Reza, from Sacramento California, discusses how he built his electric BMX with our electric bike kit to meet his stringent requirements and needs for commuting throughout the city.
Key Quotes from the text of Reza’s write-up:
“To give you an idea of the power, the motor can easily pull me up a steep driveway from a dead stop. I have ridden 30 miles with seemingly no effect on the battery. I honestly couldn’t tell you how far the battery would take me because I’ve never had to push it that far.”
“I’m using a 36 volt 10 AH LifePO4 Battery with the kit; I cannot imagine what it would be like with 48 volts.” – Reza
– Frame: 2000 Diamondback Viper XL with 3/8” drop-outs – Bars/Forks/Stem/Headset: All original stock Diamondback – Brakes: Stock Tektro set-up (front brake levers from Electric Bike Solutions) – Wheels: Redline with 3/8” axles – Tires: Odyssey 20 x 2.25 in the front and back – Cranks: Eastern Nomad – Pedals: Eastern CFRP – Gear Ratio: 2.93 (44 tooth front / 15 tooth rear) – Chain: KMC heavy duty with master link – Bontrager Rear Rack holds 25lbs. – 36 volt LifePO4 10 AH Prismatic Cells BatteryThe idea of an electric motor on a BMX bike goes against the traditional thinking of most BMX users. It’s too this or that and cannot be raced, etc…. So why do it? Well it’s light, compact, sturdy, and most of all, pure FUN! I built this bike to be a vehicle that would eliminate my car commute. As such, bicycle weight was important to maximize battery power, but I needed it to be strong enough to handle the additional weight and torque. While I’m not hitting any ramps, I am still hopping off curbs and speed bumps as I cut through parking lots and dirt fields to get to work. I went through a lot of trial and error to build this bike, so the following write-up is for anyone looking to build a BMX or a fixed gear bike. “Fixies” are all the rage these days, but they often use BMX parts so I imagine there’s a lot of overlap.
The GoCarLite.com (Electric Bike Solutions) kit install was pretty straightforward. The control unit and wiring fit pretty easily in an inexpensive under-the-seat bag. I went with a Bontrager rear rack that was $45. A little much, but it was so light and strong that I couldn’t resist (at the time). It needed modifications to be properly installed and give enough clearance for the battery to slide on and off. It was far more work than it was worth. If I had to do it over again, I’d get the lightweight multipurpose rack that Electric Bike Solutions and the Vacaville Velo Wrench Bicycle Shop now have for around $24.
The real work was playing with the gear ratio. The standard BMX gear ratio is 2.75: 44 tooth front and 16 tooth in the back for the older BMX, or the current standard of 25 tooth front and 9 tooth in back. Older BMX frames have 3/8” dropouts, meaning 15 tooth rear freewheels are the smallest you can go. Newer BMX frames have 14mm dropouts, this gives the option of cassette drivers that range from 6-14 tooth. The 14mm axles offer the most gear ratios, but you will need an adaptor ($10-$20) to screw over the axle that comes with the kit to fit the motorized hub on the frame. Again, the kit comes with a motorized wheel that fits frames with 3/8” dropouts. This size has pretty much been abandoned by newer BMX bikes so you will need an adaptor. Some BMX racing bikes still come with 3/8”dropouts, but I would use caution as these bikes are usually built thinner and use slim racing wheels that may not properly handle the weight and torque of the motor. Remember, BMX bikes can easily be built to weigh under 30 lbs. While my goal was a light bike that conserves battery power, I didn’t realize the torque and power of the motor and the need for a very sturdy setup. This is why I strongly recommend a front wheel drive hub with strong steel forks. Avoid aluminum and carbon fiber because a one foot drop at 25mph causes more stress than you realize. I’m using a 36 volt LifePO4 Battery with the kit; I cannot imagine what it would be like with 48 volts.
Now back to gear ratio. 2.93 (front tooth divided by back tooth) is the SMALLEST gear ratio that you can get away with. The standard 2.75 is useless because you will very quickly reach a speed that pedaling will have no impact. In fact, your feet will probably fly off the pedals because there is almost no resistance. 2.93 is almost perfect because it is easy enough to get going without needing the motor, but still offers some help to the motor at top speeds. To be honest, I only pedal to help the motor get going and to maintain speed up hills. Pedaling is pretty much worthless downhill as the bike is just going too fast. To give you an idea of the power, the motor can easily pull me up a steep driveway from a dead stop. I have ridden 30 miles with seemingly no effect on the battery. I honestly couldn’t tell you how far the battery would take me because I’ve never had to push it that far.
Now that we’ve covered gear ratio, frames, and forks, I want to quickly get into cranks and tires. Three-piece cranks are a must. They are much lighter and much smoother thanks to sealed bearings, and offer a plethora of options that one-piece cranks cannot compete with. It should not even be a debate. While I went with Eastern cranks because they are a reputable brand and I found them cheap on CraigsList, I would recommend racing cranks by Redline. Most BMX cranks are designed for stunts so they might feel a little awkward. Racing cranks are light, smooth, and very comfortable. Redline Device cranks are affordable and provide a very noticeable difference.
Tires are incredibly important for obvious reasons. But you also need to bear in mind the stress from extra weight, mileage, and speed. I am amazed at the stresses I put the bike under after just a week of commuting. This is why I stick with thick Odyssey (20 x 2.25) tires for the front and rear. They handle all the stress really well and offer a smooth, street friendly tread pattern that lasts. Tires matter.
Lastly, do not forget a kick stand! I found a used one that bolts right onto the axle. Life is unnecessarily difficult without a kick stand.
Reza – Sacramento CA