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  1. #1
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    Cracks in chainring?

    Hi all,

    I hope someone can help me out with something weird with a relatively new bike I have. I bought it from a LBS and have done about 500 km on it already.

    Fairly early on, it developed a very strange sound, like a rumbling or crunching sound in the driveline. I sent the bike back to the shop and 3 mechanics took a look at the bike. 1 decided to retighten the cassette, 1 retightens the bottom bracket and 1 says the pedal housing is cracked and replaces the pedals. Ok, so I took the bike back and on the 1st ride, notice the noise was still there. Since the LBS was a bit ways away, I deicided to just live with the problem for the time being since they have already checked the major components. But for the past few days, the sound seem to have worsten quite a lot (gotten louder) so I decided to do some of my own checking and this was what I found:











    There are 4 cracks are almost symetrical around the middle chainring. About 1-2 teeth difference. The pictured one is the largest of the 4. I'm not really sure are they part of the chainring design or not because these are newer ones with funny patterns stamped into them.

    Also, not really sure about this too but I don't think its normal. When the chain sits on the chainring, I can see daylight through the teeth almost around the chainring. The last time I saw this on me old bike, the chain was really worn. But this bike only has 500 km done on all these components. I only noticed this when I took the chainguard off.

    Comments and advice much appreciated.
    Bert.

  2. #2
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    The stamped chainrings do have some weird cutouts around certain teeth presumably to help with shifting. Your first two pics look normal but the last macro pic (good shot by the way) seems to be showing a bent tooth. Does your bike make the rumbling/crunching noise in all gear combination or just certain ones? Given the shape of that one tooth, I would only expect the noise to occur when using the middle chainring.

    However, there is the possibility that the problem all comes down to a misaligned rear derailler, either because the cable lost tension during break-in or because of a bent hanger. If you could take another macro shot of the position of the rear derailler when in the middle of the cassette we might be able to see if it's misaligned with the cog.

  3. #3
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    Hi joejack,

    The noise happens in all gears that I frequently use, but I can't be sure if it happens in all gears because I usually only use the big ring and the last 5 cogs behind during my normal riding. I hardly ever use the middle ring or any of the bigger cogs on the 8 speed cassette. The big ring however is welded onto the middle ring close to where the 'cracks' are. The rumbling/crunching can be felt through the pedals.

    Pic of the rear derailleur:


  4. #4
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    The alignment of the rear derailler looks reasonable and even if it was slightly off, it wouldn't cause a sensation that could be felt through the pedals.

    One other thing you can try and slipping the chain off the chainring and spinning the crank on it's own. Does it feel rough? Also try grabbing both crankarms and checking for play in the bottom bracket. If the cranks have significant movement towards and away from the bike frame and/or you get a rough feeling while spinning the cranks, you have a problem with the bottom bracket.

  5. #5
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    Hi again joejack,

    I've alraedy done all the loose/play checks prior to this to rule out any of the LBS people mis-fixes. The bottom bracket is solid all the way round. I've also free spun the cranks, there was a degree of harshness in the driveline with ticks and clicks every now and then, but that was with the chain on. The harshness didn't go away even if I freshly oil the chain. With the chain off, the cranks free spin butter smooth.

  6. #6
    DOS
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    I dont think its the chainrings. A couple of other possibilities: Make sure wheel is seating properly in dropouts; I recently had a problem where I was getting rumbling and it turned out that my wheel was not cenetered. Also, you might try running a different rear wheel to see if the problem is with the cassette or freehub. Even though the shop tightened up the cassette, since the problem has gotten worse, perhaps cassette us working itself loose somehow (e.g. defective lock ring).

  7. #7
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    In my opinion, the derailleur pic shows a hanger that is definitely bent enough to produce all sorts of noises and/or sensations. A bent hanger twists the chain in addition to encouraging the teeth of the guide pulley and the cassette cogs to hit the insides of the plates of the chain, which would explain a consistent rumbling. A DAG would I'm sure reveal significant misalignment. At the very least it should be addressed before you shift into your spokes. Your chainrings are perfectly normal.

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    DOS
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    Quote Originally Posted by nonfortuna View Post
    In my opinion, the derailleur pic shows a hanger that is definitely bent enough to produce all sorts of noises and/or sensations.
    Its hard to tell in the photo, but now that you point it out, the hanger does look to be bent in toward the wheel.

  9. #9
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    Put it on a stand. Physically look and watch the chain slowly mesh with your cogs and chainrings. If you go slow enough, you should be able to see if there is some improper alignment. I actually worked an hour on this yesterday on a bike. The chain was catching up on ONE tooth, in ONE gear. It was slightly bent (but is supposed to be for shifting) but I thought i was maybe over-bent. Fidded with it, ended up putting on a new cassette, same problem. Turns out the whole thing was just a RD adjustment issue with my HL problem. If its smooth sans chain.... its not BB or pedals... its chain and gear

    And yes, derailleur hangar looks bent. Look at the bottom pulley of the RD....sometimes staring at bottom pulley wheel is better than looking at the hangar itself

  10. #10
    Bikaholic blamp28's Avatar
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    Looks like a bent hanger to me. Carefully bend back and re-adjust rear derailleur/cable. Good as new. The bike was knocked over on the drive side or accidentally bumped while parked - would be two likely causes. The chainrings look normal to me. Those unusually shaped teeth aid in shifting. That's why the are in a symmetrical pattern.
    Trek Fuel XC MTB, Giant OCR Road Bike, Rans Screamer - Tandem

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