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The brake calipers on my front disc brakes appear to be grasping a bit further in than I would expect.

Rotors showing wear

You can see that the calipers do not grasp the outer ~15% of the rotor. I imagine that if they grasped the whole outside section, braking would be slightly stronger. Also, braking currently shudders ever so slightly (although this rotor is much better than the previous one). By not grasping so much of the inside supports, this shuddering might be reduced?

I went into my LBS, but they were a bit vague. They said they didn't "know the tolerance of SRAM rotors", although the previous rotor had the same issue. They also claimed that the fork was manufactured correctly, although "if I wanted to" they could fix the position, probably by spacing the HY-RD out by putting some M16 washers in. They didn't really say whether it was of concern or not.

  1. Are the calipers grasping at the correct position?
  2. If not, what is to blame?
  3. Is putting washers in a safe fix?

It is a 160 mm SRAM Centerline X2 rotor with a TRP HY-RD caliper (on a Soma Double Cross fork).

Also the calipers are at the limit in the z-plane, i.e. they are pushed out away from the wheel as far as the mount will allow, so there's conceivably another issue there.

Update

I changed the Shimano adapter for a TRP-branded adapter. I'm not certain if it solves the problem of the grasping at this stage. I'll leave it a few days and see if it clears the outer dirt ring on the disc. However, the (related?) problem with the lateral position of the calipers is worse. With this new mount, they are pushed out as far as possible, but the outer pad still rubs against the disc.

Update 2

Changing brands of adapters had little effect. In fact, the TRP-branded ones were probably slightly worse than the Shimano.

I took my bike to a fourth shop to try and sort out the shuddering. They decided to put washers in, moving the calipers out. However, they used the washers that have some lateral movement (I think the same as Mike Baranczak's answer). This may have slightly fixed the z-plane issues mentioned above. The shuddering is much better, but now the brakes screech terribly, and the stopping strength is still less than expected.

I will see them in a few weeks to see if they can tweak it more. I will update this question further when I have more definitive results.

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    That is not right, but it won't kill you. Check to ensure the caliper is not supposed to have CPS washers under it. – mattnz May 11 '17 at 1:43
  • @mattnz A different LBS built my bike, but they were fairly rubbish, so it's plausible they stuffed up the installation. Would you expect that information in the HY-RD installation manual? I couldn't see anything about CPS washers there. – Sparhawk May 11 '17 at 1:55
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    AFAIK you don't want any part of the brake pad to be hanging over the outermost edge of the rotor, and every part of the pad should be swept by an open space. Looks like it could go out a biit on a washer but not too far,. – Criggie May 11 '17 at 3:49
  • Make sure the mount is compatible with the rotor diameter. – Carel May 16 '17 at 9:06
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    @Criggie I updated the question 18 hours ago! I'll get back in a few days when I have something more conclusive. Don't worry, I won't abandon the question! (I have >10k rep in other SE sites, so I know how the system (should) work.) – Sparhawk Jun 12 '17 at 22:18
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  1. Some amount of unworn upper lip on the rotor is normal. For example, when you try to measure a round-edged rotor with a caliper, usually it doesn't work from the outside edge because there's some kind of lip there. The entire brake pad surface should be contacting rotor - in other words, there shouldn't normally be any lip getting worn into the pad. If there were, that would be your real sign that the pad isn't making full contact. But because it can't be totally perfect, it follows that the rotor is wear the unworn spot is going to show up. This one does look a little more extreme than normal, but I think it's fine.
  2. The first thing I would do if I suspected a problem is check firsthand that the mount is actually to spec, since it's pretty simple. Use a caliper or even a pocket rule and poke a skewer through the axle without the QR nut to give you an easy spot to measure the center of the axle. Here are the dimensions. If it's not that then it may be a peculiarity of how this rotor is getting along with this brake and adapter. In my experience, even though it's not supposed to matter, you can save yourself some grief by mismatching adapter/caliper/rotor as little as possible, and you've got a different brand for each here. I do tend to be willing to put in Shimano rotors on anything. (Just to be clear, mismatches shouldn't matter and usually don't.) I think part of what may actually be going on here is the pad type used by the HYRD is just a little taller than most or all of the SRAM/Avid pads and it's making it look like the rotor wear is going further than normal into the large cut-out section of the rotor.
  3. Up to 2-3mm or so should be fine if you want to try this. I would feel like I was doing something wrong if I stacked them up any more than that in a shop setting. I don't really think it's worth messing with upon further consideration.

Edit: Substantively changed this answer after thinking it through a bit more.

  • Thank you (+1). I'll have a measure when I get a chance, and get back to you. – Sparhawk May 11 '17 at 12:08
  • As reference, the calipers are grasping about 1.5 mm in from the outer edge of the rotor. The rotors my LBS originally used had the same offset, and they were TRP branded too! I had a measure, and think that the specs are reasonable. The distance from axle to farthest bolt is within measurement error (0.3 mm). The closer bolt is shorter by ~2 mm, but that shouldn't affect the grasping of the outer rim so much. I guess this suggests that either the calipers or the adapter is out then? – Sparhawk May 13 '17 at 0:12
  • Changed my answer. I think this is actually a slightly worse version of a pretty normal situation actually. – Nathan Knutson May 13 '17 at 15:23
  • Thank you for the update. I've had so many problems with my brakes (shuddering, squealing, etc.) that have apparently been caused by my terrible LBS set up. I'm just a bit paranoid now. It does feel pretty good now, but I guess I'd like it to be 100% perfect. I investigated the adapter more, and it's a "Shimano sm-ma90-f160p/s", which means a fourth brand! I might give the TRP-branded adapter a go (I assume this is the right one?) as it's quite a cheap test to perform. I'll report back when it arrives and I have it fit. – Sparhawk May 14 '17 at 1:51
  • My TRP-branded adapter finally came in. I'm not certain if it solves the problem of the grasping at this stage. I'll leave it a few days and see if it clears the outer dirt ring on the disc. However, the (related?) problem with the lateral position is worse. With the previous mount, the calipers were "pushed out away from the wheel as far as the mount will allow". With the new mount, they are pushed out as far as possible, but the outer pad still rubs against the disc. – Sparhawk Jun 12 '17 at 3:25
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Are the calipers grasping at the correct position?

No, but it's not a huge problem. Your stopping power is probably slightly lower than it would be otherwise.

If not, what is to blame?

The brakes that I have came with special washers that allowed the angle of the calipers to be adjusted (see photos). If I was to install the calipers without these washers, I would expect to see something like you have. But your manual looks like it's supposed to be installed without these washers, so I don't know.

Is putting washers in a safe fix?

Yes, as long as you don't space them out too much (it looks like you just need a couple millimeters, so that should be fine), and as long as the bolts are still fully engaged in the threads (you may have to get longer bolts).

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  • Thank you (+1). I don't quite understand how those washers work. How do they change the angle? Or are you saying you could use it on one side and not the other? My calipers are also at the limit in the z-plane, i.e. they are pushed out away from the wheel as far as the mount will allow, so there's conceivably another issue there. – Sparhawk May 11 '17 at 12:09
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    It's hard to explain in words, and I couldn't find a good illustration. You have a concave washer paired with a convex washer (2 of these pairs for each bolt, so 4 pairs per caliper). When the bolt is loosened, it sort of works like a ball-and-socket joint, and allows for a few degrees of tilt. When you find the right position, you tighten the bolt to fix it in that position. – Mike Baranczak May 12 '17 at 13:44
  • the TRP HY/RDs are not designed to use these washers when mounting – Paul May 12 '17 at 20:04
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    In case anybody is wondering, these concave/convex washers are called Avid CPS Washers. CPS stands for Caliper Positioning System, and in my opinion is a bit of pain to setup. It is only needed with Avid's (SRAM's) CPS calipers such as BB5/BB7. Most of SRAM's newer brakes are "standard calipers" and don't need those washers, except certain mounting configurations (such as when using a 20mm adapter/spacers to use a 180mm rotor on a 160mm post mount fork). – Gene Pavlovsky Jan 6 '18 at 19:34

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