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noodles

Rear wheel howl

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noodles
My rear wheel howled very loudly at me on the highway this morning @ 70 mph and I could smell that brake pad smell.  I pulled over checked quickly for anything really obvious and carried on to work since I was on a limited access highway.
The noise was sudden, loud, and irregular in that it would come on for a few seconds then go away. It was repeatable when I exceeded 70 mph but eventually stopped altogether along with the brake pad smell disappearing when I continued down the highway. 
 
The brake still worked when applied, the rotor wasn't glowing, the wheel turned freely when I got to work without resistance or obvious warpage. The fluid was changed a month ago and the piston was cleaned. Anyone on have an idea what I should look at first when I get home? I'm thinking rebleeding at the least and inspecting the piston, hoses, master cylinder, and disc for alignment or wear issues... 
 

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Beemer
I can't say for sure what the problem is but I can't help but get the feeling it's possibly a caliper hanging up for whatever reason. It sounds like you're on the right track to finding it. Be sure and check your brake cable/lever to make sure they are going back to where they belong after using them. I had a friend that had a problem with a sticking caliper on his car and after $1500 for all new brakes to correct the problem, but didn't, I went out and bought a $5 foot pedal spring for his car that corrected the problem. His one caliper was hanging up because the pedal wasn't being pulled back up all the way after he depressed the pedal. Just a weak spring was all it was.
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rick
Well, unless the caliper is leaking fluid, not sure why bleeding the system or doing anything at the rear master is gonna change anything.
 
I would look at pads 1st to see if maybe they've been contaminated with something that's getting hot and burning, or for any abnormal wear. Brake pads can also make noise between the piston and the steel backing or where the tabs rub/mount in the caliper that applying a thin layer of brake caliper grease will fix.
 
You might have caught a stone or maybe even a bit of plastic that burned up and is now gone.
 
I'm curious as to why you removed the pistons to that caliper? Unless a caliper's piston gets stuck, this is not really a routine maintenance thing, imo.
 
 

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noodles
Well, unless the caliper is leaking fluid, not sure why bleeding the system or doing anything at the rear master is gonna change anything.  
I would look at pads 1st to see if maybe they've been contaminated with something that's getting hot and burning, or for any abnormal wear. Brake pads can also make noise between the piston and the steel backing or where the tabs rub/mount in the caliper that applying a thin layer of brake caliper grease will fix.
 
You might have caught a stone or maybe even a bit of plastic that burned up and is now gone.
 
I'm curious as to why you removed the pistons to that caliper? Unless a caliper's piston gets stuck, this is not really a routine maintenance thing, imo.
 

I didn't remove the pistons I cleaned them by pumping them out of bore without removal then brushing gunk off then back in the bore they go after brake cleaner.  
I want to rebleed since I may not have put in enough fluid when I bled them last and if I'm going to top it off I may as well crack the bleeder to be sure the results is no air in the lines. 
 
While it was making noise I pushed up on brake pedal with my foot made no difference. At 17000 miles brake pedal never needed adjustment so I doubt it's that. Glancing at pads in parking lot at work I didn't say any obvious wear or discoloration and the disc has no scratches or discoloration. All parts are where they're supposed to be and both pins are in etc.
 
Hope I don't have to rebuild rear caliper 

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rick
Ah, ok.
 
Well, if the brake lever feels good, you've probably done a good job bleeding air out - just top up the master.
 
hmm, guess it's possible one of the pads hung up for a bit for some reason. I would pull the caliper and inspect the pads friction surface and backing plate and also have a look at the any spring clips (not been into mine yet). Also have a close look at where the pads are glued to the steel plate. BTW, if you remove a set of pads and put them back in, it's always a good idea to put them back where the side they came from so that the forces on them don't get reversed.
 
If you find/see nothing suspicious, where the pads are seated up against the body of the caliper, I'd put a small film of this stuff (or similar) https://www.permatex.com/products/lubricants/specialty-lubricants-brakes/permatex-ultra-disc-brake-caliper-lube-4/ pretty much any parts store will carry it. Don't think the pins need to have lube - unless they've become rusty - then I'd lube them as well. There will be people who say it just attracts dirt, but OE car pads will frequently come with a metallic antiseize-like stuff to go on the back pads (to cut down on squealing) as well as a small tube of this synthetic grease to lube contact spots.
 
Still think you might have caught something between the rotor and pad that made a racket and then escaped. You might see this as a scar on a pad after pulling them.
 
 
 

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noodles
Ah, ok.  
Well, if the brake lever feels good, you've probably done a good job bleeding air out - just top up the master.
 
hmm, guess it's possible one of the pads hung up for a bit for some reason. I would pull the caliper and inspect the pads friction surface and backing plate and also have a look at the any spring clips (not been into mine yet). Also have a close look at where the pads are glued to the steel plate. BTW, if you remove a set of pads and put them back in, it's always a good idea to put them back where the side they came from so that the forces on them don't get reversed.
 
If you find/see nothing suspicious, where the pads are seated up against the body of the caliper, I'd put a small film of this stuff (or similar) https://www.permatex.com/products/lubricants/specialty-lubricants-brakes/permatex-ultra-disc-brake-caliper-lube-4/ pretty much any parts store will carry it. Don't think the pins need to have lube - unless they've become rusty - then I'd lube them as well. There will be people who say it just attracts dirt, but OE car pads will frequently come with a metallic antiseize-like stuff to go on the back pads (to cut down on squealing) as well as a small tube of this synthetic grease to lube contact spots.
 
Still think you might have caught something between the rotor and pad that made a racket and then escaped. You might see this as a scar on a pad after pulling them.
 
 

I have high temp antiseize I'll put that on the back of the pads. I'd rather keep the pin dry and shiny, I remove corrosion once a year. The pads I got didn't come with shims so I reused OE shims. OE has insulators too, not sure why, but I reused them anyways. Still theyre so thin I think it's unlikely they would cause drag. We will see what the pads look like I suppose.  
Ugh the ONE day I don't bring my saddlebags w/ tools to work!! 

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rick
I've used and reused all of those clips and springs over the years w/o any issue. As long as they are clean and in good shape, should be fine to reuse all the spring clips or whatever.
 
Having that happen is a really goofy thing, so this will be interesting. That it happened w/o you applying the brakes 1st, still makes me think something got flung up in there, made some ugly noises, caused some pad smells and smoke and then ejected itself clear - maybe just fell off when you stopped to have a look.
 
 

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noodles
I've used and reused all of those clips and springs over the years w/o any issue. As long as they are clean and in good shape, should be fine to reuse all the spring clips or whatever.  
Having that happen is a really goofy thing, so this will be interesting. That it happened w/o you applying the brakes 1st, still makes me think something got flung up in there, made some ugly noises, caused some pad smells and smoke and then ejected itself clear - maybe just fell off when you stopped to have a look.
 

Yeah it scared the bejesus out of me at 75 mph on a busy interstate w/ somewhat cold tires and cold morning air & cold morning pavement w/ surrounding traffic and a gravelly shoulder!! I thought rear lockup was coming but it never did. Loud howl/hum like when a bicycle caliper is dragging you know? But about 20x louder... Almost painfully loud even thru my helmet, my earplugs, and the wind noise @ 75mph 
I installed new pads (EBC HH series) front and rear maybe 3000 miles ago w/o issue - was when I did the brake fluid change and piston clean up. Cleaned the rotors even! Didn't think I'd have to touch the brakes until next spring after the snow stops and the bikes come out to play.
 
Think what I'll do is, after I exclude pad/rotor/alignment/fluid issues, I'll get some silicone brake lube, put that on backs of pads (didn't do that when I installed them), and also gently lube the boot + slide + pins. Be totally sure this doesn't happen again. Pics when I take a look tonight ...

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r1limited
More than likely your foot was resting on the brake pedel.

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rick
Hmm, are you sure it wasn't something that got stuck between the tire and the hugger/fender thing? That would be loud and cause a vibration felt thru the bike. Would also cause some burning smells w/o causing the bike to slow down much. Have a look at you chain as well.
 
The front brake on that old /6 BMW in my Avatar would cause the wheel's spokes to go into resonance when the brakes/rotor were wet. As I would come to a stop, the entire front wheel would sing.

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noodles
More than likely your foot was resting on the brake pedel.
I do not ride with my foot on pedal unless going through traffic. But going home before I take pads off I won't even touch it just in case 

Hmm, are you sure it wasn't something that got stuck between the tire and the hugger/fender thing? That would be loud and cause a vibration felt thru the bike. Would also cause some burning smells w/o causing the bike to slow down much. Have a look at you chain as well.
This was maybe 10 minutes after starting it up after some light traffic and about 45mph cruising, I got on the highway, hit 75mph, and bam, noise from nowhere. No wobbles just noise

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r1limited
What color is your brake fluid? Clear, Tan or dark

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dmoney
You obviously need a louder exhaust ?

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shinyribs
If everything checks out with the rear brake, is it possible you may need to inspect the wheel bearings?
 
When you said it was howling and you pressed the rear brake and saw no difference...did you see no difference in the howling? Or did it seem like the pressing the rear brake made no difference in slowing the bike? Did the brake pedal move like normal, or did it feel stuck?
 
Sorry for jumping in here, but this is just very interesting. Curious to see what you find. Good luck!

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rick
Wheel bearings will cause a rumble and some vibration that you would feel more than hear, but once they go bad, they don't get better and then all of a sudden stop making noise.
 
Yeah, this will be interesting. I think there's gonna be some telltale evidence left behind somewhere.
 
Aprilia's of vintage like mine have a bad habit of getting air in the rear caliper that's hard to get out w/o pulling the caliper and raising it well up. The fluid in the master will look perfect, but the fluid in the caliper will look kinda ugly. If it gets really bad, the lever will lose all feel and do nothing when pushed to its stop(don't ask how I know), but this doesn't result in any noises - just no rear brake.

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