Weird disc brake squeal problem, a puzzler
 

3 series BMW with 4 wheel discs. This car is not driven often (once a month or so).

Brakes have always operated silently.

I put on a set of new wheels and tires. Didn’t touch the brakes.

Rotors and pads have less than 5,000 miles on them.

Immediately after putting the new wheels and tires on, i get a low speed classic disc brake squeal. Only happens at around 15-20 mph with medium brake pressure.

It’s hard for me to hear which wheel it’s coming from. So I apply the handbrake to slow the car down, and I hear the squeal. (It has the type of rear calipers that apply the brake pads when you pull the handle, rather than the type that have a separate internal drum brake for the ebrake).

So I figure I found at least which end it’s coming from.

I take the rear pads out, clean and grease everything, and put new pads in.

Oddly, the exact same squeal is still there, except this time it only is there when I use the regular brakes. When doing the handbrake, it’s silent.

So I take the front brake pads out, clean and grease, new pads.

Still the same sound.

I can’t see how that can be. The exact same sound, but with different pads. These are not performance pads, but low noise pads with anti squeal backings.

Not sure what the next step is.

Yes I did.

Did the stock pads have anti-squeal backing shims? If not, you might try removing them and see if it goes away. If all else fails you might trysome spray-on anti-rattle goo, but that's a low-rent solution.

The fronts have clipped on anti squeal shims, the rears didn't have anything removable.
Akebono rears, Bosch Quietcast in front.
High or low rent, any solution that works is good for me!

Do you still have the original wheels/tires. Put them back on for fun so you can really pull your hair out if silent.

Only long shot beyond mentioned and not counting the coincidence is was there any drag on pressing the pistons back in?

I have not seen slider pins on these cars presenting any issues.

Mating faces to the hub/new wheel clean? Suspect so. Torque sequence "by the book"?

I say reface the rear rotors but just talking.

Are the squeal clips on backwards or not fully seated?
(hey it happens)

Those shouldn't even touch until the pad is worn down though. afaik

Any chance you bent a backing plate when you put the wheels on ? maybe rubbing the rotor ?

Ah, sympathetic frequencies...

Do what you can to apply braking pressure that does not make them sing. (harder or softer pedal pressure). After a while the singing should fade.

X2 on reinstalling original wheels to verify that's what caused it, then inspect new wheels closely. Did new wheels come with different lug bolts? If so make sure they're not too long...not sure if that would cause a braking noise but it sure seems like it's related to the new wheel installation and not the brakes since there was no problem before that change. My 2 cents.

I like this one. Bent the shield on the back side of the rotor. A friend of mine did this on his car.

Wow, thanks, you guys don’t disappoint! I feel like my answer in here somewhere.

No rear backing plates in the car, but it does have fronts, I’ll see if one got bent (although wouldn’t that be a constant noise??)

I don’t have the original wheels anymore, otherwise I’d swap them back and see, for sure. I did have to get longer lug bolts. I have some shorter ones I can experiment with, also some 5 mm spacers I can try. (Although again, wouldn’t the noise be constant if this were the problem, not just when braking?)

Both of the above are good leads, because the squeal happened after putting the new wheels on, when I hadn’t changed the pads yet.

Squeal clips can’t be on backwards, they only go on one way. And they came installed already. They are fully seated.

I’ll try the shorter lugs. Check the backing plates. Then might try Island’s suggestion, since it doesn’t involve any actual work. Then go down the list from there.

Are you a skier, McLovin? Ever encounter washboard formations in the flatter run-outs? Well there's this thing where oscillations sets-up the geometry for more oscillations. The snow washboards are a macro example of what happens to brake surfaces. Another example could be dirt roads that get washboarded.

Anyway, resurfacing your rotors will give you the quickest solution. Whereas allowing the pads to slowly resurface those micro washboards is the slow path to the same solution.

It is interesting that the wheel change precipitated the problem. Natural frequencies are fundamentally based on stiffness per mass. Although typically the sympathetic frequencies happen when the brake pad wears (decreasing mass and increasing stiffness) - that's when the singing occurs.

meh. It would be interesting to hear if you still have the squealing after reinstalling the original wheels. I expect that you would, but would like to know. That is, I expect you've already 'cut a record' so to speak. and the disk surface is now predisposed to grab/slip of that squeal frequency. But maybe not. Perhaps the old wheels (and lower stiffness higher weight) damp the grab/slip of that squeal frequency at the pad.

I don’t have the old wheels, unfortunately.

I’m pretty sure I’ve isolated it to the rear driver side wheel. I’ll confirm later when I have an assistant.

I put in 3mm shorter lug bolts, no difference. They screw in around 4.5 rotations, so they’re not too long.

There’s no backing plate.

When I put on the new wheels, I also put on a 5 mm hub centric spacer.

Listening closely this morning at very slow speed (walking speed) the squeak (it’s more of a squeak than a squeal) seems rotational. In other words, it only makes the noise at one spot in the wheel rotation.

It is almost like there is metal on metal contact in one spot of the rotation, when the brakes are on. But it’s not a grinding sound at all, it’s a high pitched squeak.

Before I changed the pads, it would make the noise when I applied the rear brakes with the handbrake. It doesn’t do that anymore. Only with the foot brake now.

(I could have this backwards but)

1). Organic pads are softer and conform to a rotor surface. More dust.

2). Inorganic pads ("semi-metallic") are harder and might level out small inconsistencies with normal use. More rotor wear.

Existing rotor thickness with run-out and style of driving will probably affect all of this.

Hmm, maybe Island wins the prize.

I went out for a half an hour and did 20 or so stops from 55 mph.

By the end, the noise was 99% gone.

I’ll try again tomorrow after is cooled back down.