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The front disc brake grinds after a light accident with full braking and plunge.

To give an idea of the forces involved:

The bike fell to the left, mostly cushioned by me. The bike had no more linear momentum. The brake lever and handlebar band has minor scratches. The handlebar or stem is slightly skewed.

  1. It look like the wheel/rim is not deformed/buckled, it's not visibly wobbling.
  2. The disc is slightly deformed, wobbling at most a couple of 100µm. The bending cause by the pads looks more than a millimeter.
  3. The gaps between disc brake pads are not even, one pad is always touching the disc. The disc seems to be not centered in the brake.
  4. The handlebar is slightly skewed (de-justified stem/steer tube).

The animation shows the brake photographed from the front toggling with brake released and slightly, but not fully pressed:

Animation: detail shot of the brake toggling between braked, slightly braked

From the animation you can see that the brake pads are not evenly spaced. On the left (inner) side there's a (wide) gap, while on the right (outer) side the pad is already touching the disc. Also, the disc is being pushed and bent by the outer pad moving.

(Note that the animation was created from two photos taken with the bike standing but being unfixated and the levers being operated. This causes some minor movement of the whole bike. If you think this makes the judgement impossible, tell me and I'll try to make a better pair of pictures.)

The animation shows the wheel spinning in 90° steps, photographed with sharp angle to (along) the fork:

Animation: wheel spinning in 90° steps - the disc is wobbling slightly

The wobbling is quite visible.

There's no more than these scratches caused by the plunge:

Minor scratches on the handlebar and levers

Technical data

  • Gravel bike
  • Carbon fork
  • Through-axle
  • Aluminium drop bar
  • Shimano 105 BR-R7070 with dual pistons
  • 1200 km driven, 4 months old
  • Bike: 10 kg
  • Saddle bag and water bottle: 3 kg
  • Me including backpack: 65 kg

Course of events

This is after full braking and a - presumably - light plunge from standing. The front wheel locked, the rear wheel lifted off the ground. The bike was upside down (angled 45° - 90°) and I had the saddle in my spine before I fell fore- and sidewards.

So I pushed downwards and sidewards (along the axle, not in driving direction) into the stem with my body weight and a bit remaining momentum. There probably was torque by the fork into the axle.

Questions and guidance

Is is possible that the braking energy itself might have

  • deformed the brake?
  • deformed the disc?
  • damaged the wheel hub?
  • deformed/invisibly fractured the carbon fork?

I'm pretty sure the bike pluging did no major damage.

What was stressed most during braking?

What components shall I replace (apart from re-justifying) to be safe?

Apart from asking a professional bicycle technican for an estimate, are there things I can check for myself?

Might the fork be damaged by the aforementioned forces?

More background

This happened during a light traffic accident.

Ask for details

Ask for details, more part numbers, measurements or other photos (i.e. of the scratches) if required to gauge this. If you think there's too much distracting information, tell me and I'll remove it.

History of later clarifications/additions:

Clarified: double pistons. Rephrased/deduplicated "Course of events" section.

  • Can't the brake be just shifted? Did you try to reset it? – Vladimir F Oct 27 '19 at 18:41
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    +1 for great photos. – Criggie Oct 27 '19 at 18:52
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    Excellently constructed question (IMHO) and the photo-animations prove helpful. Wonderful idea, there. – Jeff Oct 27 '19 at 20:51
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Two possibilities - the wheel itself have shifted, unlikely with a though axle though. The crash does not seem to have be bad enough for a major damage like deformed hub to damaged carbon fork (Carbon is far tougher than many people make out and tends to fail catastrophically). However, if you have concerns over the fork, get it checked by a bike shop. While the risk of damage might be slim, the consequences if the fork fails are about as bad as it gets.

Its likely that the caliper has taken a knock and shifted on its mounts. All that will be required if this is the case is aligning the caliper. Refer Park tools website for detailed instructions https://www.parktool.com/blog/repair-help/hydraulic-disc-brake-alignment

The short version is to loosen the caliper bolts, apply the brakes gently hile spinning the shell, as you tighten the bolts slowly and carefully. This will center the caliper and and alight the pads with the disk.

  • 2
    No shift of the caliper can explain a wobble of the disk. A wobble of the disk must mean that the disk is out of alignment with its (the wheel's) axle. – Craig McQueen Oct 28 '19 at 2:51
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    The amount of wobble I see is probably not enough to cause a correctly aligned caliper to rub, so if the caliper has moved, it might have been like that before the prang. The only way to know is to (try to) align the caliper. If that cannot be done with rubbing, the disc may need truing (see park tool "How to true a disc rotor - youtube.com/watch?v=O0c2Ez2v0PU) – mattnz Oct 28 '19 at 3:18
  • I might take measurements or macro photographs of the brake to judge whether the disc is still centered within the caliper but the pads are unequally worn or the pistons unequally shifted. Because it might be the case the disc - although wobbling in 100s of micrometers - is actually centered, isn't it? – try-catch-finally Oct 29 '19 at 5:25
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    @try-catch-finally I think you overestimate the manufacturing precision of bicycles. Caliper brake pads can move based on oil pressure and resistance against the seal. You just need one seal to be stiffer (or dirty) to have some unevenness. Baring no clear frame damage, I would simply re-align the caliper and get the park tool to straighten the rotor then go ride it. Rotors get knocked and become out of true all the time. It is a bicycle not a space craft. – Rider_X Oct 29 '19 at 17:19
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Based on your description of the accident (thankfully it wasn't more severe to you or the bike), especially the lack of contact save the left sided ground landing, the odds are quite favorable that the carbon fork is unharmed structurally. A carbon composite can become weakened if it's taken an impact which has broken the resin bond between fibers. This delamination results in weakened areas as delamination can spread out from the area initially damaged.

I'll defer to others here to elucidate the physics involved, but generally, the braking forces acting upon the fork are concentrated along the forward, leading edge and the backside, trailing edge of the fork and it's steerer tube. The emergency braking resulted in enough force to momentarily defeat gravity and begin a loaded rotation around the hub/rotor area, but carbon composite forks are designed (significantly, by direction of weave and thickness of laminate at fore and aft edges) to withstand far greater force than this. The absence of plasticity in carbon fiber means the fork---all carbon aspects of the fork if not full carbon--is in no way deformed from the powerful braking episode. The presence of a thru axle was helpful to the scenario, allowing force to be spread over a larger area as well as diminishing potentially damaging twisting forces upon the fork.

Regarding the rotor: take it off and have it trued. Reinstall and then go about the simple, straight forward process of realigning the caliper. Here's a Park Tool article that covers the mechanical details that need to be taken to do those jobs. A note about the caliper: certainly something significant could have affected the caliper because of the extreme braking instance but it's much more likely to be unharmed. Probably designed to tolerate far greater forces, the caliper wouldn't even wear out much faster if every brake activation generated the degree of force experienced here.

Finally, I offer you this q&a from this site that may guide you to the next steps to take in order to build confidence that all is well.

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Thanks to Jeff and Matt for your thorough answers.

Here's a report of the repair ...

Repair results

  1. The brake was tightly bolted to the fork, the screws weren't loose at all. After partially undoing the screws I realigned the brake by pulling the brake levers. That did it already, the disc was not grinding anymore. I had to spin the wheel and carefully brake it to a stop for proper realignment, static braking didn't do it.

  2. The brake pads looked good, evenly worn and intact.

  3. Then I checked the pistons. They had a bit dust around them. The caliper and pistons were cleaned using Q-tips, a towel and alcohol. The pistons were pushed back in using a plastic tool. I exercised them by braking and pushing them back. After that they (still) moved evenly on both sides.

  4. Screwing the through axle back in cause a squeaking sound and wasn't that easy. I've cleaned the inside thread from minor burr with a brush and the axle was now happier. A small portion of the inside threads looked a bit "blunt" but not all of them.

  5. The handlebar was realigned. In the process I made two silly mistakes:

    1. Using too much torque on the steering tube clamp bolts
    2. Not asking for clarification beforehand
  • Thank you very much for posting your results! – gschenk Jan 18 at 22:31
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That rotor is warped. This is a sign that it got too hot. Replace the disk. It sounds like you came within a hair's breadth of serious injury. If it was me, every time I used that brake I would be wondering if this was the time the rotor would fail. Every little noise or vibration would eat at me. You needed every bit of brake performance you had, and that will never be as good or as reliable as it was before until you fix the damage. If the pads are replaceable do them too. Your safety and peace of mind is worth it.

  • 1
    This seems a bit overwrought; there are tools for straightening a bent rotor, so a bit of warping is not cause for throwing it away. Besides it doesn't need a lot of lateral strength anyway. (In fact, unless it's perfectly centred and you have opposed-piston brakes, it will flex sideways every time you brake.) Are you confusing the rotor with the pads? – DavidW Oct 29 '19 at 14:05

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