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My bike is equipped with disk-brakes. It also has a fork with suspension.

When I apply the front brake, my fork vibrates violently, as can be seen below:

I've adjusted the brake-pads, the cable is in good working order.

Is this shudder caused by a faulty brake or a faulty fork?

The shaking happens at any speed, by the way, and any brake-pressure (other than feather-light brake.)

brake is Shimano, cable-operated.

bike is eWind classic.

fork is TGs 068E "Top Gun."

UPDATE:

  • When flipped upside down, the braking seems fine, but I am not sure if this is a fair comparison? When riding, the inertia of forward moving mass of the bike will resist the brake. When on ground, there is no such force.

  • There is no free play in the steering head or fork tubes, that I can see.

  • The wheel is true.

  • The calipers are securely fixed to the frame.

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UPDATE 2:

The test suggested by @andrew-hendle:

UPDATE 3:

I investigated this some more, and closely watched the shaking fork while I'm riding and braking.

It turns out that top-side, at the frame, the fork is perfectly still.

Only the bottom-end of the fork, at the wheel, does it violently shake, forwards, and backwards.

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  • when you flip the bike upside down and use your hand to turn the wheel, does it do the same or similar thing when you apply the brake then? Is the rotor secure? Is the brake caliper secure? Are the pads clamping on the rotor in the correct place/angle? Are the pads and rotor clean? Is your wheel true?
    – Armand
    Jul 14 at 3:31
  • It sounds like someone put a playingcard in between the spokes..
    – Luuklag
    Jul 14 at 9:44
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    Whatever you do, do not ride this bike until the issue is fixed: Such strong vibrations have the tendency to induce fatigue into the material rather quickly. And you definitely do not want your fork to fail. Jul 14 at 11:21
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    Can you please post a picture of your fork and caliper? I recently answered a question where the issue might be somehow related or similar to yours. See the first half of: bicycles.stackexchange.com/a/77483/2177
    – Jahaziel
    Jul 14 at 18:02
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    Did this happen all the time with this bike or start at since point? Did it start at once or slowly develop?
    – gschenk
    Jul 14 at 20:13
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Insufficiently rigid disc forks can always shudder. In this case it looks like there's a ton of bushing play. If someone wants to post an accurate treatise on how exactly the feedback loop is working, they would certainly deserve the win more, but: there is some kind of feedback loop from things flexing when you actuate the brake that causes the catch and release to happen at high frequency, which is another wording for fork shutter. You need a fork that sufficently transcends fork-shaped-object status to eliminate the problem. It would seem to not be the caliper's fault because all it's doing wrong is squeezing the rotor good enough to cause the problem. If there was something going on with the brake causing it to grab unevenly in different parts of the rotors rotation, maybe you could get it to do something like this, but that would be unusual and extreme.

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Check for something loose. Something is moving or flexing.

Is the brake mounted tightly? Are all the brake components properly torqued?

Is the headset loose? Hold the bike upright on its wheels, squeeze the front brake to hold the front wheel in place. Push and pull forward and backward on the saddle. Does the frame move back and forth more than a very small number of millimeters because of slop in the headset? It shouldn't.

Are there any loose spokes? Check the tension on all the spokes on your wheels.

There's a good chance the shuddering is caused by something loose.

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  • 2
    The clip is a bit short to isolate the source. I'm thinking of a loose headset.
    – Carel
    Jul 14 at 15:51
  • I added a video of the test you proposed. It is unclear to me if it passed or failed the test? Thx.
    – Bram
    Jul 15 at 2:46
  • @Bram That rattling sound you hear in that video? Something is loose, and it sounds like a loose headset. That would go a long way towards explaining the shudder. But it could be something else - make sure that there's nothing else loose on your bike that could make a noise like that, such as a hanging saddle bag. Jul 15 at 12:02
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You have "play" in the system somewhere.

The brake is working fine in stopping the wheel when the bike is upside-down, but the wheel alone weighs far less than the rest of the bike plus you.

So, when the brake stops the wheel, it moves backward slightly, which flexes the area with play, changing the bike's geometry, which releases the play, and sets up an OSCILLATION that's what you're feeling instead of a VIBRATION

I suspect two main possibilities:

  1. there is wear in the fork, so the lower stanchions move around inside the uppers. If the fork was not serviced for a long time (or ever), then this could be wear in bushings. I also observe rust on the chromed part - that's a sign of a long hard life and torn seals allowing water in. Your test of compressing the fork shows that it doesn't compress very much at all.
    Test for this by getting an assistant to squeeze the front brake lever, and rock the bike forward and backward. Meanwhile hold your hand around the chromed parts and the lowers. If you feel play, thats a cause or a contributing factor.
    A fork service might fix this, but could cost as much as a similar low-end fork new.
  2. Headset bearings loose. The top and bottom of your heatset tube have a ring of bearings, the bottom holds your weight and the top one provides compression/preload. Again, this could set up an oscillation when under front braking.
    The test is the same as previous, but hold your hand around the stem, where it enters your bikeframe's headtube. There should be no/minimal play. If you feel it rocking like its two parts, then its too loose.
    Fix here is to tighten the preload.

Relatively unlikely causes of the oscillation:

  1. I've seen this exact behaviour on an old rim brake bike, when the brake pads were super old and hardened. The disk brakes have a higher pressure and probably won't suffer the same way, but do take the pads out and visually check them for metal particles or strange wear.
  2. Check for a loose front bearing by wiggling the tyre sideways. It shouldn't move at all.
  3. Examine the frame for damage, like cracks. Lacking a structural member or brace could certainly give rise to an oscillation, but you'd definitely notice this.
  4. Just confirm your front light mount and fender/mudguard is tight. They look fine, but they're also right where the problem is and any looseness can contribute to oscillations.

I agree with other suggestions that this bike needs to be fixed before you ride it again. As-is, a hard brake could act unexpectedly, and depending on the cause, it could get worse real quick.

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