Replacing a hub motor is one of the bigger jobs you can do at home, and one of the easiest to spend money on by mistake. A problem that looks like a dead motor is often something else: a connector, a brake cutoff sensor or the controller. So this guide starts with confirming the fault, then the measurements a new motor has to match, then the swap itself.
First, confirm the motor is the problem
Run the free checks in our e-bike diagnosis before you buy anything. If the motor shudders, grinds or will not turn, test the Hall sensors and phase wires as described in our Hall sensor test guide and the Hall sensor and phase wire page. Water in the motor plug is a common cause too; see inspecting the motor connector.
What the new motor must match
- Voltage and power. The motor has to suit your battery voltage and your controller.
- Dropout width and axle. The distance between the frame slots and the axle size and flats must match. Measure with our dropout width and axle guide. A frame built for a thru-axle generally will not take a bolt-on hub motor.
- Gears. A thread-on freewheel and a cassette are not interchangeable, so the new motor must take the type you plan to run, with no more speeds than it is built for.
- Motor plug. The connector and its pin count have to match your controller cable, or you need the right extension. Our plug guide helps you identify it.
- Brake mount and wheel size, if you are buying a motor already built into a wheel.
Deciding between a complete motor wheel and a bare motor? Our comparison of motor wheel versus bare hub motor covers the trade-offs. If you send us photos, we can check the fit first; here are the 4 photos we need.
What you need
Everything from the rear wheel job (axle wrench, torque wrench, hex keys, cable ties), plus a freewheel remover or a cassette lockring tool and chain whip, a Torx key for the brake rotor bolts, thread-locking compound if your rotor bolts call for it, tire levers and a pump, and a multimeter for the final checks.
Swapping in a complete motor wheel
- Turn the bike off and remove the battery, and keep it out for the whole job.
- Take the old wheel out following our rear hub motor wheel guide.
- Move the tire and tube to the new wheel if you are reusing them; the flat tire guide shows how to get them off and on.
- Move the brake rotor. Undo the rotor bolts, fit the rotor to the new motor the same way round, and tighten the bolts to the torque in your motor or rotor instructions.
- Move the freewheel or cassette. A freewheel unthreads counterclockwise with the correct large-bore remover; a cassette comes off with a lockring tool while a chain whip holds the sprockets. Grease the threads before refitting. Our freewheel removal guide explains which tool fits a hub motor.
- Fit the wheel with its washers and torque arm. Fit a torque arm whenever the hub motor is rated 500 W or more, whenever it is a front hub motor, and whenever the dropouts are aluminum. Then tighten the axle nuts to the torque your new motor specifies. Never leave them hand-tight.
- Connect the motor cable, arrows lined up and fully seated, with a gentle loop at the axle and ties keeping it away from the spokes.
- Run the motor with the wheel off the ground. Refit the battery, use the lowest assist level and a touch of throttle. If the wheel runs backward, shudders or will not turn, switch off and check the wiring order; many controllers have a self-learning function, and our self-learning guide explains each result. The phase and Hall combination guide covers the manual method.
- Test ride, then check again. Ride slowly in an open area, testing the brake cutoff first. After the first few rides, check that the axle nuts are still at full torque and the wheel still sits straight.
Fitting a bare motor into your old rim
A bare motor has to be laced into a rim with new spokes. Hub motor flanges are a different size from a normal bicycle hub, so the spoke length must be calculated for your exact motor and rim, and the wheel then needs truing and tensioning. Unless you have built wheels before, this is a job for a local wheel builder. You still need to match every item in the list above.
Safety notes
- Keep the battery out until the very last test. With a controller powered, the phase wires can carry battery voltage.
- Motor wheels are heavy. Lift with your legs and support the bike so it cannot fall while the wheel is out.
- If the motor ever runs on its own, switch off, remove the battery and read what to do when the motor runs by itself.