Brake Harmonic Frequency

Has anyone battled with a vibration issue with their brakes? It seems as though I've got a resonant frequency I need to break through. On sustained descents, the rotor begins to vibrate (it looks blurry) and the vibrations carry through the frame and HOWL loudly (not wet pad screeching, this is a brutal howl). I've changed pads, rotors, re-bled, re-centered.I have Zee calipers + metal pads + (1) 203 shimano RT66 and (1) jagwire 203mm rotor .

I found a shop in whistler that could actually give me an answer better than "well your rotor may not be true". They then explained this resonant frequency issue that is likely starting at the caliper. Their suggestion is to clean all metal mounting interfaces to the frame, consider adding an anti-seeze backing grease to the pads (like the stuff you use on cars) and look into adding a small weight to the caliper.

Has anyone else lived through this issue and have a solution to share? I'm curious about options and success rates.

I wonder if it's an issue with the mating surface between the caliper and the fork? Sometimes an excess bit of paint can build up around those spots and result in mating surfaces that aren't completely flat. On long descents as the caliper heats up this difference could be increased due to the heat, causing the caliper to vibrate enough to create the howl. Considering everything else you've checked/change that could be it. 

You could take a fine file and make a few passes on the mating surfaces of the caliper and the fork and then get the wife to shuttle you up to the Mushroom lot for a quick blast down the hill to see if the filing sorted things out.

Good thought - but the issue is on the rear brake. The same thought applies though about the mating surface.

What about changing rotor sizes? Adding or removing a brake adapter may be enough to change the frequency and kill the noise. (Not sure if it works; just a thought)

I had this on my hardtail (carbon chameleon) crazy vibration through the whole frame. Have you tried different *types of pads? I tried several rotors including different sizes, a few sets of pads (but all metallic). A swap to organic pads fixed it.

A friend of mine had a squeal last year that turned out to be an aftermarket adaptor that was a touch too short. The outside of the rotor rubbed and sang when heat from braking caused it to expand radially. Years ago I had a Giant  Trance that had a howling rear brake. A layer of painters tape on the back of the pads helped but was temporary. Changing from SRAM brakes to Shimano XT cured it.

All good thoughts;

- I've used 3 different sets of brake pads (sintered, organic and a combo set with a bit of everythign in it) - they all howled

- I had the issue with 180mm shimano on the rear (same howl developed), I switched to a 203mm jagwire (all they had in stock at the LBS) + a proper shimano adapter. Howled - I put the Jagwire up front and transferred a shimano 203 to the rear (from the front) - still howled, but less often

Meaning the issue is not pad or rotor related - albeit that the jagwire rotor does warp when it gets hot, hence the idea to switch it to be up front. It is going to take a few weeks to be sure that I've fixed it, but I plan to try to anti-seeze and find a lead weight from a tire-balancing place...then cross my fingers. I'll try to mount it on the stay and caliper separately and see. Other idea I'm debating is cutting an old tube and making rubber spacers for under the caliper + adapter to try to isolate the vibration away from the frame.

I did find a thread on mtbr from ~8 yrs ago that swear by putting weights on the seat-stay.

somebody recently told me padz have a coating you want to burn off ?

I fixed vibrating howling brakes on a new  FAT bike by sanding the padz then taking the bike to a super long steep down hill did panic stops all the way down to brake ( pun intended) them in

the front brake on my yeti ( SLX/ DT SWISS/ FOX 36) would vibrate but only at certian speeds usually when I was on the brakes in the middle of a berm, I tried swapping padz front to rear, sanding padz, sanding rotors, several times I thot I had it but never quite, I did solve the problem by selling the bike and since it was a Yeti to a medical professional but not a dentist

I found a easy way to sand rotors with a flap sanding wheel for a power drill, rotate the bike wheel as you run the drill, depending on how you tilt the drill it will rotate the wheel as it sands

The flapper idea is a good one.

I did sand rotors + pads as part of troubleshooting. I have not done a burn using a blowtorch or our oven.

https://www.amazon.ca/Flap-Sanding-Wheels-Kit-Drill/dp/B07D3C8GSD/ref=asc_df_B07D3C8GSD?tag=bingshopdesk-20&linkCode=df0&hvadid=80745476896042&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584345021576277&psc=1

yeah these ^^^ they will rotate the wheel as they sand

I had this BITD on a DH bike (sounded like Chewbacca being given a good rogering.....). I changed rotor size (as mentioned above) and the noise went away. It was definitely a resonance issue rather than contamination. Power was good and the noise was a low vibration rather the higher pitch and loss of power from contamination.

Tried adding a wheel-balancing weight to the stay (then moved it to the caliper adaptor, then the caliper itself) and cleaning (sanding) all the interfaces around the brake caliper - still sounding like Chewbacca (great descriptor btw). It is a carbon rear triangle, so I'm still confused as to the source. I'm going to get a replacement seat-stay and see if that solves it. I am at a loss on this one.

is harmonics the reason that Shimano centrelock rotors have a rubber plug bolted to the back of one arm?

Who knows? In this case, I'm using 6-bolt rotors (mix of shimano and jagwire - both vibrate as rear-brakes).