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  1. #1
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    Need troubleshooting advice

    Ride a Fisher Cobia 29'er, it's about 4 years old and I ride almost everyday. I weigh about 230 and frequently ride rocky, rooty trails, and it's not uncommon for me to be bunny-hopping logs and ledges and such...

    Now, I've been having a real hard time with my rear derailleur lately, when I'm on techy uphills (lots of rocks) or trying to come out of a flowy section, as soon as I really start putting it to the pedals, my gears are all over the place. Up a gear, down a gear, up 2 , down 2,... it's rather irritating. My rear wheel seems true, and has forever. I run wide enough tires that any deflection would cause the tire to rub the frame.

    When on the stand or in parking lot, shifts are perfect, and the derailleur is lining up where it's suppose to be.

    Also, there is a fresh wear spot on the frame, adjacent to the rear rotor.

    Any ideas? I blew the fork up on this bike 2 weeks ago and my 2nd set of bottom bracket bearings are completely shot. Replacements are in my garage waiting for installation. Frame? Wheel?

  2. #2
    turtles make me hot
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    I guess it's possible you have a cracked frame or you're possibly flexing it enough to change gears. I would remove the rear wheel and do a careful visual inspection of the frame. If all checks out ok, go ahead and do the bottom bracket and make sure the rear wheel bearings are good. Make sure the derailler cable is ok, the derailler swings smoothly... Lots of stuff to check back there.
    I built my daughter a Cobia and that frame is pretty light. I think if it were cracked, it would fail altogether with you hammerin' on it at 230 pounds.
    BTW, I ruined a couple of bottom brackets too. I switched to Chris King and bought the grease tool. Had the same one for a couple of years now.
    Last edited by NYrr496; 09-17-2011 at 08:50 AM.
    I like turtles

  3. #3
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    Got the wheel off last night and the hub looks fine. There has been significant rubbing of the rear rotor on the frame (paint still stuck to the disc), as well as the tire on the frame on both sides. I couldn't find any cracks, the derailleur hanger looks straight, and the derailleur isn't bent and doesn't appear to have taken a hit of any kind. I'm beginning to think that my rear triangle is just flexing under load.

  4. #4
    turtles make me hot
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    Actually, I just had a thought. How's the spoke tension on your rear wheel? That could move your tire all over. Not so much the brake or the shifting...
    I like turtles

  5. #5
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    Quote Originally Posted by NYrr496 View Post
    Actually, I just had a thought. How's the spoke tension on your rear wheel? That could move your tire all over. Not so much the brake or the shifting...
    Dunno, to be honest, I've not had it trued in over a year.

  6. #6
    @adelorenzo
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    It's not in the spokes... It's in the hub or the dropouts. Broken axle? Cracked dropout? I've never heard of a rotor rubbing a frame before.

  7. #7
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    Well, I took apart the hub greased the bearings, put it back together. Everything looks fine. I can't find any cracks or anything either. I think I'm just going to put it back together and ride it until something breaks. In the meantime I'll start accumulating my "new frame" fund.

  8. #8
    Former Bike Wrench
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    If your rotor is rubbing your frame, then something is broken or worn. Check the drop outs very carefully for cracks. Look at where the axle sits, is it a tight fit or are they "hollowed" out like the axle has been moving around there. Check the chainstays near the bottom bracket for cracks there, a very common spot that could cause flex.

  9. #9
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    Found the issue. Rear hub was loose enough to cause some movement under load, but not enough to show on the stand. I rode it another couple weeks, and the rear hub became noticeably very loose (could feel play by hand). Tightened up the hub and everything is fine for now.

    Thanks again for the advice. It was much appreciated.

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