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The sprocket for my coaster brake hub is shaped concavely in the middle, it is not symmetrical. I've seen some people on Youtube who had it mounted on the hub facing inward, and others facing outward. What is the purpose for this concave shape? What is the correct orientation? (My guess is that you can flip it around to adjust the chain line, but I'm not entirely sure)

Video showing my hub:

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Your guess is right, it's a way of dialing chainline. Most commonly you use it in the more outboard position. It's easy to just measure front chainline and both rear chainline options and see which position is optimal.

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Besides offering 2 chainline options, the concave shape also allows a plastic guard to be mounted easier, keeping it away from rubbing the chain. So the cog can be flipped based on where the guard is wanted, on the inside or outside.

enter image description here Image from: https://bike.shimano.com/en-EU/product/component/alfine-s7000s700/CS-S500.html

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  • And what would that guard be guarding? Is that just for looks, or does it serve an actual purpose?
    – cmaster - reinstate monica
    Commented Oct 25, 2018 at 9:51
  • @cmaster I believe it would prevent the chain from slipping off if it's a bit loose. With coaster brakes chain slip is dangerous, because you cannot backpedal anymore to activate the brake. The Shimano Alfine hub has guards on both sides of the rear cog. It would also keep away the straps of the panniers, loose cables, broken spokes, screws mounted incorrectly, or any other objects. There may be other purposes, I'm not entirely sure.
    – Robert
    Commented Oct 25, 2018 at 17:36
  • Ah, good point about pannier straps. As to the chain jumping off, that would not strike me as a danger for single speed chains: Single speed chains need a lot of slack before they run any danger of jumping off the sprocket. And when they do jump off, well, you got a second brake to stop the bike, don't you? I mean, the coaster brake should be only for backup purposes anyway (danger of overheating, ease to block rear wheel). So that's why I didn't think that you'd need a guard to stop the chain from jumping. But panniers with straps are definitely worth a consideration if you use them.
    – cmaster - reinstate monica
    Commented Oct 25, 2018 at 19:30

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