Cause of severe brake judder?

Question

I've got a 2018 Canyon Exceed (XC mountain bike) that has recently developed alarming judder in the front brake (SRAM Level T), to the point that it makes the forks "twang" backwards and forwards, depending on speed and braking effort. The headset adjustment is fine, it's just that the forks are lightweight XC model (SID) and therefore a little flexy and they really don't like the brake pulsing.

Holding a constant, light pressure on the brake lever while rotating the wheel by hand reveals that the brake grabs as each of the rotor arms pass through the calliper. Inspecting the rotor shows that the pad coverage extends a surprisingly long way down the arm:

Same thing but different lighting:

I was surprised by the inconsistency of the pad surface too, which seems to correspond to which bits of pad the long cooling slots pass over:

The bikes got a little over 700km on it, most of them quite dry. The last 50 were wetter. The rotors are the originals, but are well away from their wear limit (1.55mm) and don't vary in thickness (1.87mm all the way around). I did wonder whether the calliper spacer was on the wrong way around, leading to the pads being closer to the wheel centre than they should be, but according to the direction arrow, it's correct (from factory).

I can't explain why this has suddenly become a problem. My trail bike has similar brakes (Level TL), also still on the original rotors, has lead a much harder life (3x the mileage, more severe terrain), pad also extends down the rotor arm, but it has never shown this problem. The same light-pressure, turn the wheel by hand test shows no hint of grabbing.

Is the solution just a new set of pads? Bedded in differently? How can I avoid this problem returning?

Answer 1

Some rotors have a very even distribution of contact with the pads. Others, like yours, don't. My rear, and old front, rotors are very even (diagonal slots in a pattern that mean all areas of the pad see almost the same amount of contact). The front rotor on my new dynamo wheel is uneven with the brakes on that bike, but nowhere near as much as yours I get a tiny bit of judder, more heard than felt, on that new front rotor.

The effect of uneven contact is first uneven wear, then juddering braking as the less worn parts of the pad come into contact with the rotor. If the pads are uneven, either in height or surface quality, it's likely to be these less worn parts that brake more than you'd expect. This might suddenly be noticeable after a particularly hard ride that glazes or abrades the pads where they're most heavily used.

The remedy is simply to resurface the pads (unless they're worn enough to be worth replacing). Fine sandpaper on a flat surface does a nice job, but be sure to remove all abrasive particles afterwards. Bedding in afterwards shouldn't be too onerous, as one significant part of bedding in is transferring pad material to the rotor, which is unaffected. However as most bedding in recipes involve planned hard braking, and that's a good way to test whether the brakes are behaving themselves, it's a good idea to do it on the first ride.

When replacing the rotors, if this is something that troubles you, it should be possible to get something that makes more even contact, probably just by avoiding those long circumferential slots.

Answer 2

Severe juddering causing movement in the fork when braking is unlikely to be because of the pads or rotors. I would recommend checking the tightness of your headset immediately. Even if the bolt doesn't seem loose while you're adjusting it, tighten it up a bit anyway, then take it for a ride to see if it solves your issue.