Loud bang in rear hub.

[ruffruff]

I have a mid drive Bafang.
When I start to pedal or apply throttle I get a very loud BANG from the rear.
It is not a grinding, it happens about 8 out of 10 times and it definitely sounds like hammering metal on metal.
It just started doing it after a ride.

It's not chain skip, I cleaned lubed and adjusted everything. It shifts smooth.

If I freewheel the cassette it feels rough and like there is sand in there.
The bike has 300 miles on it.
Any ideas.

 

[Sierratim]

I have a mid drive Bafang.
When I start to pedal or apply throttle I get a very loud BANG from the rear.
It is not a grinding, it happens about 8 out of 10 times and it definitely sounds like hammering metal on metal.
It just started doing it after a ride.

It's not chain skip, I cleaned lubed and adjusted everything. It shifts smooth.

If I freewheel the cassette it feels rough and like there is sand in there.
The bike has 300 miles on it.
Any ideas.

I assume you have a freehub vs a freewheel?

Sounds like one or more of the 'paws' in the hub many be broken. The paws allow the hub to freewheel making that familiar clicking or ratchetting noise. Loosing one or more paws will cause the hub to ratchet forward to a good paw making a sound that could be a 'bang' with the added torque of the mid-drive motor.

Park Tool,probably has video on this. Here's a writeup on servicing several types of hubs; https://www.parktool.com/blog/repair-help/freehub-service .

Seems like the next step is to do a tear down on the hub...

 

[ruffruff]

I've got a ticket in for service. Will see what they say.
But I think you are right on. My guess is a pawl is shot and it's rotating to the next one. That extra rotation gives it time to spin up and really hammer the next pawl.