Knocking sound from front disc brake?

[Chibbie]

Hi all,

My Vintage Electric Cafe bike has developed a metallic knocking sound (and accompanying pulsation that I can feel through the handlebars) coming from the front wheel when applying the front brake. It only happens with the front brake; if I apply the rear brake alone, I do not get the knocking sound or pulsation. It also doesn’t happen while coasting: only when applying the front brake. And the sound gets louder/stronger in proportion to how hard I’m apply the brakes. The brakes are “Promax Lucid Hydraulic disc brakes.”

I am a complete bike newbie, so I lack the tools and skills to try to diagnose or fix it myself. I’m nonetheless wondering what might be causing this, both for my own learning and so that I can tell my local bike shop what specific things to check.

Thanks in advance for any insights!

 

[AHicks]

Is there any lining left on the brake pads?

 

[Chibbie]

I wouldn’t know how to tell if there’s lining left on the brake pads, but the brake pads are brand new. The local bike shop just installed them last week (but the problem didn’t manifest until today, a week after the new pads were installed).

 

[JGcycle]

Sounds like you might have a warped brake rotor.

 

[scrambler]

Hang the bike, spin the front wheel freely while looking at the front rotor, and see if the rotor is flat and if it stays free of contact during rotation.

Then while spinning the wheel, slowly apply the front brake while looking at the rotor and caliper and try to see what generates the noise.

 

[Chibbie]

Thanks, y’all! (I’ll see if I can come up with a way to hang the bike: I don’t have a bike stand or hanging hooks).

 

[m@Robertson]

This happens on my Magura MT5's using Tektro downhill rotors in the most extreme, steady downhill use. We're talking slow descents where the hill is so steep I start leaning on the brakes while speed is still very slow and I am clamping the bejesus out of the levers to keep it that way.

There is nothing wrong with anything and this happens on multiple bikes going down the hill I am thinking of. It can be an absolutely brand new rotor and set of pads.

Its something that only occurs under these circumstances, and near as I can tell its the pads being jammed so hard against the rotors I am getting feedback from the pads intersecting with the rotor holes. Possibly the leading edges of the pads pressing in so hard they 'bump' the edge of the rotor hole and go just a hair inside of it, hence the mild thunking.

 

[PCeBiker]

I'm just guessing, but maybe your rotors got contaminated?
I know that can cause a pulsation but I don't know about an associated noise?
Cleaning the rotors with alcohol or acetone is simple and easy to do. (careful with the acetone, it eats paint)
Apparently, even fingerprints can contaminate a rotor.

 

[Chibbie]

This happens on my Magura MT5's using Tektro downhill rotors in the most extreme, steady downhill use. We're talking slow descents where the hill is so steep I start leaning on the brakes while speed is still very slow and I am clamping the bejesus out of the levers to keep it that way.

There is nothing wrong with anything and this happens on multiple bikes going down the hill I am thinking of. It can be an absolutely brand new rotor and set of pads.

Its something that only occurs under these circumstances, and near as I can tell its the pads being jammed so hard against the rotors I am getting feedback from the pads intersecting with the rotor holes. Possibly the leading edges of the pads pressing in so hard they 'bump' the edge of the rotor hole and go just a hair inside of it, hence the mild thunking.

Interesting! I was in fact going down a steep hill when this issue began.

Just so i understand: are you saying in your case that the thunking began in this steep downhill scenario and continued permanently thereafter? Or that it began in the steep downhill scenario but resolved itself later through normal usage of the brakes on relatively flat rides? I ask because in my case, the thunking continued once I was back riding on level ground. (I’m gonna go out now for a ride around the block to see if it’s still happening.)

 

[Dave Rocks]

I wouldn’t know how to tell if there’s lining left on the brake pads, but the brake pads are brand new. The local bike shop just installed them last week (but the problem didn’t manifest until today, a week after the new pads were installed).

Then the bike shop screwed up your front brake.

 

[Chibbie]

Then the bike shop screwed up your front brake.

That’s certainly possible and occurred to me too. But then, wouldn’t the problem have manifested right away rather than a week later?