Bicycles Asked on July 23, 2021
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.
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.
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.
Answered by Nathan Knutson on July 23, 2021
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.
Answered by Andrew Henle on July 23, 2021
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:
Relatively unlikely causes of the oscillation:
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.
Answered by Criggie on July 23, 2021
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