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  1. #1
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    Bonking ... not feelin' well Pulsing disk brake

    Hello,

    Last week I purchased a 2012 Kona Kahuna (brand new from shop).

    Been riding it almost every day. Today after a high speed descent and hard braking (which was fine) any
    subsequent braking results in "pulsing".

    I have Avid Elixir I that came with the bike. Mind you there is no sound except for the fork being knocked back and forth so its not the vibration I have seen people describe. Its a pulsing that causes my entire front fork bend back and forth as if I was rapidly pulling and releasing the brake.

    The pulsing happens multiple times per revolution. Almost like matching the patter on the disk.

    I tried cleaning the rotors and pads. The rotor is not warped at all, at no point does it even wobble a bit when
    I spin the wheel (and of course never touches the pads).

    The rotor seems to be lined up (visually at least) between the pads and its exactly in the midle of the two pads as well.

    Any ideas?

    I have had absolutely no problems with it up until now.

  2. #2
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    BLEGH!

    I got rid of my bso precisely to avoid wasting time with this kind of crap....

    After looking more at the bike I realized that the tire on the fork was almost touching the left side of the lower..... Upon checking the quick release it was, loose to say the least. I am surprised the wheel didnt eject.... I never touched it so I am guessing the LBS where I got it didn't tighten it properly.

    So I tightened the quick-release and re-aligned the pads but its still pulsing and now its giving this chirping sound as well. I am guessing the whole wheel/rotor being tilted from being pushed up and out on the left side quick release destroyed a good chunk of the pads.

    So I am guessing I'll have to buy new pads, or maybe I will try to get the LBS do it at no charge. They should really have tightened that quick-release properly...

  3. #3
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    I have the same issue going on right now as well! The rear disc is doing the same too... Same bike as you and same year... Looking to see what is going on. After I cleaned the pads they were ok for about a mile and they started to do the same thing. Headed to the LBS tomorrow to see what is going on. Ill let you know what they say.

  4. #4
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    Fixed!

    Mine was fixed last week. Took it to a different shop and the guy did it for $15. Fixed the first time. He said it was a very basic probelm and was shocked the other place couldnt fix it. A couple things I noticed along the way.

    When I got the bike back from the place I bought it from and couldn't fix it one of the pads was advanced further than the other and always rubbing. Supposedly this prevented the pads from properly being centered during the caliper adjustment.

    The pads were re-sanded flat. Rotors cleaned with alcohol. Regular cleaning is not recomended though, only of needed. Pad material depositing on the rotor is a good thing. Makes the breaks work better and removing it can cause problems (which I did).

    After the partial ejection all the paint from the fork dropout on the left side was ripped off, requiring a readjustment of the calipers. Supposedly the thick paint layer being removed changed things enough.

    The guy who fixed it didn't tell me much more.


    The pad advancement problem I had since then again, after removing the wheel myself. I figured out how to fix it. I found a wedge of perfect size to push the pads back to starting position without touching the padding material. I can take pictures if you want me to show how to do this. The pads are self adjusting and it seems the slightest bump on the break level while the wheel is not on will cause one of my pads to advance too far requiring this manual reset of them being pushed back. This seems to be problem one. Once its advanced further then the calibration is not good any more, causing one of the pads to dig into the disk pattern more, causing the pulsation. Fortunately its a very easy and quick fix so even at a trailhead after pulling the bike out of the car it should be completely doable, and its something that should be checked every time QR is used.

    Re-adjusting the calipers is definetly something you can try too yourself, plenty of videos on youtube, just make sure the pads have equal "extension" on both pads. If one sticks out further than the other, refer to previous paragraph. Again like cleaning, it shouldnt be done unless there actually is a problem.

    On a side note this new mechanic seemed to have left the QR looser than I like as well which I found interesting. I like to have the QR start "gripping" when its in line with the axle. Maybe this is not right? I don't know. But for both mechanics grip started at a 30-45 degrees from parallel to the axle. By grip I mean when you just feel the QR mechanisms latch just touch the fork on both sides.

    Since then the front breaks are perfect. The rear one started singing though. Its not the the pulsating type though, but more like a tuning fork type sound. This might simply be normal and possibly fixed by changing the resonant frequency of the rear triangle or calipers by attaching pieces of rubber plus possibly re-calibration of the calipers. I'll let you know how this goes.

    Meanwhile if you can get more info out of the mechanic to see what he does to resolve it it might be helpful. Don't get suckered into a bleed though, its probably unnecessary.

  5. #5
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    Wow thanks for the in depth reply!!!!!

    I am pretty mechanically inclined and the brake rotors and calipers are just like auto brakes. It seems as though I tried everything that you posted except sanding the pads flat which now makes sense.

    Also about the fork QR... My paint has worn pretty good over the last year and a half where the QR mounts I the fork. It now seems that when I grab a lot if front brake the wheel actually moves in the fork.. Could be an optical illusion but I have had it rub the brake line before from flexing. Not sure if that is just how much the fork flexes or what.

    My LbS is super cool and the mech I'm sure will let me know what he did. I have done all maintenance on it and they even complimented on how clean my bike still is. I do ride the heck out of it too. I left the bike at the shop tonight and should get it back tomorrow or Thursday. I will keep you updated.

    Still not sure what is up with the rear but the mech rode it and couldn't believe how bad it was! He thought the rotor was cracked as bad as it was vibrating and pulsating... We will see.

    Thanks again for the help. If you could post a few pics of that wedge that might help others as well. It is just compressing the pistons back and then re centering them but others may want to see as well.

    Next upgrade is a new fork for sure!

  6. #6
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    Just for reference, there are generally two things that will cause pulsing brakes, warped rotors or rotors with thickness variations. Thickness variations can be caused by excessive pad material deposited in one spot; or, in the case of cars, rust building up everywhere except where the pad is. When the rust is knocked down the rotor ends up thicker where the pad was.

    If cleaning the rotor fixed the issue it's likely you had an area of thick pad material buidup on the rotor (this is why they tell you not to come to a complete stop when bedding in certain rotors).
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  7. #7
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    These are pretty obvious problems.

    Mind you my bike is brand new, the disk has absolutely no visible wobble when I spin the wheels, and the thickness is even too.

    It started doing it again. The QR came loose again... Fixed that and after a couple brakings the pulsation went away. If it was the thickness it would never go away.

    T-Byrd any news?

    Quote Originally Posted by scatterbrained View Post
    Just for reference, there are generally two things that will cause pulsing brakes, warped rotors or rotors with thickness variations. Thickness variations can be caused by excessive pad material deposited in one spot; or, in the case of cars, rust building up everywhere except where the pad is. When the rust is knocked down the rotor ends up thicker where the pad was.

    If cleaning the rotor fixed the issue it's likely you had an area of thick pad material buidup on the rotor (this is why they tell you not to come to a complete stop when bedding in certain rotors).

  8. #8
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    Battling with the LBS right now... The mechs acted like they have never seen this before. They cleaned them and said there is nothing else wrong. I called an hour later letting then know they were squealing again. They said oh, well we need to send them to avid then. Bring the bike in.. I have a ride this weekend so I will take it in next week.. Prolly going to look at upgrading to shimano sxt's. Not happy with how the LbS is luring to me about the known avid issue though.

  9. #9
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    This is why I don,t use QR,s on a mountain bike nor would I let someone else's build go un checked. When I had QR wheels they were checked before each ride. Also I would only run Shimano XT QR.

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