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I have the NCM Moscow 27.5 that since purchase has always showed this issue, that is the rear rotor isn't in total contact with the pads when breaking. The company said this is not an issue but a design choice. Since I don't really trust them I'm asking you if you think this could happen on some bikes or if it needs to be corrected (and how?).

enter image description here

This is the wear result of my rear pads that came with the bike. I never touched the caliper, wheel or anything else to make it not touch.

The brake is Tektro MD280 on a 160mm rotor.

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    I was so free and resized and uploaded the image from the original dropbox link and remove the latter. External links may vanish and are generally not very trustworthy, so please rather use the image upload functionality of the platform (max. file size is 2 MB, so you potentially need to edit high-res picture in and additional step).
    – DoNuT
    Commented Nov 3, 2023 at 16:50

1 Answer 1

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When the overhang is about this amount, say 0-1.5mm, it's not ideal but usually won't cause serious problems in power or functionality.

Presuming a correctly made caliper (which most of them are beyond the really cheap brake-shaped-object type brakes, and the one you have is fine), the problem originates from the mounting position of the brake caliper not being exactly to spec.

In some cases you can fix this by taking material off the posts (or the adapter in an IS installation), then facing. As evidenced by the wedge shape of the lip on the left, there's an angular component to the orientation that would need correcting as well as a height one, which means you can't always fix it because what you can't change is the angular orientation of the post threads, and the conical washer interface that some caliper mounting bolts use can only compensate so much. Another issue that can come up is the caliper running out of clearance with the frame/fork/adapter.

Because it's a job that involves cutting into the frame/fork and the outcome can be a little uncertain unless an unusual amount of modeling/study is done, this is something most shops are going to be unwilling or reluctant to do. The brake post facer needed is a fairly expensive shop-level tool, so actually getting this fixed can be tricky or expensive despite being technically possible in many cases.

In some cases this problem can be caused by someone determining they needed to stick in some kind of shim between the caliper and one or both brake posts to avoid some other problem, like a clearance issue as above or issues with the post indenting into the caliper. Since the lost braking surface is not a critical issue when it's only this much and otherwise the brake might not be able to work, this can be an appropriate thing to do at times, though it's unfortunate.

Edit: Criggie makes an excellent point in the comments. When the lips develop sufficiently that they meet each other under force, wouldn't that reduce or eliminate the clamping force on the rotor? It certainly can't help braking power, and the math bears out that they will meet at some point since the total thickness of the two pads is quite a bit greater than the thickness of the rotor. There's not really anything plausible to suggest that the lip surface gets reduced incidentally somehow in these situations. Since I've seen a decent number of brakes in the wild that develop small lips like this, and total or severe loss of braking power is not something I've seen go along with it, at this point I'm going with the hypothesis that what happens is when there would be dead space that prevents clamping the rotor, the pistons get angled up to take that gap up. They can do this since they don't move through a rigid pathway but rather through rubber seals, though the pistons and the seals both tend to work better over time if they're not asked to function like this. This situation would also likely cause some further loss in braking power, since at the moment the lip contact starts happening, it will only be the lower edge of each pad that contacts the rotor. The pad surface will then begin to wear at an angle to the backing plate. It also bears pointing out that though it's observable that brake pistons have some ability to operate at an angle like this, there will be a limit to it where the piston does interfere with the bore it moves through, and if the consequence of exceeding that limit were catastrophic loss of braking power, it then becomes appropriate to do more than just guess that's never going to happen. In light of all this I suspect I'll probably do more of the above mentioned brake post surgery than in the past, though it's also true there are bikes that have no good solution to this issue.

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  • Thanks for taking the time. I think I'm just going with "not being made to specs" explanation. Commented Nov 3, 2023 at 17:20
  • Is there a potential for the two unworn lips of braking material to touch? IE, if each has a height half that of the rotor thickness, they would touch and reduce the pressure applied to the rotor's face? Or would they be squishy having never been bedded in ?
    – Criggie
    Commented Nov 3, 2023 at 22:06
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    @Criggie Quickly looking up specs says that Tektro rotors are 1,8 mm thick and their pads have "5 mm pad thickness" ... not sure if the latter is the whole pad or just the wearing/raised part but if I do the math quickly, 1 mm of wear each side might already be enough. Not sure if that would become a real-life problem, but it sounds feasible...
    – DoNuT
    Commented Nov 4, 2023 at 11:47
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    @Criggie That's an excellent question and I admit I've never thought about it. I've also never seen it happen, at least not to the extent needed to disable the brake, despite seeing a decent number of brakes with unworn lips like in the picture. I suspect (but don't know) the mechanism here is that power tends to be maintained because the pistons have some ability to angle themselves up whenever that air gap would be present as a result of the lips touching. This may cause me to reconsider how acceptable the lip really is, since that kind of thing is bad for the seals/pistons. Commented Nov 4, 2023 at 23:07

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