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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:

front brake rotor

Same thing but different lighting:

enter image description here

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:

worn brake pad

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.

same rotor, different bike

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

  • Do you know what pad material you have? How much thickness left? Same material on both bikes? Terrain may be less important than conditions when it comes to wear. Testing new pads is cheap. – Chris H Nov 18 '18 at 13:16
  • ... In fact you may be able to try the back pads on the front as a test, or sand down the current front pads – Chris H Nov 18 '18 at 13:25
  • There's plenty of thickness left on the Canyon. The other bike's on maybe it's 3rd set of Uberbikes pads, so definitely different. I might try the front to back swap first and if that doesn't help, move to new pads. – Chris Nov 18 '18 at 14:11
  • Trying the back pads on the front (on the stand) sounds like a good start, and if that helps, sanding down the front pads might be good. Unless you've got a very convenient bike shop that stocks the right pads, I'd want a spare set handy – Chris H Nov 18 '18 at 14:15
  • I went straight for the sanding option (sandpaper on a large flat surface) and it now passes the turns-smoothly-by-hand test, but I haven't had a chance to bed-in again and ride for real. I do have a small stock of Uber pads, but would like to understand why the originals had "gone bad" with so much life left in them. – Chris Nov 18 '18 at 19:46
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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.

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    Isn't it quite bad rotor design when the pads wear unevenly? Why would engineers do that on purpose? – Nobody Nov 19 '18 at 11:38
  • @Nobody I'm inclined to agree with you. I can understand prioritising cooling, but this seems like prioritising style to me. Or maybe they just want to sell more pads. I run metal pads on my tourer and don't do much mileage on my MTB, so haven't found it to be much of an issue myself – Chris H Nov 19 '18 at 11:47
  • Is there a resolution with the problem with sanding the pads? This is a very common brake/ pad/ rotor combination and a very uncommon issue so I'm interested if it has been resolved. – DWGKNZ Nov 20 '18 at 0:34
  • As wrong as it looks, it does seem to be deliberate - this is SRAM brakes on SRAM rotors on SRAM (well, RockShox) forks with factory adaptors. I have a similar setup on bikes from two un-related manufacturers (SID + Level T on one, Yari + Level TL on the other) and they're both using the same (original) rotors. The trail bike sees more varied conditions and gets through pads quicker. The XC one did most of it's miles completely dry, then did an atrociously wet and often gritty ride - maybe somehow that accounts for weird pad state? Sanding the pads has made them consistent again. – Chris Nov 20 '18 at 10:29
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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.

  • There was no play in the headset (tested by grasping the top headset cup while rocking the bike on the front brake), but a little more pre-load perhaps wouldn't do any harm. However, this absolutely was a pads+rotors problem. During turn-the-wheel-with-constant-brake-pressure test I describe earlier, the contrast in braking force between when the pad was between rotor arms and when it was covering a rotor arm was huge. Whatever circumstances had caused the striped appearance of the pads meant the pad on the main rotor was very ineffective in comparison to the pad on the rotor arm. – Chris Nov 20 '18 at 10:21

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