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My bike has a coaster brake (pedal brake).

I am not sure how it started, but it has just started to brake itself even if I don't do back pedal.

It works weirdly that, it is kind of latching the brake when I use the back pedal. Sometimes it gets fine especially when I do a strong pedal brake. But, it usually brakes itself and the strength of the brake varies, and time-to-time it gets fine itself for a while. It does not have a consistent state.

Does anyone have an idea what might be wrong with it and if it is something that I can fix it?

EDIT: Picture of the brake arm: enter image description here enter image description here

  • Did you check if the reaction arm for the coaster brake is securely attached? – Batman Aug 18 '16 at 15:11
  • I don't know the mechanism well, that's why I've just uploaded the picture of the arm. It feels secure on the nuts (on the arm and the yellow nut on the first picture), the arm doesn't vibrate or move. – Angs Aug 18 '16 at 15:21
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    Please check your spelling of brake. I can figure out what you mean but "break itself" is a step too far. – Chris H Aug 18 '16 at 18:06
  • Yeah, my guess is that the tire is rubbing the frame, because the axle is loose. – Daniel R Hicks Aug 18 '16 at 18:30
  • That chain is looking awfully droopy. It won't be causing the brake problem though it will provide slack for the brake to be shocked when you first apply the brake. – Criggie Aug 18 '16 at 20:03
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Are you sure that the friction comes from the hub, not from tyre rubbing against frame? Axle mounting point looks as it is not tightened properly.

Second thing to check is to assure that hub is not screwed too tight.

I would personally unscrew arm-side of the hub and check the clutch and its "spring" condition. It's easy, no special tools required. Check the very first minute of this video.

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Could be the that clutch inside the hub is worn down and is only functioning correctly some of the time. I'm no master of coaster brakes but this image may help you understand how they work and ultimately diagnose the problem.

enter image description here

Basically the thing labeled Drive/Brake Cone is the clutch, it slides left when back brake is applied pushing the brake shoes out against the hub slowing the bike down much like a drum brake on a car. If it gets to worn it becomes a lot more smooth and therefor generates less friction, and is not as large so exerts less force on the brake shoes, which can also be worn down. As long as the arm is functioning like it is suppose to and is firmly attached then this is what i would look at.

How many miles on the bike? Might be time for a rebuild of the hub.

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