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A friend of mine just got her bike back from the store after getting new brake pads, a new fork and a new cassette. After riding a few meters I noticed a weird behaviour while using the front brakes and they seem to be mounted incorrectly to me as the brake pad is not completly on the disk but only like the inner forth of it. Is this mounting alright or was something done wrong?

detail image of the brake pad and disc

Update: more pictures

overview picture of the brake

picture of the bike

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  • 2
    Are those metal filings all over the place or does your friend use glitter? Nov 3, 2023 at 6:48
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    @MarkMorganLloyd that's just dust from the garage, as it was very windy and blew everything around
    – andii1997
    Nov 3, 2023 at 9:37
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    Just checking, but I thought it a question worth asking in view of the extreme wear showing on the brakepads. Nov 3, 2023 at 16:13
  • @MarkMorganLloyd sure. The brake pads are new and were driven about half an hour. The extreme wear might be because only a small part of the pad is used
    – andii1997
    Nov 3, 2023 at 17:12

4 Answers 4

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The brake pads are too high and barely contacting the rotor, which is wrong. It's also dangerous and the bike shouldn't be used in this condition. If the wheel in the picture is properly in place in the dropouts, the problem is either that the wrong adapter was used, or an adapter was used where none should be. (The latter is not likely, because it appears that in this installation, not having an adapter would go past the "just right" point and instead the caliper would be too low. In other words it looks like it's missing roughly 10mm of rotor contact but taking off the adapter would drop it more than that. Note here that adapters basically make a radius correction in how far away the caliper is from the wheel center, but they're named in terms of the diameter correction they make in rotor sizes, so you have to bear that in mind at times in the following.)

The adapter in the picture is a post-to-post adapter of the type that offsets the mounting bolts for the caliper up instead of keeping them in line and simply using longer bolts. Thus it is likely a +40 or +43 adapter, which are typically all of the offset design, and +20 or +23 adapters are typically all of the inline type. You don't provide the fork model or rotor size, which would be helpful to know, but because it's typically only the +40/+43 designs that use the bolt offset, it's possible to get to near-certainty that it's the wrong adapter for the installation.

For reference, here is a picture of a common +40 adapter (TRP P40) and then a common +20 (TRP P20).

P40 P20

Most every MTB brake brand has adapters like this and they mostly do the same thing and look similar.

It's likely that what happened here is the original fork was a post mount 160 fork and the rotor is 200 or 203. So, if the rotor is a 200, it would have been set up with a +40 adapter, or a +43 if it was 203. Then it was switched to a post mount 180 fork by someone who is not a bike mechanic. "Post mount 180" means that 180 is the native rotor size that the fork would take with no adapter, so you would use only a +20 to go to 200mm, and so forth. The appropriate thing to do here that would have avoided this problem is take away the +40 or +43 and sell your friend the appropriate +20 or +23, plus longer bolts (usually included).

The other permutation we could be looking at is if the rotor is a 180, the new fork is PM160, the original fork was PM180, originally there was no adapter, and someone put that one on because they found they needed something to avoid physical interference, and it was all they had. Same basic problem.

It's important to understand that no aspect of the work done by the person who did this should be trusted for any length of time or under any circumstances. This error is almost impossible to let out the door by someone well-intentioned. All you have to do is look at it. Re-adjusting the brake is one of the fundamental parts of installing a new fork, and one can't just miss the problems here when doing so, even if one doesn't understand the nuances of brake adapter compatibility. You must absolutely second guess everything else about the installation, which includes whether the cut on the steerer was made correctly, whether the steerer/stem gap was set correctly, whether the stem hardware was handled and torqued properly, whether the stem height/position was recreated faithfully, whether the crown race made it on to the new fork properly and in good condition, headset adjustment, whether the wheel was installed correctly, caliper mounting bolts torqued, hose length appropriate, hose routed correctly, etc. Replacing a fork is a job with a lot of steps that are all pretty safety-critical. One mistake this bad is often accompanied by more. Getting eyes on the stem gap, the steerer cut, and the crown race is important and unfortunately may require a trip to a different shop.

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    Commenting here to state my agreement with the last paragraph. Find a better shop, explain to them what happened, and be prepared to pay for their time in exchange for a working bike and peace of mind that the bike is safe to ride.
    – Paul H
    Nov 2, 2023 at 18:35
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    Last paragraph is well put, I imagine required Herculean self-restraint.
    – mattnz
    Nov 2, 2023 at 23:59
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    Agree, that's really bad meching, it's fundamental stuff that would have reached someone's eyeballs, that fact that nothing was done about it since then... 🤦🏻‍♂️ Nov 3, 2023 at 4:18
  • I'm sure I remember on of my bikes it was possible to fit the adapter upside down, which moved the caliper relative to the disk. I swear I almost did it every time I changed pads - almost. Nov 3, 2023 at 9:27
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Yes, that caliper is mounted incorrectly. As others have noted, the pads are not contacting the rotor appropriately.

Perhaps, the caliper is mounted using an adapter where none is needed. I have had forks that do not require an adapter when used with the smaller rotor size. This seems to be the case, so, a probable solution is to mount the caliper directly to the fork posts. The other possible solution is to change the adapter, since some component combinations require different adapters. I have needed adapters that are none but 10mm post "extensions" and correspondingly longer bolts (or so I remember).

I would recommend not to ride the bike in this state and return as soon as possible to the store and demand they reinstall the caliper. Ask for a different technician to inspect the work done and also, inspect the installation of the other parts that where changed. This seems to me a mistake that an inexperienced technician would make, and if so, other mistakes may have happened in the rest of the bike.

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This does not look right at all. The pads are just gripping the very outer part of the disc as you yourself have noted. I'm not sure about the cause. Perhaps incompatible parts(after a swap of forks) or faulty assembly. Either way, this is no good.

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No, the brake pads are not contacting the disk in the right place - the pads are too far away from the disk.

Looking at the disk you can see where the pads use to contact the disk - the wear path of the old pads. This is where the new pads should also contact the disk.

Since the pads were replaced but the disks were not, the disk should be the original size and a new disk should not be needed.

It's possible that the wheel axle is not seated in the drop outs correctly preventing the disk from being in the correct place. If the wheel is seated correctly then the shop needs to make this right.

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  • Added a picture of the whole bike and the brake mount. The axle should be right, i guess it's an unnecessary or wrong adaptor.
    – andii1997
    Nov 4, 2023 at 11:07

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