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Broken shimano m447 brakes?

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agreenbike

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Discussion starter · #1 ·
I've had issues with my rear brake quite often, and now it really does seem like something is major wrong with the brakes. Today I was riding and almost at the end of the downhill part of my ride when I started to hear a grinding noise. Going home and checking the brake pad, the top part of the pad still had about 1/2-3/4 of the pad left while the bottom was totally gone and the brake disc apparently damaged as a result. Is this normal? When the brakes were new over 2 years ago I never remember such a problem.

The front (shimano m447, 180mm sm-rt56 rotor) has always worked fine but been kind of underwhelming (it gets the job done, lets put it that way) but I had this issue happen once to the rear brake before, along with needed to keep adding fluid since the brakes would occasionally start to go soft to the point that the rear would not even work. The rear has a XT SM-RT86 rotor (160mm, 6 bolt) that I think now will have to be replaced.

Anybody know what is going on and if I can fix it or if I should just get new brakes? I've been thinking about getting xt brakes for a long time but it seems those also have some inconsistency issues too, at least from 2016 on. I just need some reliable brakes that hopefully are a little stronger than my current ones, they have got overworked on intense yet short dh sections in the past.

I attached some photos of the very inconsistent wear and also of what appears to be rotor damage. Its worth noting that I have gone through about 4 brake pads in the time I have gone through less than 1 up front and I only use the rear brake a tiny bit more than the front, certainly not 4x more.

edit: for some reason I add other images but it just keeps the one and doesn't add the others even though they have loaded 100%, must be some sort of glitch. But this photo shows the brake pad uneven wear clear enough I guess, and the rotor does have a dark spot where it looks like the pad rubbed off the top surface of the rotor.
 

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Discussion starter · #4 ·
phreeky said:
Maybe the brake caliper mounts (the frame) are crooked and need to be machined and made square?
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Maybe, if anything maybe it was something with the rotor a;ready being warped?
Mr Pig said:
That pad should have been replaced long ago. Letting them wear down that much is asking for it.

Also, the pads should not under any circumstances overlap the top of the rotor.
Click to expand...
The top of the pad still has half the original thickness though while the bottom is totally gone, even though it does not look like it in the photo. From the top it does not appear to need to be changed.

Also, what do you mean by overlapping the top of the rotor?
 
#5 ·
agreenbike said:
What do you mean by overlapping the top of the rotor?
Click to expand...
The thing that has caused you this problem!

If the pads stick up over the edge of the rotor there will be a lip of pad that is not worn away. Eventually the lips meet as the rest of the pad wears away and this happens. I've seen brakes fail completely because of this.

New pads, new rotor if this one is gubbed, and you should be back in the game. Remember to fix the positioning of the pads though or this will just happen again.
 
#6 ·
It's hard to tell from the photo of because it's blurry at the top edge of the pad.

I think pig is generally right though.

1. Your pads should never get that low
2. The wear looks uneven, so either the pads aren't hitting the whole rotor, or the Pistons aren't pushing evenly.

New pads and maybe a bleed would probably change things for you, but if you don't like them in the first place you could always look to something else.

Sent from my XT1049 using Tapatalk
 
Discussion starter · #7 ·
Mr Pig said:
The thing that has caused you this problem!

If the pads stick up over the edge of the rotor there will be a lip of pad that is not worn away. Eventually the lips meet as the rest of the pad wears away and this happens. I've seen brakes fail completely because of this.

New pads, new rotor if this one is gubbed, and you should be back in the game. Remember to fix the positioning of the pads though or this will just happen again.
Click to expand...
I understand now, replaced the pads today and they top of the pad is pretty much right at the level of the top of the rotor, it is possible before that the pad got bumped when changing tires or something and led to the pad hanging over the top, causing uneven wear. I replaced the front pad too and it had completely even wear. There is some finish missing on a baby spot of the rotor, it should still be fine but I will have to see how it performs this weekend.
tfinator said:
It's hard to tell from the photo of because it's blurry at the top edge of the pad.

I think pig is generally right though.

1. Your pads should never get that low
2. The wear looks uneven, so either the pads aren't hitting the whole rotor, or the Pistons aren't pushing evenly.

New pads and maybe a bleed would probably change things for you, but if you don't like them in the first place you could always look to something else.

Sent from my XT1049 using Tapatalk
Click to expand...
Wish I could upload the other photos, I add them then once they hit 100% they dissapear and won't let me add them. Otherwise I would have loaded better photos of the pad than that one.
 
#8 ·
agreenbike said:
I understand now, replaced the pads today and they top of the pad is pretty much right at the level of the top of the rotor, it is possible before that the pad got bumped when changing tires or something and led to the pad hanging over the top, causing uneven wear. I replaced the front pad too and it had completely even wear. There is some finish missing on a baby spot of the rotor, it should still be fine but I will have to see how it performs this weekend.

Wish I could upload the other photos, I add them then once they hit 100% they dissapear and won't let me add them. Otherwise I would have loaded better photos of the pad than that one.
Click to expand...
Before you ride, bed the pads in. Pedal up to speed and brake hard 15 times or so for each brake.

Sent from my XT1049 using Tapatalk
 
Discussion starter · #9 ·
I thought about it more today, and I was curious again about Mr Pig's explanation of the wear on my pads, if that case did happen, then shouldn't the wear be on the top of the pad near the pin area and the pads be unworn at the bottom? Basically i'd expect that if the pads were over the top of the rotor, they would also kind of angle themselves and sit at an "^" shape and have lots of wear near the top and the bottom would get unworn. Instead the wear is at the bottom which means the pads would be at a "V" shape to the rotor instead of being parallel to it. I could see the pads just hanging over the top of the rotor a little but i'd feel like the pads would be angled some or something to get the uneven wear.

edit: Actually I guess his explanation still makes sense, just trying to think about it though.

And with that being said, the bike worked fine on some trail riding with some downhills, didn't have to that much intense braking but it seems like the brakes work fine now with no obvious significant damage to the rotors. Will have to keep demoing it.
 
#10 ·
agreenbike said:
I was curious again about Mr Pig's explanation of the wear on my pads, if that case did happen, then shouldn't the wear be on the top of the pad near the pin area and the pads be unworn at the bottom?
Click to expand...
No, the pads will still wear relatively evenly, until the unworn lips at the top of the pad touch each other. Once they do you will either loose all braking or the pads will wear at an angle although even then braking will be compromised.

Letting the pads get worn as much as your is pushing your luck. You can see that the lower edge of the backing plate has been rubbing the rotor, that's pretty bad. Apart from the damage it can do to the rotor the metal plate can catch on the rotor arms and bad things happen. Destroy the rotor, snap the calliper, put you over the bars, bad things. Also when the friction material gets really thin it can fall off the back-plate. Metal to metal, zero brakes.

All you're looking at here is neglect, both in the set-up of the brake and changing of the pads. Shimano brakes are super reliable but it's like a car or any other machine. If you don't take care of the basic maintenance they'll fail.
 
#11 ·
I'm probably overdue on replacing my pads on this set. I believe they are inferior to the XT brakes on my other bike, after riding both down my hill and being scared with these. Does it make sense to try to get metallic or semi-metallic pads for these? I've been getting the rotors that don't say resin only, I think the RT66.

I'm getting another set for a kids bike. These have been mostly problem free, except when they iced up one time but that was because I crashed through a thin layer of ice over a puddle on a cold morning ride. The brakes came back further down the trail. But I believe the XT has better modulation, but maybe I should replace the pads and compare again.
 
#13 ·
Sorry to bring this one back from the dead but searching for an answer to uneven wear on my bike found this post. I have a '16 Trek Fuel EX 8 with the same m447 calipers. After a bunch of digging and seeing the Shimano compatibility charts, it appears Trek paired a narrow Shimano rotor (SM-RT66) instead of the wide rotors (SM-RT56) on my bike. It appears you have the same issue as the RT-86 is listed as a narrow rotor. Probably too late to solve your issue but hope this helps someone else in the future. I have an email into Trek to see if they'll warranty my brakes and the wife's since she has the identical bike/issue.
 
#15 ·
Mr Pig said:
This isn't an issue. As long as the pads don't overlap the outer edge of the rotor, you don't have a problem.
Click to expand...
Not sure I follow. Reading your older post about the backing plate catching a rotor arm seems like it wouldn't occur had a wide caliper/wide rotor been used. I would imagine that the pad would wear more evenly and not have to remove the wheel or caliper to figure out how much pad is left. Second, I question why Shimano went through the effort of creating the compatibility chart if it doesn't matter. I get what you're saying about outer overlap though. That would be a true pucker moment.
 
#16 ·
SlowTri said:
Reading your older post about the backing plate catching a rotor arm seems like it wouldn't occur had a wide caliper/wide rotor been used.
Click to expand...
No, the backing plate shouldn't get anywhere near the rotor no matter what sort it is. The pads should be changed before that happens. If you run the pads down to the metal it doesn't matter what sort of rotor you have, it'll be bad.

A few years ago a guy brought a bike to me which had lost all front braking. Turned out the pads overlapped the outer edge of the rotor, big time.

If the pads overlap the arms of the rotor though it doesn't seem to cause any problems at all. I know, you would think it would, but it doesn't. Exactly why that is I'm not sure but it's true.
 
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