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When I brake on a road bike I feel staggered braking—similar, but much lesser in amplitude yet higher in frequency, to what you'd feel when ABS brakes engage in a car.

When I brake on an MTB, I sometimes hear a grinding noise, as if some pollutant occasionally interferes with braking.

For both problems, it appears that disk brake pads and/or rotor cleaning may solve the issue.

What is the right procedure for cleaning disk brake pads and rotors? The distance ridden is 250 and 500 km, and so replacing either part is almost surely wasteful.

Specifically
  1. Will isopropyl alcohol (using the large supply of left-over disinfectant?) remove the brake pad material from the rotors, before restarting the bed-in procedure?
  2. Is there a tell-tale sign that a rotor has been ruined beyond repair?
  3. Does the pin holding the brake pads in place sustain multiple bendings back-and-forth, or do I need to have a supply of new pins for replacement?
Details
  1. The rotors are perfectly true.
  2. Braking power remains perfectly fine.
  3. The gaps are just right and need no adjustment.
Update
(Update following Jeff's and Renaud's answers)

Can you confirm that staggered braking is normal? All (?) rotors have holes. How these holes do not act as an abrasive material that rapidly shave the surface of the brake pads is a mystery, but is a separate issue. Yet I have only felt staggered braking on this one bike. Does it eventually go away? Chamfering of the holes is surely not done at the factory; could chamfering be done by users using the brake pads while braking, and then the staggering will fade away?

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  • Just to reiterate - DON'T use "leftover disinfectant" for any bike work unless it is pure isopropyl alcohol or a mixture with water with no additives. Most commercial disinfectants (including both hand sanitizers and surface disinfectants) will have addtives that you don't want anywhere near your brakes. BTW, acetone should also be fine for cleaning both pads and rotors (in fact many brand brake cleaners eg Muc Off are just acetone + ethanol plus some proprietary additives). Jul 11, 2023 at 15:59

2 Answers 2

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Based on your reported symptoms of "staggered braking," yet having true rotors and normal braking power, I don't feel any intervention is necessary. You may want to remove the pads and.check for uneven wear. This would be unevenness on one pad. Comparison of the pads together you would see the pad to the inside wearing faster (typically). This is normal. Uneven wear on an individual pad is not normal.

While the pads are out, you can clean the piston areas of the caliper. You can activate the lever (gently) and observe the pistons for any sticking. Use care that pistons don't come entirely out of the calipers housing. After cleaning, use a tire lever or similar to push the pistons back in. They can go all the way in as they'll reset to the proper spacing when the pads are returned and you throw the lever a few times.

Regarding the mountain bikes brakes, they do at times get some debris on the pads that will make itself known by the noise. In my experience, using the brake a time or two generally cleans it out. One of the functions of the holes in the rotor is to clean the pad of debris, if the grinding persists through several braking cycles, I would stop and check for larger debris stuck in there. Rarely a chunk of pad can break off and cause abnormal sounds under braking.

The best (and only thing I use) to clean pads and rotors if necessary due to contamination is isopropyl alcohol. I typically get the 92% stuff, but the 70% IPA that's normally found in stores is ok too. Just be sure there are no other adulterants in it or you'll risk contaminating the system.

Sometimes after general bike maintenance--especially after lubing the chain if I wasn't able to cover the rotor with a plastic bag-- I'll prophylactically wipe the rotor down with a clean rag and isopropyl alcohol. I don't feel that it's necessary to clean pads or rotors regularly. Especially if the braking power is normal.

Obvious signs of rotor wear or damage requiring replacement include a thickness of 1.5mm or less; any aluminum showing on the braking surface (mid to high level rotors are often a sandwich of an aluminum core under hardened stainless steel); cracks in the rotor, a bent rotor refractory to truing attempts. There are certainly other reasons left out, but those would be the most common ones I'd say

Regarding the pad fixing pin, Shimano, neither in their Dealer's Manual (DM) for General Operations nor the DM for several specific MTB brake models, do they mention any advise regarding reusing the split pin that secures the pads. It's common practice when using split pins (aka cotter pins) in other use cases to reuse the pins through multiple cycles of removal and replacement. In fact, it's desirable to not so deform the cotter pin on installation that it makes it difficult to straighten the pin for removal and allowing its reuse. Being steel, the pin will take many, many cycles of bending/straightening without generating a critical fatigue in the material.

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My answer is only valid for Magura and Shimano, I didn't check for SRAM - that use a different braking fluid, so it may impact the answer.

  1. Neither Shimano nor Magura recommend to clean the pistons with iso-propyl alcool. They do recommend to use alcool or detergents on some parts (rotors, hoses, external parts of the caliper,...) but not on the pistons. Considering how conservative are usually user manuals, I would take this as a sign that is not necessary (as part of the routine maintenance though).
  2. Thickness is identified as the indicator that a rotor is worn, both by Magura and Shimano. The thickness is written on the rotor (on my Shimano rotors, 1.6mm, on the Magura's 1.8mm, but this may vary depending on the model).
  3. Shimano official recommendation is replace the cotter pin after each use (Magura uses screws instead of pins). But as explained by Jeff, it may be an too conservative recommendation - I change them when I change the pads, because they are provided and I prefer to discard an old part than a new one.

The general recommendations are only to check for pad and rotor wear, leaks, and consistent behaviour (not too mushy or too hard).

As for the staggered braking on the road bike, it seems to me that this kind of behaviour is linked to the size of the holes on the rotor and the continuity of the braking surface on the rotor. Almost non-existent on the stock rotors of the fun-bike (basic Shimano SM-RT56, only small holes), started to appear when I upgraded the wheels and rotors (SM-RT54, that have a few large holes) and the behaviour is even more marked on my commuter bike, that has only large openings (Magura MDR-C rotor - and smaller pads, as it is a 4 pistons caliper).

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