Monday, 13 February 2017

Build a BMX EBike

Build a BMX EBike  - Intro

If you are looking for a cheap bike to convert in to an ebike, then a BMX can make a good option. They are tough, small and soon fall out of favor with maturing kids, meaning then can be picked up very cheaply on ebay, and small ads.

Perhaps the biggest advantage is the small wheels, which on a ebike means greater torque (for a lower speed) for a motor of a given power.

BMX Ebike - Quirky . . . 

This makes them ideal for modifying for BMX cargo bike applications.

Build a BMX EBike  - Ebike Kit

Order this first as delivery can be slow.

The cheapest way to get the motor throttle and controller is to order it directly from china. This could cost as little at £150 / $250 with postage.

If you live in a flat area go for a direct drive hub motor, or geared in a hilly area. Choose a power level that is legal (for road use) or to suite your application. Choose upwards of 750w from off-road use.

My bike (orange) uses a MY1020Z motor, a chain drive option on the front wheel. A hub motor would be a better idea.

Build a BMX EBike  - Batteries

Lead Acid are a cheap option, but they are heavy and short lived. Lithium are expensive. Battery choice comes down to budget. Buy what you can afford. Battery voltage should match your motor / controller voltage.

Build a BMX EBike  - Mounting the Battery

Finding a spot for the battery is tricky. BMX frames are small, and finding spot that will not result in problems is . . . .tricky.

Ebike BMX - Battery Mount Options - 2016
"Perfected" - 2018

Rear Mounted 

If you mount the battery at the rear, it will have to be thin and long in shape, so as to sit between your heels. If the battery is wide and square it will have to be set very far back, other wise your heels will hit the battery when you pedal.

Mount High at Rear to Avoid Heel Strike

Front Mounted

The few BMX ebikes which you see on the web, typically have the batteries mounted towards the front of the top tube. As you can see below I have welded a "basket" to put my bulky battery in. But you could use webbing pouches, or for light lipo (lithium polymer) packs, even duct tape can be used!
A lead acid set up, using a none welded bracket.

To be sure my choice would work I taped a cardboard box of a similar size in to position and cycled around to see if it got in the way.

Build a BMX EBike - Comfort

For comfort I would suggest fitting a maximum length seat post (400mm) if you are reasonably tall, and raising the handle bar height as tall as possible. I welded extra length in to mine, but you could fit some "chopper" handle bars for the same effect.

High Chopper Handlebars - Can increase comfort.

Build a BMX EBike - Tires

You will get more range from your battery using smooth tires. Knobbly tires on road being the worst option. If you are using the bike for practical reasons, why not install some tire slime / sealant so you won't get punctures.

Build a BMX EBike -  Limitations

If your are keeping pedal power just for the sake of legality, then a BMX is a good choice for the above mentioned reasons, but if you intended to do a lot of pedaling or need gears then opt for another type of bike.

Carrying stuff in panniers etc. is also tricky. Heel strike at the rear of the bike being a real problem. A saddle mounted bag might be an option for small items.

The main problem with this design is the lack of weight on the front wheel. This if ine over flat ground but a gravel uphill section will cause problems.

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