Tektro hydraulic disc brakes chatter under load

Question

I bought a second-hand bike in good condition, however the hydraulic brakes chatter under braking load.

Check it out: https://youtube.com/shorts/hdDmeGFhjgk?feature=share

It's a Kona Splice with Tektro hydraulic brakes.

The front rotor vibrates worse than the rear, but both suffer this problem.

I have properly cleaned with IPA and sanded off the glazing on the pads, as well as the rotors. The problem here is that the chattering affects the bedding in process, so I've not been able to improve the feel.

It feels like the pads are slipping under braking, but the rotor bolts are tight.

Some online posts mention a loose headset could be the cause, however my rear suffers in the same way, so it's not so likely.

I have front suspension forks, but the chatter occurs when locked out or open.

I haven't touched the brake reservoir yet, so I'm hopeful that it's just a fluid issue.

Could it be caused simply by low fluid or air in the system? Will a full re-bleed fix it?

Answer 1

The cause of brake chatter can vary, and you'll need to do some troubleshooting to pinpoint the underlying cause.

First, is the brake track uniform along the entire circumference? If it's been sitting unused for a while, the surface might have oxidized where the pads were, causing a local change in the friction coefficient and therefore brake chatter. Small rust buildup should clear after a few brake usages, but more persistent contaminants might not be affected.

Second, check if the rotors run true. If they are bent, the force between the rotors and the pads will vary during a single revolution, which might be enough for them to start slipping and sliding.

Third, make sure that the rotors' thickness does not vary along the circumference. It might be the case that the brakes have not been bedded in correctly, causing uneven brake pad material transfer onto the rotor, in which case you simply need to swap the rotors and pads.

Also, make sure that the wheel axles are tightened correctly and that there's no excessive play in the wheel bearings. It would also help to know if the occurrence of chatter depends on the braking force applied. Does the chatter start only above a certain threshold, or is present even during very light braking?

Anyway, I doubt the problem is related to air in the brake lines - that would present itself with a mushy feeling of the lever and decreased stopping power. And it definitely isn't caused by the presence of the fork.

Answer 2

Upon further inspection, it appears the chattering was caused by loose hub bearings.

When the brakes were applied, I was able to move the front wheel laterally in the forks and when unmounted, both front and back hub nuts were easily unscrewed by hand.

I have tightened them enough just to verify the fix and can confirm the brakes are more responsive, without chatter.

I have no idea how long the bike was run like this before I bought it, but I saw metallic particles in the bearing grease, caused likely from the play in the hub.

I will research how to check the condition of the bearings, and hopefully repack them with grease to save the cost of buying replacements!