Rim seems to be shedding metal chunks

Question

I have rim brakes on my mountain bike that's outfitted for urban duty.

Two weeks ago, I noticed that my rear brakes were weaker and louder than normal. I found that, although my pads still indicated 1/16" of wear left, the metal "internal" part of the brake had become exposed.

I immediately replaced the pads.

Since I installed the new pads, about every 40-50 miles I'll notice my brakes suddenly get noisy and weak. I'll get off my bike, and find a little metal chunk in one of my brake pads. After I dig it out, my brakes behave normally again.

I'm pretty sure that it's not from junk on the road. This never happens when I'm riding my other BSOs.

My rim looks and feels (to my finger) fine.

It seems like my rim is shedding metal bits. Is this the case? And, if so, what can I do to stop it? I've tried wiping my rim with a rag, but it hasn't made a difference.

Edit: I forgot to add that this is happening with both rims, not just the rear one that had the worn pad incident.

Answer 1

If you've used the brake pads up to the point where the metal is exposed, you've used them way too long and possibly caused some rim damage. There are wear indicator grooves on brake pads -- when they're gone, put on new pads.

As for having metal embedded in the brake pads, I defer to this question. You might want to try switching pads (Promax stuff is generally low quality -- try Kool Stop), and checking the wear indicators on the rims.

Answer 2

Without seeing the rim it's hard to tell exactly what is going on but I suspect you are in a loop where your rim surface got some rough spots which stuck to the brake pads which ground or pulled up some rough spots... and repeat.

Since the brake pad change didn't work I'd suggest one of two courses of action. Or both courses but then you start losing track of the variables.

Option 1 try different brake pads. The Koolstop salmon pads have a great reputation. Throw them on and see if they work. They are made of pretty hard material and they may accomplish what I outline below.

Option 2 is work on your rims and once worked on use new brake pads on them.

The work you need to do can be done with those green Scotch brand scrubby pads. You are going to use these to "machine" your sidewalls. Vigorous scrubbing around the circumference will smooth things out and remove trouble spots. You can go so far as to stuff the scrubby between the brake pad and sidewall and then ride 50 ft or so. Problem with that technique is that you only abrade the width of the brake pad so you miss some spots on the rim.

Some people wet sand the sidewall with 220 or higher sandpaper. I would only do this if the scrubby doesn't work.

The danger is manually machining the sidewalls is that you remove material. Remove enough material and the rim fails. Sometimes in spectacular fashion. Of course, grinding away with metal chunked brake pads also removes material so pick your poison!