Brake screeches like a Nazgul
 

I have a 2015 Trek Boone 9 with cantilever brakes. I am sure problems like this have been posted in the past but couldn't find one that solved the problem.

I have isolated the screech to the rear left brake.
Actions taken so far:

• Cleaned rim and pad with alcohol
• sanded pad down
• switched front and rear pads to see if a different pad would work
• tightened bolts
• toed the brake
• realigned the clamp
• ran out of ideas

And still the screech that would make even the toughest of warriors roll on the ground covering their ears.

Any other advice?

Fork mounted cable stop or mini Vs.

My guess is that the problem is fork shudder.

https://www.sheldonbrown.com/rim-brakes.html

Could you show pictures of the pad, and the braking surface? How long have the wheels and pads been in use?

Depending on the cantilever setup, perhaps the arms need to be re-centered so the pads have equal force on the rim. Use the spring adjustment bolts to move that left arm a little closer to the rim. You could try loosening the pinch bolt and pulling a little more cable through to reduce travel at the lever, allowing more modulation too.

I think it was Sheldon that explained that screeching brakes are simply a resonant vibration. Toe-in was the solution, though the previous poster mentioned cantilever-shudder, like when a frame flexes along the fork, and that solution was to use a fork mounted stop.

You've already handled everything else I can think of. I'm wondering if you have something embedded in the pad or the holder contacting the rim. I doubt it though.

You could try toe again. Calvin Jones recommended a rubber band at the rear of the pad as a spacing aid, and it's genius.

The fork crown mounted cable stop, while good up front, will not help with the OP's rear brake noise.

Another thought:

Maybe the pivot bolt that fixes the arm to the frame boss has loosened, and introduced unwanted motion in the arm during braking? Maybe the brake arm is internally worn and you need new brakes?

This is mounted Cable Stop. and it is the Rear break. Ill look into that.

unfortunately I don't have "10 posts" so it rejects every time I try to load, but the brakes pad has no grooves or abnormalities and has pretty even wear. I even changed the pad and got the same sound. The wheel spins true and has no clear abnormality in the braking surface.

I aligned the pads to be equal force and the travel is pretty minimal.

I will try toe in again and see where it gets me.

For the first point, I took apart the brake and put it back together and made sure all bolts were tight, and still got the same sound. I'm concerned about the second idea. Any way to test it with out buying completely new bake sets?

unfortunately I don't have "10 posts" so it rejects every time I try to load, but the brakes pad has no grooves or abnormalities and has pretty even wear. I even changed the pad and got the same sound. The wheel spins true and has no clear abnormality in the braking surface.

I aligned the pads to be equal force and the travel is pretty minimal.

I will try toe in again and see where it gets me.

If it was just a banshee, I'd say enjoy the warning you're giving people on MUPs and other drivers.

Since it's more like a Nazgul, maybe a different way of cleaning brakes and rim surface is called for. Find a good, deep, mud puddle, ride through there, immediately brake hard. Rinse everything off with clean water and go for a ride. If it continues, don't brake so much. :)

Do you mean that the pad is completely smooth? That could indicate that all pads need to be replaced, since there's either a line on the top of the pad (usually 1.5-2mm from the carrier) or a groove cut into the pad that indicates when the pad is due to be replaced. That doesn't explain why your left rear pad is the one squealing though.

To test for play, see if you can move the brake arm in any direction other than slightly up and down the frame boss and pivoting around the frame boss. Use another brake arm for reference, but there shouldn't be much play other than slight movement parallel to the boss and pivoting around that point. When did the screeching start?

Perhaps a pad with a different compound would change the system enough to break up the resonance.

"Unca Sam's" suggestion has merit. For years I had a problem with canti's screeching on my Klein. I tried various brands of brakes, various brands of pads, more toe-in, less toe-in, etc. The solution for my bike was a set of canti's that fit the frame boss very tightly. If there is play between the brake and the frame boss, the brake will vibrate (repeatedly stick then slip) which generates the squealing.

In response to all of these...

- I tried different pads and it was the same rear left brake making the sound. I had roughed up the old pad with some sand paper.

- I used wiggled the brake to see if there was a lot of play, maybe the slightest bit more than the others but not much if any.

- I bought the bike a week ago (the price was more than right) thinking after some research I could fix a screech. He said he had taken it to a mechanic before and that had fixed it about a year ago, but it came back and he has been riding his other bikes so it just didn't get fixed this time. He said he was gonna reach out to the guy who fixed it and get back to me but I have not heard back.


New Attempts and Diagnosis.

- I unhooked a brake are one at a time and pulled the brakes against the wheel as it spun and only the left rear made the screech.

- I then re-hooked the brakes up and held the brake lever closed and forced the wheel through at various pressure points and it seemed to make the screech no matter the pressure or speed of the wheel.

- I then tried holding the brake with my hand as a little bit extra stabilizing support and got the same screech.

- The brake pads are hitting the wheel at the same time and are equidistant to the rim when released.

-I am new to this forum, so it still wont let me load pictures.

- my conclusion is that it might be the left side of the rim rather than the brake that is the problem?

- I cleaned the inside of the rim with Alcohol, but got the screech. I heard baby powder might help but I would be concerned about stopping power.

- any other suggestions? This might be the point I cave and take it to a shop...though I am not sure they will have much luck.

It is not a solution, but you should use your front brake the majority of the time anyway, maybe just deal with the squealing when you need to use the back one. It will certainly let folks know that you are coming!
As to the talcum powder, give it a try; your rear brakes's limit is locking the wheel, if you can do that it is the best stopping power you are going to get.

You could always swap the wheels or flip them and see if it still screeches. That'll eliminate or confirm the left brake surface as the cause. Frankly, if you lightly sanded the sidewall, it won't be that. @Jean_TX's explanation of the cause of screeching is accurate, and if there's extra play in the brake, you might simply replace the brake arms.
@dsbrantjr is giving good advice in using the front brake most of the time. It's the quickest way to stop.

go introduce yourself in the introductions thread, and greet some other new members, and you'll be up to 10 posts in no time.

Yeah we really need pics at this point. Sounds interesting. Usually a good bit of toe in will stop squeals...

Resolved:

I gave up and put brand new pads on. I didn't think this would work, because I had switched the used pads and still had the screech in the same location. I tried switching the wheels and still had the screech as well. But with the wheels back on and brand new pads, the screech decreased significantly, but it was still there. I did some work to align the brakes better...all my fiddling probably made the alignment worse. Then pulled the pads off and took the top layer off with some sandpaper.

Now I can sneak up on a hobbit and they would never know I was coming....

Thank you all for your help on this one.