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Recently I hit a curb on my bike. I heard a cracking sound, my front disk brake (cable) felt stiff, and I could feel a lot of resistance from the front wheel. I left it alone for a few minutes, and when I came back, the brake was back to normal, but I could hear some grinding in the front bearings. I also soon realized that I couldn't stop as quickly as before. Today I took the bearing apart, cleaned it, applied fresh grease and the grinding disappeared. The brakes still were noticeably weaker. I tried smoothing the rotor with sandpaper, but that did not help. What could be the reason?

Additional information:
I was moving at around 25 km/h, the curb was about 3-5 cm
The axle is straight, the dropouts seem to be fine
The cable shroud seems to be a bit deformed, but that deformation does not change how far it can retract
I took the whole caliper apart, sanded the rotor and pads. After reassembling, I think it is a little better, but not as much as it was before. Or that's just placebo, I'm not sure.

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    Impossible to say, absent more information.
    – Daniel R Hicks
    Commented Apr 29, 2021 at 17:41
  • What kind of information? I'll happily add more info once I know what you need
    – adamski234
    Commented Apr 29, 2021 at 18:12
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    Some clear photos might help. Sounds like you had enough momentum in your collision to damage several parts of your bike. Must have been moving fast
    – Criggie ♦
    Commented Apr 29, 2021 at 19:33
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    The only time a similar thing happened to me, was after a crash, the brake cable housing had dislodged from its stop and ended somehow misplaced in a weird way, so I could not activate the brake. Afer a quick inspection I found and corrected the issue.A similar thing may have happened to your brake. Check cable routing and bake pad positioning.
    – Jahaziel
    Commented Apr 29, 2021 at 19:42
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    @WeiwenNg If there was any chance of that, it was corrected after reinstalling the wheel. It doesn't look shifted
    – adamski234
    Commented Apr 30, 2021 at 19:25

1 Answer 1

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Based on the second photo, I'd guess the rotor is no longer parallel to the brake caliper.

Thus instead of having flat contact between the pads and the rotor like

=| |x| |=

you now have smaller area of contact.

=/ |x| /=

Sanding the pads has rounded them off a little, increasing the area of contact a little.

The best fix I can come up with is re-aligning the caliper with the rotor.

  1. Take out the front wheel and fit a slip of paper over the rotor, then reinstall so the paper is in the caliper.
  2. Back off both bolts holding the caliper to the fork, enough so it can slide.
  3. Press and hold the brake lever on the handlebars and help the caliper find its center.
  4. Tighten the two bolts sufficiently (not excessively)
  5. Release the brake lever
  6. Spin the wheel to pop the paper out, and test. You're looking for no-rubbing and that the pads touch the rotor all-at-once.

Small changes will have a big effect on the brakes.

The problem may be if the little mounts that support the caliper have been bent, or if the caliper's mounting tabs have bent, or if the mounting bolts are bent.

You might need to replace both bolts if they are not straight.

I would not recommend attempting to straighten the caliper if its bent - they will probably break, or if not will be weakened.

If the fork's mount points are bent, you may be able to file/mill/sand the top surfaces to be flat and at right angles again, but it is hard to get perfect.

Upshot - avoid riding your bike into kerbs/curbs.

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    I tried re-aligning the caliper. Unfortunately, I couldn't notice an improvement. I think that the mounting tabs were bent. Guess I'll just have to limit my speed now. Thanks for the help though, it's much appreciated.
    – adamski234
    Commented May 3, 2021 at 14:04
  • @adamski234 why limit your speed? Keep working to solve the problem and improve the braking performance. I had a twitchy BB5 caliper that would never work well, so I replaced the whole thing with the bottom-grade shimano hydraulic pre-bled unit. It was night and day, and a relatively inexpensive upgrade. That presumes your damage is in the caliper. If its the rotor or wheel, rubbing or otherwise, need more diagnosis. Good luck!
    – Criggie ♦
    Commented May 31, 2021 at 2:36
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    With decreased braking performance, I won't be able to stop as quickly as I did before. So I can't really take as many risks as I would usually do. And I learned that the hard way just a few days ago, by nearly falling into water
    – adamski234
    Commented May 31, 2021 at 19:40
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    @adamski234 my point was more about fix the problem and then you can feel confident riding fast again.
    – Criggie ♦
    Commented May 31, 2021 at 19:42
  • I get that. However, the bike is quite old, and too small, so I'm already looking for a replacement. I don't really want to make investments into a bike I might replace this year. Though I guess I could fit the new parts into the new bike, and not be at loss
    – adamski234
    Commented May 31, 2021 at 19:45

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