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My son's bike has a SRAM i3 internal gear + coaster brake hub. It has recently started sometimes only activating the coaster brake after backpedalling a good one or two full revolutions. When in this situation, it sounds somewhat clunky or grindy when the brake finally activates. However, sometimes, it kicks in straight away. I haven't found a solid pattern to when it works and doesn't, but it seems that the brake is more reliable in gear 1 than in gear 3, though I might be wrong about this. Can this adjusted/fixed, or do I need to replace the mechanism?

EDIT: After experimenting with the brakes and gearing with the bike in a stand (previously I had been trying while riding it) it seems that in gear 1, the brake always works fine. In gear 3, it requires almost a full rotation backwards before the gear engages. While in gear 2, it's somewhere between - about half a rotation.

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    Is the "arm" from the hub tightly attached to both hub and frame? – Daniel R Hicks May 19 '19 at 0:42
  • the brake arm is tightly attached to the chainstay. As for the hub, I can't tell as I haven't got a thin spanner as large as the nut that's there, but there is a locknut, and it seems tight, but I can't be certain. However, I would be surprised if it had come loose on its own as I haven't touched it, and it's been running fine for ages. – askvictor May 19 '19 at 1:44
  • How much mileage has this gearbox done in its life. How long/far since the last oil change ? – Criggie May 19 '19 at 4:37
  • I bought it second hand, so don't know the previous history, but it's a kid's bike, so it won't be massive mileage. Since we've had it, it's done less that 1000km. Never had an oil change/service since I've had it, so I suppose that's what I'll look at doing. – askvictor May 23 '19 at 0:17
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    I have the same hub, and the same problem. After 6 years of no problems. Btw. these hubs don't need oil replacement. – Wes Green Oct 20 '19 at 22:06
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SRAM recalled 7,700 of these hubs due to brake failure in 2019.
Evidently the brake pawls were not lubricated properly.

According to the company, some of the affected hubs may have been manufactured with grease that gets sticky over time, especially if the bike sits unused for a while or is stored in a hot area. When the grease becomes sticky, the brake pawls in the hub can fail to engage.

Further

The recall applies to all bikes with this cruiser brake model that do not have independent front brakes.

The logic appears to be that if you have a front brake your hub does not need to be recalled.

According to the SRAM manual for this hub there are no user serviceable parts.

Only a qualified bike dealer should perform any necessary work on the gear hub, shifter, and brakes. SRAM i-Motion 3 User Manual

The manual says that "the brake jacket has to be lubricated with special grease. This work must be performed by a qualified bike dealer"

First I'd read the manual and make sure everything is adjusted correctly.
If that does not work then you can choose between:

  • Fixing it yourself
  • Finding someone else to fix it
  • Replacing the wheel

The easy way to get rear brakes might be to add a hand brake to this bike if it will take one.

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    Note that it's lethal folly to ride a bike with only a single functioning brake. Especially hand operated brakes can and do break, and you don't want to be on a bike where the one and only brake fails you. As such, I see the part about not recalling bikes with a hand brake with disgust. – cmaster - reinstate monica Dec 16 '20 at 19:32

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