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What's the channel in the brake track of this rim for? (I think it's in the brake track, anyway, and that the brake pad would cover it.) I would have thought it better to have an unchanneled surface for better brake contact.

Note that rims have a channel on the inside for seating the tire bead, but here I'm talking about the one on the outside of the rim.

Rim with channel in brake track

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    Its a bit backwards really - when the indicator is not visible its worn out. A car tyre has wear bars that indicate replacement when they are flush with the rest of the tread. A better solution would be to have a bright-coloured "pill" embedded inside the rim, so wear uncovers and shows the indicator, or when braking it makes a sound, so you notice and look and then do something about it.
    – Criggie ♦
    Nov 1, 2017 at 21:26
  • Yeah, I agree, that would be better.
    – compton
    Nov 1, 2017 at 22:52
  • @Criggie That would be nice but, in reality, it would be somewhere between very difficult and impossible and would almost certainly be a weak point in the rim. And you should probably be replacing the rim before the groove has completely eroded away, so you shouldn't ever be looking at a rim wondering if it ever had a groove.
    – David Richerby
    Nov 2, 2017 at 22:56
  • Does this mean that pro road racers use rims without the wear indicator channel, because they have a team with calipers who can check wear and can instead get the benefit of a larger braking surface? Or is the braking surface already big enough even with channel and it doesn't matter? (No idea if these guys are all on disc brakes now anyway.)
    – compton
    Nov 3, 2017 at 13:02
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    @compton Besides the fact that pro riders have pro mechanics, nearly all pro riders race on tubular tires, so the tire does not exert pressure on the rim in the same manner as a clincher tire.
    – GLNN.LRSN
    Nov 16, 2017 at 20:33

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It's a wear indicator. The friction of the brakes gradually abrades the rim away and the track lets you see how fast that's happening. Once the rim has worn enough that the track is very shallow, the wheel is starting to lose strength and should be replaced.

For wheels that don't have a wear indicator groove, you can detect wear by holding a straight edge radially against the rim. Worn rims will have a concave profile, since the brake blocks don't rub evenly across the whole surface.

Of course, this is irrelevant if you're using anything other than rim brakes.

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    Some rims have just a few small recessed spots instead of a groove. The rim is worn out when the spots are no longer visible.
    – Christian Lindig
    Nov 1, 2017 at 14:55

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