I was concerned with the chain action on the Enertia I’m testing when it’s pushed around unpowered. It felt like what happens when a chain and sprocket get really worn- very close to the point of breaking- on a gas bike. I wasn’t all too concerned, as soon as you put the power to it, it rides nice and smooth.
I got a great explanation from the guys at Brammo, it turns out, as I’d hoped, it’s the natural characteristic of a PMAC motor. Here’s the video showing the effect:
Here’s the explanation:
(The Enertia has) a synchronous permanent magnet alternating current motor or “PMAC motor”. This type of motor has strong rare-earth magnets bonded to an internal rotor, and the copper-wound stator is external. When the motor is not powered, and the rotor is forced by external motion, the magnetic reluctance path is closed as the magnets pass by the stator poles.
This reluctance path tries to hold the rotor in place and this effect is called “magnetic cogging”.
(That’s one snappy end-title page, huh?)