Help me understand how derailleurs work with rear hub motor please

[ElectricJoe]

Cant get my head round this.
It's obvious how they work if the power comes from the main chainwheel, eg a bottom bracket motor, or just your own legs, the smaller the cog on the derailleur the faster the wheel turns for each rotation of the cog.
But when the motor is on the same wheel /hub as the derailleur cogs, and thus seemingly connected to it, what's going on?
Probably I'm missing something obvious here.
Thanks

 

[vfr400]

There's a freewheel between the driving cogs and the wheel just like any bike, so the motor turns completely independently. Imagine the motor as something that is attached to the stationary axle, and the hub that the spokes are attached to as being separate. There is another freewheel between the motor and the hub. That means that you can pedal without the motor working or it can turn the wheel independently from you.. It's a fantastic system and it works perfectly, exactly how you'd want it to.

 

[Woosh]

you have two types of hub motors: direct drive and geared hub.
Direct drive, as in the name, the rotor is connected directly to the rim with spokes. The motor supplies part of the energy needed to move the bike forward, the rider adds to it via chain and cogs. The good point is simplicity, not much to go wrong. The bad point is when the power is off, you have to supply more energy than just to move the bike, you have to move the rotor too and the rotor has magnets that generates a bit of electricity when the battery is off. You feel clearly a drag in the wheel.
Geared hub motors solve the problem with this drag by having an internal clutch between the rotor and the wheel hub. The clutch only engages the rotor if you ride on electric. Geared hub also has an internal planetary gearbox to increase the torque of the motor, very useful when you climb. Typically, the gear reduction ratio is about 1:4-1:5 but the current tendency is to have higher reduction ratio 1:11-1:13 for even higher climbing ability.

This is how it's arranged inside a hub motor:

https://wooshbikes.co.uk/2019/hubkits/hubmotor.jpg

 

[Andy-Mat]

Cant get my head round this.
It's obvious how they work if the power comes from the main chainwheel, eg a bottom bracket motor, or just your own legs, the smaller the cog on the derailleur the faster the wheel turns for each rotation of the cog.
But when the motor is on the same wheel /hub as the derailleur cogs, and thus seemingly connected to it, what's going on?
Probably I'm missing something obvious here.
Thanks

I trust you understand it better now, as it works well for most people, and hub motors (front or back), do not suffer from the mid motor's "Single point of failure", which is the chain, as all the propulsive energy to the back wheel, has to go through the same chain, both electric and muscle.
On a hub bike, if the chain breaks, most good bikes now react still to the PAS sensor and/or throttle, and supply electricity to the motor, assuming the battery is not empty, to get you home.
Conversely, if the battery is empty, you use the cranks and the chain to get home!!
Also, generally speaking, on a hub bike, with reasonable average chain maintenance, the chains last far, far longer, even relatively cheap ones from ebay, I have had really long life from!
Regards
Andy