Converting a Folding Bike to an E-Bike
A folding bike can be converted, but it is a compromise. The small wheels rule out many standard kits, there is little room at the bottom bracket, and the frame often has no space for a battery. For the example bike the configurator's verdict is "convert with work": every motor type has at least one yellow flag.
What makes a folding bike
Typical features are 20-inch wheels, either ETRTO 406 or the narrow 451, a hinged aluminium frame, rim brakes, 3 to 8 gears and a weight of around 14 kg. Our example bike: 406, aluminium, mechanical rim brakes, 7-speed, no room for a frame battery.
What usually works
- Hub spacing: 100 mm at the front and 135 mm at the rear are standard on the example bike.
- Light bike, short rides: the example bike covers 10 km a day on the flat. A 250 W kit is enough for that.
- Front or rear motor in 20 inch: kits in the right size exist. For the example bike the configurator rates them yellow, with one of them as first choice. Front and rear motor score close together.
What it fails on
Small wheels. Many kits are built for 622. 406 and 451 are different rims that you must not mix up, see Wheel sizes. Read the ETRTO number off the tyre.
Bottom bracket and frame shape. For a mid-drive the configurator rates yellow because of the frame shape. The hinge and fold mechanism leave little room, see Measure the bottom bracket.
Battery. There is no space in the frame triangle. The battery has to go on a rack, in a bag or on the handlebar. Some kits drop out because of this: anything that needs a frame battery turns red for the example bike. See Battery.
Brakes. Mechanical rim brakes reach their limits with extra weight and speed (GIGA). Plan for better pads or brake levers with a motor cut-off, see Brakes. With hub motors a torque arm is added, see Torque arm.
Fold mechanism and weight. A kit with battery often weighs 6 to 8 kg. A folding bike you carry to the train becomes a much heavier bike. Hinges are also not designed for extra load. That is an assumption Fabio is still checking; ask the manufacturer if in doubt.
Warranty. The manufacturer's warranty mostly lapses after a conversion (GIGA).
Which motor type fits
| Motor type | Outlook | Catch |
|---|---|---|
| Front motor | often the simplest solution in 20 inch | weight at the front, the small fork has to carry it |
| Rear motor | good traction | check cassette, gear count and hub spacing |
| Mid-drive | only if bottom bracket and frame shape fit | little room, battery needed |
For the example bike the configurator has front and rear motors narrowly ahead. The comparison in detail: Motor types.
What to measure first
- Rim size (406 or 451) on the tyre, see Wheel sizes.
- Hub spacing at the front and rear and the axle type, see Hub spacing.
- Room for the battery: rack or bag, see Battery.
- Bottom bracket, if you are considering a mid-drive, see Measure the bottom bracket.
- Weight of bike and kit, see Frame and weight.
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 you mostly carry and fold the bike, because then the extra weight is a constant annoyance. Leave it too if you cannot find room for a battery, or if your wheels are 451 and no kit fits. A ready-made folding e-bike or a used e-bike is often the calmer solution here. And if the brakes are already weak without a motor and you do not want to improve them, the conversion is not safe.
How sure is this?
On brakes and warranty the statements come from the sources listed. On the fold mechanism, the frame shape at the bottom bracket and carrying weight they are assumptions that Fabio is still checking. The assessment of the example bike comes from our configurator.
Traffic light for an example bike
Example bike – yours may differ.
Calculated with our example bike “Folding bike 20 inch (406)”:
Recommendation for the example bike: Rear hub motor
Motor types at a glance
- Rear hub motorNeeds workYour rim brakes are only just enough for 25 km/h and about 7 kg extra.
- Front hub motorNeeds workYour rim brakes are only just enough for 25 km/h and about 7 kg extra.
- Mid-driveNeeds workWith this frame shape there is often too little room around the bottom bracket.
- Friction roller motorNeeds workYour rim brakes are only just enough for 25 km/h and about 7 kg extra.
Main findings
- Needs workYour rim brakes are only just enough for 25 km/h and about 7 kg extra.What helps: At least good pads and fresh cables, better disc brakes.
- NoteAluminium dropouts need a torque arm with a rear hub motor.
- NoteThe bike maker's warranty usually ends after the conversion.