws
  • 4 M3*16mm and nuts
  • wood glue
  • bolt and nut M6*20

throttle and controller (if necessary)

  • PLA for printing
  • copper plate for prototyping (20*20 mm)
  • telephone wire (with 3 or 4 conductor)
  • attiny45
  • smd capacitor

Step 2: Make the Battery Box and Motor Mounting

If you have access to a laser cutter, you can just download the files and cut it (just check that your tube diameter are like mine). If not just make a wood box a bit larger than your battery and a mounting for the motor. On this bike I had no rear break but if you have one, it should be possible to secure the motor on the same screw. The motor diameter is only 56mm so you can put it on the other side (between the frame and the wheel). I added a transparent window on the top because this battery has a charge indicator directly on it.

Step 3: Connect and Test Your Electronic

In case you are motived to make a controller for the ESC here are the files. This controller takes the throttle value (it is a Hall sensor value in my case but it can be a potentiometer) and send a servo command to the ESC: a pulse between 1 and 2 ms every 20ms. The advantage of this controller is that you can choose the exact throttle response. For example, the fisrt throttle I bought was super non linear so I linearized the signal in order to have a very accurate response. Also you can average the signal on few values in order to smooth the throttle answer. You can load my code with arduino.

Otherwise just connect and test.

Step 4: Make a Throttle (optional)

It consist in two parts: the trigger and the collet which simply handle a Hall sensor and a magnet. The magnet is in the trigger and the hall sensor just in front in the collet. A spring pushes the trigger away.

Step 5: Just for Fun: the First Version

It had a 250w BLDC with reductor 4:1. A belt and crank also acted as a 5:1 reductor. I had a lipo battery 8Ah.

It was not bad but the motor holder was too fragile because I couldn't mill an aluminium version and I had some problem with the battery: I was regularly cutting after 10 minutes. Maybe it was just because of the cold weather but comparing with the battery that I have now who can deliver up to 50A, even if it is only 5Ah, it last much longer. I will recycle it into an electric skateboard and see how it behave.

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    4 Discussions

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    AdrienRmoross

    Reply 2 months ago

    нет не жалко. я уже один месяц катался и все нормально

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    tnoten

    Question 2 months ago on Step 5

    Hi. Great idea.

    When the battery is empty, does the motor get in free mode, so that it needs no(or very few) extra energy to ride the bike.

    How much extra kmh gives this setup. And how long?

    1 more answer
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    AdrienRtnoten

    Answer 2 months ago

    No the motor doesn't get in free mode but I don't really feel it. It doesn't break so much. I haven't measured how much km/h I can reach without peddling, I would say around 30km/h. I can ride one hour but it depend a lot of your riding style. If I use it all the time, I don't think that it would last one hour but it is enough for commuting.