"Misdemeanor" Electric BMX
- November 27, 2015
- Barent Hoffman
- BMX, RWD, Kids
When you have a family, inevitably, riding an ebike on family outings becomes a bit of a sore subject.
For Mom and Dad, there are lots of options for ebikes, but for kids… not so much.
That's why we built electric BMX bikes for our two youngest kids.
Are they legal?
It depends.
While most state laws prohibit anyone younger than 16 years old from riding an electric bicycle,
we note that Razor makes a variety of electric powered vehicles for kid use, including a few models
of electric dirt bikes capable of speeds of 14 MPH and higher. Our recommendation is that if you
do decide to build an electric bicycle for your kid, you want to make it very low powered and
only allow them to ride it on safe trails and closed courses. We DO NOT recommend allowing your
underage kids to ride an electric powered bicycle on the street where cars are present. And
of course they should ALWAYS wear a good helmet.
So how did we build it?
The main challenge with a BMX frame is that the drop-out is narrower than a standard adult bicycle.
BMX bikes are typically 110mm between the rear drop-outs, while adult bikes have a 135mm width
of which most hubs are designed to fit.
However, a front direct drive hub motor will typically fit a 100mm drop-out since this width is
common to full size mountain bikes' front forks. Therefore it is relatively easy to install a
front direct drive hub motor into the rear drop-out of a BMX frame. However, front motors do
not usually have the ability to install a freewheel sprocket.
For these BMX builds, we special ordered a front hub motor, and - request a rear motor side cover
which allows us to mount a freewheel sprocket on the motor. For this build we opted to install
a 16T single speed freewheel sprocket, and the bike pedals normally even without the motor powered.
For the rest of the components we used the standard Xie Chang 6FET 25A programmable controller,
with throttle and small Lithium Polymer battery which has a nominal voltage of 29.6V (8S) and
5 Amp-hours of capacity. Even with this very small LiPo battery the bike still has a range of
13 miles at 14 mph. Specialized custom-ordered small 18650-based battery packs are possible
and suitable in this application if you are not comfortable using LiPo. Batteries as small as
36V/6Ah can be made with as few as 20 cells (Samsung 30Q 18650 cells) and managed by a 25A BMS.
For Dad's that are adrenaline junkies, it's easy to install a better controller and a higher
voltage battery to get speeds of over 30 mph with this same set-up.
If this project interests you, contact us and we will special order the motor. This is
typically a non-stock item, very uncommon to source state-side by ANY ebike reseller, including
the Kinaye website. However, it's a very fun project that you may wish to build with your
kids and inspire them to be part of the electric bike revolution!