Interesting read below from the customer that built this bike using one of our 20″ direct drive kit setups….
If an MTB E-bike conversion is like an F150, and a beach cruiser conversion is like a Cadillac, then a good BMX 20-inch “Urban Bike” conversion is the Mazda Miata of E-Bikes. You might not want to take a thousand mile cross-country trip on one, but the day-to-day driving will be very enjoyable. My experience bears this out! I’d like to persuade you to consider this format for your next e-bike project.In general, the stronger, stiffer frame and wheels are part of the appeal, especially if you choose the “freestyle” type BMX frame over a lighter “race” type frame; precise handling is a great benefit with this setup. Also, when compared to a 26″ or 700c bike wheel, there is a real strong torque/thrust advantage. It is also nice for us older riders to be able to place both feet on the ground at a stop, making mounting and dismounting easier.
One more feature: the smaller (20-inch) wheel not only produces more thrust, but the motor runs at a more efficient rpm, enhancing performance and range.To be fair, there are also certain drawbacks inherent to the BMX frame concept; limited space overall for battery placement being one. The narrow rear dropout width (110mm) makes a rear drive motor problematic but not impossible. Still, with good planning and execution, a really outstanding urban bike is easy to achieve.
FYI – my BMX/Urban E-Bike conversion features: a Trek TR10 steel frame (20″ top bar and 14mm dropouts), a GT steel front fork (3/8″ dropouts), a stock Trek 48-spoke rear wheel, and Maxxis “Miracle” ribbed 20″ x 2.1″ tires (85 to 110 lbs. pressure rating) at about 90 lbs. pressure. I have a two-speed derailleur on my dual front sprockets (40/48 teeth) and a 15-tooth rear sprocket. I use standard V-brakes. (A big, wide, coil spring suspension saddle from cloud nine takes care of the bumps for my old back.)
Let me say here that I got my conversion kit from Doug Daut at Electric Bike Kit Solutions (gocarlite.com.), and my experience was great! Doug quickly and patiently answered every email, ordered my kit promptly, and kept me well informed as to progress in locating and shipping. It arrived in excellent condition, and I was soon up and running. This is my second AmpedBike conversion kit, and I am considering ordering a third. These kits come with everything needed for an easy and trouble-free conversion. The AmpedBikes direct drive electric hub motors have always been ultra-reliable for me.
Back to the bike: I have a front-mount direct-drive motor, which comes laced into a good quality 20 inch aluminium rim. I am using the stock supplied 35-amp (max) 36-60volt controller, thumb throttle, brake levers (with built-in motor power cut-off), and on-off master switch.
I am running with a hand assembled Turnigy 10 amp-hour (5S2P x 6 – 20C) – 55.5 volt nominal Lithium-Polymer battery pack (high of 63 volts, low of 48 volts). Getting into LiPo can be expensive compared to lead-acid batteries, but it is very well worth the extra expense. (please take my word for this: my first e-bike used lead-acid which offered far more weight for far less range and much less service life)My battery pack weighs a tiny bit more than ten pounds when wired and bagged for riding, and is mounted on a TopPeak rear touring rack capable of 50 pounds load. My controller is mounted underneath the rack. The extremely heavy rear dropouts made it simple to drill and tap holes for mounting the rack support legs.
I also operate two LED battery operated headlights, two ebike.ca halogen 10 watt headlights that run off the main battery, and a Planet Bike LED battery operated taillight. Makes it possible to enjoy night rides
This complete package weighs in at slightly less than 69 pounds on the bathroom scale. My measured range, when I ride at normal bicycle speeds and help by pedalling, is more than 30 miles. Absolute top speed is about 28 mph on the flats @ 63 volts right off the charger. That doesn’t last long! More realistically, top speed is a nominal 25 mph. AVERAGE speed possible in my subdivision, with no stop signs, up hill and down, maxes out at a measured 21 mph. This level of power is enjoyable, true, but I more typically ride at about 12 – 15 miles per hour. Fast enough!
This bike tackles the hills with verve, yet has plenty of speed for the flats, and I only pedal when I want to. Rarely is the hill so steep that I feel like I HAVE to pedal. (That was not the case with my old 36-volt 26″ lead-acid MTB.)
There seems to be enough rake and trail with the GT fork that the steering is not at all twitchy. Potholes are handled with aplomb due to the strong small wheels.
Interestingly enough, the “magnetic drag” issue is less of a factor on this bike with 20 inch wheels than it was on my MTB with 26 inch wheels. I don’t know why, but it is not nearly so apparent when coasting down steep hills. I would have thought the opposite would prove true.
The high-pressure Maxxis Miracle ribbed tires help with good steering and braking, excellent handling and a smooth ride. The tires are rated for 85 to 110 pounds pressure, and I roll at about 90 pounds inflated pressure. These tires seem to help with the inherent rolling resistance of the smaller 20” wheels, and my range is excellent as a result.
This is not an off-road bike! The smooth hard tires will not get much traction on wet grass or in mud, and the ride is rough. On pavement, which this bike is intended for, the traction is good and the ride is acceptably smooth. I really enjoy the precise handling.
1) start with a BMX frame that features 3/8” dropouts on the fork and rear. Mine had 14mm dropouts, and I had to find a different front fork with the smaller dropouts before I could proceed with the build.
2) consider fastening a 1” diameter aluminium tube just below your top tube, as a “runway” for the wiring between the handlebars, motor, battery pack and controller. Looks much neater! You can spray-paint the tube to match your bike frame, and most people won’t even notice it.
3) you do not have to build up your own battery pack, as I did, to get into LiPo batteries at a good price. Talk to Doug Daut at Electric Bike Kit Solutions (gocarlite.com.). He can supply what you need in a very user-friendly package – at a very fair price.
4) the hardest part of using a BMX frame to build your e-bike lies in overcoming the mental image of “an overgrown male child riding his bike to destruction while performing dare-devil stunts”………We fear we won’t “look cool” on the smaller bike. Once I got past my personal prejudice, I found the advantages of this concept far outweigh any disadvantages. Give it a try!Hopefully, this narrative will encourage more of you to take on the BMX-20-inch build for your next e-bike. I’m really glad I did! I’d like to read about your BMX frame build experiences.
Bill – age 59, Greater Atlanta area, State of Georgia, USA