What contributes to disk brake noise when wet, and how could I minimize it?

Question

The accepted answer in Why do my disc brakes squeal when wet? states that some noise on disk brakes when wet is to be expected. However, I'm interested in factors that might affect the volume of the sound.

On my commute there's a steep downhill that goes past a school yard. When wet, my brand new gravel bike's brakes make such a loud noise that every single kid in that yard will turn to look. People along my regular route must be planning emigration by now just to avoid the noise. It's lucky that there are no dormant Balrog nearby, for they would have surely been awoken. If a car were to suddenly swerve in front of me, I'd be faced with the difficult choice between the squeal or just taking the hit.

So while it might be impossible to completely eliminate the noise, I'd just like to find ways to lower the volume before hearing loss fixes the problem for me. My bike is a 3T Exploro Team (carbon fiber frame), with SRAM Rival brakes. The rotors are SRAM Paceline 160mm, mounted with centerlock to Fulcrum Rapid Red wheels.

I suspect changing the brake pads and the rotors are the most viable options. For rotors, there are 1-piece and 2-piece designs with different style hole patterns - are some of these designs known to be less noisy than others? If getting another wheelset (which I'm planning to do anyway), would a 6-hole mount be more rigid than centerlock? Are there other variables I could change?

EDIT - Clarifying the intent of this question

In the answers and discussion of the question linked in the beginning, the consensus seems to be that even completely clean and well set-up brakes can be noisy when wet, simply because water acts as a lubricant. When water reduces the friction between brake pads and rotors, the system vibrates, causing noise.

My bike has done this since it's first ride, and has been consistently making noise over the first 300km or so - but only on wet rides, dry rides have been totally quiet. The bike was delivered factory fresh and assembled and set up by a reputable bike shop. That said, I'll start with the cleaning and centering procedure described in Jeff's great answer, and I will update with the results as soon as I get a chance to test.

However, I find it unlikely that the noise would completely stop with this procedure, and there are certainly riders that have reported that when wet, their system is loud despite all maintenance measures. As pointed out in Vladimir F's comment on Jeff's answer, even professional cyclocross racers' brakes squeal, and they should certainly be contaminant-free and well set up.

So specifically I'm looking for information on how the components of the entire brake system - the caliper, the frame it is attached to, the pads, the rotor and the hub contribute to the loudness. Intuitively it would seem that if vibrations are the cause of the noise, a more rigid system would help mitigate the volume of that noise.

Jeff's answer states that the rotor attachment style would probably have no contribution to the noise, but the material of the brake pads does. I will look into getting some resin pads for this bike. But if the noise persists after that, I would next look at changing the brake rotor.

The rotors I have are 1-piece design, meaning that they are simply cut from a single piece of steel. There are also rotors that have an aluminum spider, to which the steel outer braking surface is attached. Is there a categorical difference in the rigidity of these designs? Or is there something else in the construction of the rotor, such as how the holes are cut, that might affect the noise volume?

I would very much appreciate if someone had specific understanding of the mechanics of how wet disk brake noise is generated, and how the hardware of the system affects that noise.

Answer 1

I've been obsessing over this issue for years now. I'm an 85kg rider who rides in all conditions in Scotland, so my brakes are frequently cold and wet.

I can confirm that specifically wet weather squeal does not appear to be mitigated by any of the following:

1. Pad type. I have tried all compounds and a variety of manufacturers including OEM SRAM/TRP/Shimano, organic/sintered/ceramic.

2. Brake brand. I have tried SRAM hydraulic, Shimano Hydraulic, TRP mechanical.

3. Rotor size and construction. I have tried 160mm and 140mm, very skeletal Magura Storm rotors and thicker Magura e-Bihe rotors. Shimano 6 bolt 2-piece XT rotors, SRAM one piece Centreline, 140mm centrelock Swissstop Catalyst.

4. Any number of modifications to pads including sipes cut in the pad surface, bevels filed on the leading and trailing edges, automotive anti-squeal 'shims' (adhesive pads) on pad backs

5. 'Advanced' bedding in, e.g. bedding rotors in on long, steep MTB descents then swapping rotors and pads to gravel/road bike.

None of the above reduced squealing in wet conditions. Brakes were all very powerful and quiet in the dry though.

I think the squealing mechanism is pad vibration that is set up by stick/slip friction on a wet rotor. This resonates through the rotors/frame/forks/wheels.

To reduce this I guess the pad either needs to be fixed to the piston in such a way that it can't vibrate (mechanical fixing would mean much bulkier pads and callipers) or some sort of magical pad material which doesn't stick/slip, or is soft enough to absorb the vibrations.

I think the most realistic solution might be longer pads and/or 4 pot brakes, I would be interested to try the latest Hope RX4s but have already sank too much money into the project! 😎

Answer 2

To handle the last part of your question first, simply because I don't believe there is a solid answer, my opinion is that there is little difference in the potential for brake squeal between a 6 bolt mount or center lock. Each has an equally good chance of being part of an offending system. Being certain that your rotor bolts are all torqued up to spec is certainly important and should be a part of routine maintenance.

Related to this equipment selection as a means to reduce brake squeal, is your choice of brake pad compounds. Basically two types, resin and metal, are available to choose from. Resin pads are known to be quieter, especially when wet. While the stopping performance is below that of metal pads, resin can be used with any rotor and the most economical rotors are typically for "resin only" pads. Resin pads have better modulation characteristics which may reduce noise, however I feel that's a bit misleading (better modulation) because one can certainly have excellent modulation with metal pads.

Almost all brake squeal can be attributed to two general problems: contaminated rotor or pads and/or poorly set up brake system which includes one that is overdue for maintenance. Regarding contamination, oils from the road, various parts of the bike including the calipers themselves, and fingers will create a situation that can lead to brake squeal. Remove the wheel and remove the pads from the caliper. Use only isopropyl alcohol and a clean rag. Pour some alcohol on the rag and go around the rotor several times using a clean area of the rag and more alcohol every couple laps. To the pads: note the presence of any glazing and/or discoloration on the braking surface. These are signs of contamination or too zealous of a bedding in process that has overheated. The clean rag and alcohol should be used to wipe the pad. I have the alcohol in a spray bottle and squirt the pads several times after an initial wipe or two. Gives it a little pressure wash action. Careful with this. Control the splatter to avoid your eyes and sensitive (painted) surfaces around your work area. Again wipe the pads with clean parts of the rag. I go until the rag comes away clean. You can use fine grain sand paper as well to remove any glazing and kind of recondition the pads.

While the pads are out, inspect the piston side for dark, oily rings that may indicate a leaking caliper seal leading to contamination. This may be reason to change out the caliper. At the caliper, complete a thorough cleaning of it including the pistons. Mild soapy water on a rag is good. The alcohol should be fine too, but one must be aware of the sensitive nature of the piston seals. Harsh cleaners and degreasers can damage the seals. After cleaning and air drying apply some mineral oil to the circumference of the pistons. If you're careful you can activate the lever to move the pistons a little ways out and push them back in with flat tool (tire levers or tongue depressors work well) after lubing the sides of the pistons with brake fluid. The pistons should move out at the same time. If one seems to stick, give some attention and lubrication to it and activate it again after pushing it back in. If you're not careful the piston can come clear out of the caliper. Avoid this.

After cleaning all the parts, replace the pads (making sure to have pushed the pistons back into the caliper). Mount the wheel making sure it is fully in the drop-outs and centered. Recenter the caliper over the rotor. Loosen the caliper mounting bolts (not the bolts, if any, for the adapter) so the caliper is free to move side to side. Actually wiggle it to break any excess friction that prevents free movement. Give the wheel a spin and pull the lever and hold it in the closed position. Alternately tighten the caliper fixing bolts a few turns on one then switch to the other until they're fully tight. It's important not to move the caliper while doing this. I use a Velcro strap to keep the lever closed so I can have two hands and a better body position to work on the caliper. Release the lever and spin the wheel. The rotor should run true equidistant from each pad. Sometimes the above centering method needs to be augmented by hand if things aren't lining up. Additionally only one caliper mounting bolt can be loosened (the other being not final tight but enough so that end of the caliper can't move) and complete the wheel spin, close lever and tighten fixing bolt. A bedding in process should be undertaken after the reconditioning. Get up to speed and firmly apply the brakes, rolling to near stop and again several repetitions until the braking becomes normalized.

A clean, properly adjusted disc brake system is generally noiseless. Certainly not like you've described. Wet conditions will cause many systems to sound off, especially metallic pad set-ups but even then return to silence.

Answer 3

I found a few things that slightly mitigate brake noise on wet rides:

• washing off sand and grit with a powerful squirt of water from a bidon (I did this when transitioning from gravel to tarmac)

• lightly dragging the brake before braking to strip off the water film from the rotors

• I shipped sintered pads aft and organics (swissstop) or semi-metal (stock SRAM) in summer and winter, respectively. On wet rides I do all light braking and feathering at the rear brake and keep the front brake just ready for serious braking. While I do this to reduce wear it mitigates brake noise a little.

Answer 4

Are you okay to attach to your bike some of that noise-damping sticky-backed pad that is used in automotive applications? Downside is that its heavy and can be messy to cut.

But if you can identify a spot on your bike that resonates/vibrates while braking, then this may help. Obviously you can't stick this stuff to your rotor brake surface or other functional surfaces, but if there's clearance on the rotor's spokes then a tiny dot may make a difference.

Would be easier if you can replicate the wet-squeak in a workstand too.