TiHabanero

Customer has a 2020 Diverge with Sram disk brakes. The front disk brake produces a harmonic vibration that is very loud and annoying. Brakes stop just fine, but produce a sound that can only be explained as a harmonic wave or vibration. Pull the wheel off and I can reproduce the same sound exactly with my fingers in a paper towel with alcohol on it. Grasp the rotor with two fingers and pull them along to produce the noise.

I replaced the Sram rotor with a Shimano rotor and the noise was gone, but after a few miles after break-in it and out on a gravel ride the noise came back, albeit at a much lower volume than with the Sram rotor. Replaced pads, clean rotor, but it did not change anything. Sram suggested checking the mounting bolts for torque. Did that and they are OK at 18nm.

Any experience with this? Ideas outside of replacing with new caliper?

niknak

Perhaps the vibrations are amplified in the fork. There were some old Specialized mountain bikes that resonated oddly in the swing arm. If I remember correctly, people would tape things like magnets wrapped in inner tubes to the swing arm to reduce the harmonic resonance. Perhaps as a test you can slide a few layers of old inner tube on the NDS fork blade and attach a spring clamp and do a parking lot test.

Good luck and please respond if you figure it out!

fietsbob

Tried different pads? I fit a TRP HyRd front caliper.. stock semi metallic pad chirps when wet, Organic compound pad does not..

2old

I had the same problem with a front SRAM (Avid) XO. Tried organic pads which didn't help. Also, tried putting grease behind the pad (recommended on one forum) without success. Procured a TRP mechanical brake. Viola, and no appreciable loss of power. The Avid discs on the Trek I purchased for my son (before they were bought out by SRAM) have worked perfectly for many year, but no more AVID hydraulics for me.

TiHabanero

We have multiple stores so I called around and found two other Diverge bikes with the same Force hydro brakes had the same issue. The resolution was replacing the rotor with a Shimano rotor. Unfortunately for me this quick fix has not worked. I am suspecting the caliper as mentioned in several posts above as everything I have done up to this point has failed. Honestly, the more I deal with Sram products the less I like them as a mechanic.

Gresp15C

A new Surly Straggler in the family fleet has exactly this problem. I have narrowed it down to a torsional resonance with the spokes. In fact if I pluck the spokes I get roughly the same note as the tone of the howling. You can hear it all the way down the block. I performed a couple of experiments. One was to wrap an old innertube between the disc and the spokes. It made the resonance go away, but I was super nervous about riding it that way -- if the innertube came undone, it would be curtains for the rider, as the problem is only on the front wheel. I made a similar vibration damper by cutting a piece of a heavy black rubber plumbing adapter, and it works too. For now it's not installed, and not being an engineer, I'd hesitate to recommend messing with any kind of "fix" of such a safety critical component.

Now this is a particularly well trued wheel -- all of the spokes ring with the same note. I've thought of randomly tweaking the tensions and seeing if I can make the resonance go away, but have not gotten permission from the rider.

The shop tried all of the usual things, cleaning the discs with alcohol, etc. I tried new "organic" pads. I swapped discs with another bike. Still the same problem.

Ferrouscious 

Things to try/check:

-contaminated pads?
-loose pad securing bolt/clip/whatcha-ma-call-it?
-thin o-rings between the caliper and the frame
-ensuring perfectly flat mounting surface for caliper
-carefully grease mounting bolts with thick grease (marine grease or thicker)
-file bevel onto leading edges of brake pads (like on a car)
-file groove in middle of pad perpendicular to brake rotor
-brake "silencer" adhesive (from AutoZone or the like) on back of pads. Glues springs to pads to change res freq.
-last resort- attach weights to fork or hub


Spend 20 minutes centering a SRAM disc brake as opposed to 5 for a Shimano. Servo-Wave on disc brakes is a beautiful thing...

TiHabanero

I never suspected the wheel as the source. One thing I did notice is that the Shimano rotor locking ring has a thin metal "spacer" between the locking ring and "teeth" on the hub, which may account for the quieter operation of the Shimano disk because the Sram rotor has nothing between locking ring and hub. Going to play with it this coming week.

Amt0571

Try adjusting the wheel bearing preload or cup and cones depending on the type of wheel.

I had this issue on a bike and it was solved this way. The wheel had a slight play before adjustment and that caused the vibration which in turn made a terrible noise when braking.