Introduction: Electric BMX SE Bikes "So Cal Flyer" 25mph EBike

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E-bike conversion of a 24" SE "So Cal Flyer".

  • 25mph
  • 500w front hub motor
  • 400rpm
  • 36v
  • 10amp-hour

Supplies

  • Socket set
  • Hex (Allen key) socket set
  • Plyers
  • Zip Ties
  • Drill and drill bits
  • Phillips Screw driver

Step 1: Finding a Bike

I found this bike in poor, used condition at a garage sale. It was rusty and had spokes pulled through the rim. These bikes can also be found used on Craigslist and Facebook Marketplace. They are sold new on sebikes.com. The extra-wide chromed handlebars were replaced with standard size black BMX bars.


These bikes are made to be light, strong, and single speed.

Step 2: Buying a Kit

This 36v kit was sourced from LFaster Store on Aliexpress. It contains a 500w Brainpower controller, S866 LCD screen, twist grip throttle, brake levers, peddle-assist sensor, and front hub motor (for 100mm+ forks).

e-Bike Kit: https://www.aliexpress.us/item/2251832605582648.html

If you want to go faster than 25mph get a 48v 750w kit. Max speed is calculated by max RPM * wheel circumference.


Vehicle speed (miles per hour) calculation:

= Wheels RPM × Tire diameter × π × 60 / 63360

= 400rpm * 24 inches * 3.14 * 60 / 63360 = 28.5 mph

https://www.omnicalculator.com/everyday-life/rpm#how-to-calculate-rpm

Step 3: Swapping Out Rims

The SE Bike rims were damaged so I bought a used Sun Bicycles Revolutions 24" beach cruiser for parts and put those rims onto the SE Bike. The beach cruiser has a rear foot brake (coaster / bendix). It allows stopping the bike by pedaling in reverse. The spoke holes were too small for 10g spokes so I drilled them just big enough for the larger spokes to fit through.


Donor bike: https://www.bobsbicycles.com/product/sun-bicycles-revolutions-24-192270-1.htm

Step 4: Adding Spokes to Build a Wheel

Spokes and the rim are not included with the e-bike kit. I had to buy them separately and chose 10 gauge (3.2mm) stainless steel spokes. The hub motor was not designed for such thick spokes so the holes were drilled a bit larger so the spokes would fit. Spoke length was calculated from https://ebikes.ca/tools/spoke-calc.html and was found to be 177mm. It could have been about +5mm longer without an issue. The wheel was trued and balanced at a local bike shop. The tires it has are 2.5" Maxxis Hookworm and take a "fat tire tube 2.5 to 3 inch" sold on Amazon. Tire caps are aluminum and rocket shaped.


Spoke length calculator: https://ebikes.ca/tools/spoke-calc.html

Step 5: Mounting the Wheel

Make sure to put the safety washer into place so the motor shaft cannot spin while in the front fork. Or else it could twist off the motor cable after a few spins. Zip tie the motor cable to the frame with enough slack to turn the handlebars. If the motor rubs against the forks, put a washer in between to give a little extra space.

Step 6: Brakes

A U-brake was added to the front wheel using the front fork reflector hole. It's a standard BMX hole for front brakes. Be careful that the brake calipers don't rub on 2.5" fat tires (low clearance).

The rear wheel is a stock v-brake that most SE Bikes have.

I changed the rear rim to use a coaster brake so that pedaling backwards will slow down the bike.

Test your brakes and make them strong enough to stop the bike when it's going full speed.

Step 7: Building a Battery Pack

The battery is made from four 18v 5amp-hour Lithium Polymer quad-copter batteries. They are lighter and provide more powerful than 18650 packs. Two 18v batteries are joined in series to make 36v. Then two 36v packs are joined in parallel. The batteries are encased inside a 3d printed rubbery material. The pack stays inside a fire-resistant bag all the time. I bring the pack inside to charge and leave the bike outside. You could put two 4-battery packs in parallel for double the capacity. The connectors will likely spark when the battery is connected to the controller. You can prevent wearing out your terminals by using a heavy duty, high current switch; or by limiting current (amps) temporarily using a resistor in parallel with a power switch.


Rubbery battery pack enclosure: https://www.thingiverse.com/thing:5511860

Batteries used: https://hobbyking.com/en_us/turnigy-5000mah-5s-30c-lipo-pack-xt-90.html

Packs joined with XT90 "serial" and "parallel" cables, and terminated with female Dean's T adapter connector.

Step 8: Front Basket for Electronics

Using zip-ties strap a metal basket to the front handlebars. This fits the controller, extra wiring, and battery. Other stuff can be placed on top and strapped down with luggage straps. Reusable zip ties can be used for items that may need to be taken off. For carrying more weight, I recommend strong nylon non-reusable zip ties.

Step 9: Programming Controller

The following values were used to program the controller. If you use the Reset connector, be ready for the wheel to start spinning as it does calibration. Include the tire when measuring wheel size.

Step 10: Lights and Reflectors

I added a headlight / taillight kit that hooks into the e-bike controller and runs on 36v to 60v. Found on aliexpress. Also has turn signals and a loud horn. I also have a simple bell that makes a standard ring-ring noise. A quiet, standard bell is less scary to pedestrians.

Step 11: Secondary Mounting Bar

This 11 inch tube was mounted so I can put more stuff on the handlebars.

Step 12: Triangle Bag

Black triangle bag attaches to the frame using hook/loop tape and is opened using a zipper. Contains tools to fix the bike in the field. Good for holding an adjustable wrench, Leatherman tool, hex key set, flashlight.

Step 13: Seat and Seat Post

Seat post was changed to a Mongoose suspension seat post. Standard shims didn't fit inside the SE seat post tube in the frame so I made one from a piece of 0.35mm tin. Seat is Bell Ergo memory foam. Added quick-release clamp seat post.

Step 14: Chain Replacement

Chain was replaced with a blue painted version. It was cut to size and installed using a chain breaker tool. Rear axle chain tensioners were installed to keep the chain from moving and getting loose.

Step 15: Pedals

Plastic Nylon pedals were replaced with metal Aluminum. These are 5/8 inch not 1/2 inch.

Step 16: Kickstand

The SE So Cal Flyer bike doesn't come with a kickstand. I added a cheap frame kickstand but it broke after only a couple weeks. Will be trying U-shaped double kickstand mounted to front wheel.

Step 17: Charging

The charger has been adjusted using the potentiometer (screw knob) inside the case to be 41.1v maximum voltage instead of 42v. The batteries will puff up and get really hot if you go above 42v (4.2v per cell). So it's safer to charge to a max of 41v. They charge at 5 amp without a BMS and it takes about 3 hours to charge. Connector is a Deans / T type. A BMS might allow the battery to have more charge/discharge cycles.

Stay in the same room when charging your battery, or charge it outside away from anything else using an extension cord. If there's battery smoke, unplug it and put it outside ASAP.

Step 18: Conclusion

The controller can be programmed. It can limit the bike to 20mph (33 kilometers per hour). I recommend running in PAS 9 mode with maximum start power set to 80% or less. At 100% power the bike will spin the front wheel before gaining traction. Rapid acceleration uses up the battery faster. More braking uses up brake pads faster.

Check your state or country bike and e-bike laws.

Step 19: Troubleshooting

If the motor turns a little bit and makes a whining noise, it's probably not detecting the motor position sensors. My motor cable had a bad connector that had to be bent to the proper position.

For questions, I recommend IRC www.libera.chat rooms ##electronics or ##cycling.

My contact info can be found on www.jbcse.com

Don't mount your cell phone to the handlebars because it will rattle your phone and could break it.