Rims, Brake Pads and Punctures

Curious Yellow

Had a bit of a front wheel washout during my ride last night. It was pretty rainy and dark. The cycle path I'm on was obscured by leaves and I slipped on the division between the pavement and the path. Didn't fall, but the front wheel did have a scrape and a twist.

Took it fairly easy after that. Cautious braking and so on. Anyway, once I got home, I noticed quite a large amount of braking compound (rubber?) on the wheel's braking surface. I've also developed a slow-ish puncture overnight.

My questions are:

- Is this normal for cantilever brakes? I've had this bike for two years and I'm pretty fastidious with the cleaning. I've never noticed this much braking material on the rims before and have cycled in heavy rain and snow quite a few times before this.

- Could a scrape cause a puncture? Looking at the tyre I cannot see any tears/rips in the sidewall. The puncture was also a slow one.

- How do I know when I should replace the rims when the braking surface is worn? I've read that there's a groove on them which you could look for wear on. However, I don't recall my rims ever having had such a groove.

Thanks in advance.

sungod

gundge on the rims can build up after a few wet rides, just give them a scrub

if there's no visible damage to the tyre, it's probaby just a bit of glass/flint/etc., the edges of the road usually have the most cack in them, if it's also wet it'll lubricate the blade so it penetrates the tyre more easily

it's good practice to check the position of the hole in the tube vs. the tyre and inspect it to see if there's still something stuck in it, i had repeat slow punctures on a hire bike, i eventually found a bit of wire about 2mm long buried in the tyre, invisible as it only poked out under pressure+being ridden on

some rims have grooves or a couple of dimples in them, or even a hidden cavity, when the rim has been worn far enough to remove the dimples/groove or reveal the cavity it has had it, but many rims have no wear indicators, you can look for signs such as the rims having a clear concave area worn into them, or look for any bulging, or the sides no longer being parallel under pressure