Converting a Cargo Bike to an E-Bike

On a cargo bike, a mid-drive is usually the right choice. It carries loads and climbs hills, and that is exactly where a motor at the bottom bracket, using your gears, helps. The conversion is doable, but here the total weight decides, not the motor: frame, brakes and wheels have to cope with the load and the kit.

What makes a cargo bike

Long wheelbase, steel frame, often smaller wheels (26" with ETRTO 559, or 20" with 406), mechanical or hydraulic disc brakes and a high unladen weight. Our example bike, a long john: steel, 30 kg own weight, 559 wheels, 8-speed, mechanical disc brakes, permitted total weight 200 kg, 500 mm down tube.

What usually works

• Steel frame: sturdy, well suited.
• Bottom bracket: 68 mm BSA on the example bike, so the common mid-drives fit, see Measure the bottom bracket.
• Room on the down tube: the long down tube leaves space for a frame battery.
• Gears: the derailleur stays.

For the example bike the configurator rates the mid-drive green and the front motor yellow. The verdict is "convert". A rear motor is also rated green for the example bike when the wheel size fits the kit; with 559 that is not the case for every kit, see Wheel sizes.

What it fails on

Weight. Bike, kit, load and rider add up. Kit and battery often weigh 6 to 8 kg (GIGA). Check the permitted total weight with the manufacturer, see Frame and weight. If it is already maxed out without a motor, there is no room left for the kit.

Brakes. Mechanical disc brakes reach their limits quickly under load. Hydraulic disc brakes are best, so budget for the upgrade (GIGA), see Brakes.

Front motor. The configurator rates it yellow for the example bike because of traction. On a bike whose weight is already spread between the front wheel and the load, a motor in the front hub changes the ride noticeably. That is an assumption Fabio is still checking.

Friction drive. It has too little power for loads. The configurator rates it yellow for the example bike.

Wheel size. 559 does not fit every kit. Small wheels (406) are an even more common deal-breaker.

Wear. A mid-drive puts more load on chain and sprockets. Under constant load, chain, cassette and brake pads are wear parts you replace sooner. See Common problems.

Which motor type fits

Mid-drive: plenty of torque on hills, and it uses your gears (Upway). Hub motors usually deliver 30 to 45 Nm, mid-drives 65 to 160 Nm; these are rules of thumb. With a load on a gradient the difference is noticeable. More in Mid-drive vs hub motor and Motor types.

What to measure first

1. Permitted total weight from the manufacturer, plus your own weight, load and kit.
2. Bottom bracket shell (width and type), see Measure the bottom bracket.
3. Wheel size (ETRTO on the tyre), see Wheel sizes.
4. Room on the down tube and for the battery, see Battery.
5. Brakes: type and condition, see Brakes.

The law in short

Pedelec means: at most 250 W rated continuous power, assistance only while you pedal, up to 25 km/h. A throttle is only allowed as a walking aid up to 6 km/h, anything above is a motor vehicle. See Pedelec law.

When you should leave it

Leave it if the permitted total weight is already used up without the kit, if you carry children and the brakes do not get clearly better, or if the frame shows cracks or rust. When carrying children, a ready-made cargo e-bike with the manufacturer's approval is better than a conversion. The manufacturer's warranty mostly lapses, and some workshops refuse conversions (Fahrrad XXL).

How sure is this?

The statements on kit weight, brakes and warranty come from the sources listed. The torque ranges, the wear under constant load and the remark on ride feel with a front motor are assumptions and rules of thumb that Fabio is still checking. The assessment of the example bike comes from our configurator.

Does a conversion fit your bike?