ST19 rim guides

[drossall]

Does anyone know how these work? Sounds a daft question, but I had expected the channel to decrease in depth as the rim wore. When there's no channel, the rim is worn out. In practice, however, I'm seeing some rim wear and the channel is left projecting somewhat above the rim, which makes it harder to see how bad the wear is.

[Brucey]

this wear pattern is commonly seen when groove is narrow (and the brake arms are a bit flexy I think).

What happens is that under light braking the brake block wears so as to give a ridge that fits in the groove. However the first time the brakes are used hard, the ridge gets torn off the brake block as the arms flex and this leaves a groove in the face of the brake block. The rim then doesn't wear as much either side of the groove, leaving the groove set in a slightly proud ridge of the brake block.

Like you say the rim can then be worn out but still with a visible groove. Putting a straight edge against the rim may tell you well enough but otherwise you are going to have to measure the rim in the conventional way if you want to be 100% sure.

cheers