Loud Brake Squeal When Wet

[3X3=5]

My M Sport 35i came with 19"s which I don't really like, so I bought a set of 21" 612Ms which I used last summer. Didn't really use them in much wet weather. Late last fall I put the 19"s back on and would have occasional very minor brake squeal when wet, but it wasn't bad. Weather has already warmed here in Seattle so I put the 21"s back on. It's still rainy, and the brake squeal is awful when they get wet. There is a huge difference between the minimal squeal I have on the 19"s and long/loud squeal I get on the 21"s. The wheels themselves are the only thing that has changed, so clearly something about the larger / more open wheel is amplifying whatever brake squeal was already present. It burns off after a few good hard stomps on the brake but it's obnoxious until then.

Has anyone else experienced this problem and any solutions for what I can do reduce it? Anti-seize between the wheel and hub? Or a gasket? Worth noting I also have 15mm/5mm H&R spacers installed, and I did apply anti-seize to the backs of those.

[fosterelli]

I can make some jokes about this thread's title...but I won't.

Larger wheels, wet, major squeals. Smaller wheels, wet, minor squeals. What do they both do when dry?

Very strange though, there should be zero braking sound difference when swapping your winter/summer setups. Could it be something is rubbing from fitment or damage (backing plate bent, backspacing wrong, spacer hitting something) or something is loose, etc?

But would that make sense if your noise only happens when wet? This is a fun one.

Shapes, machined areas, material thicknesses, etc. could make a larger wheel rub where the smaller one doesn't, or vice versa. Like maybe your 21s have backside angles/materials that are slightly rubbing on a backing plate or bracket, etc. where the 19s have less angle or material at that rub spot...if that makes any sense.

You could go from 18s to 26s and shouldn't hear any "squeal" difference.

I have zero squeal on mine, ever. Even in our frequent monsoons down here. Wet or dry, mine have never made a noise.

Being a lifelong PNW resident myself, I can attest you'll have more brake dust from the hills compared to us down in the flats...but that should be consistent between your wheel setups.

Between your wheel and your hub you should have zero movement or rotation, that would be a big problem. The wheel should be firmly locked to the hub via studs/lugnuts, so anti-seize would only assist with removing them later.

Not sure what you could gasket either if you could elaborate on what you're envisioning.

Spacers also should never move, would also be catastrophic and you should feel some vibrations and/or wobble before disaster strikes.

So, I've helped zero, but I was thinking with my fingers...like thinking out loud.

[ABZ101]

lol i just had the same thought today, i have 20s and breaks were squiking super loud today while wet. I think its normal overall, my car is 1 month old.

[3X3=5]

I actually had this same issue on my E90 335i. When I changed from stock 18"s to aftermarket 19"s the wet brake squeal was AWFUL. Literally as loud as honking the horn. Two trips to the dealer and the confirmed that it was caused by the wheels amplifying the brake squeal that would normally be deadened through the OEM wheel. But, in this case, I changed from one OEM wheel to another.

The wheels are seated and torqued properly. Nothing is damaged, and there should be no fitment issues as these are genuine 612Ms, not replicas. My best guess is that somehow the larger open wheel design doesn't muffle brake squeal like the smaller 19".

[fosterelli]

I would hope the BMW's soundproofing would be sufficient to not allow slightly larger wheel openings to cause any noise intrusion. Still don't know why you're squeaking loudly at all. I'd say something is wrong...and in all my life I've never heard of anyone (including dealerships) blaming brake noise on wheel choice.

We've owned almost 40 vehicles in the past 25 years and never once put up with brake noise other than the very occasional brake dust chirp. And we changed to smaller wheels w/ studded snow tires in winter, summer large wheels in summer, in Portland, Oregon, for 15 of those 25 years...very similar weather and terrain to Seattle, same vehicles and wheel/tire ranges.

I'm lost for help but can't wait to hear the diagnosis.