1. The most important thing about buying a new bike is to make sure it fits. The only way you'll know if the bike is right for you is to size up the bike and make sure that the bike's geometry matches your body's geometry. Ask questions and do some research.
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  1. #1
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    Reputation: chuckeieio's Avatar
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    What's going on here? BB/ chainring issues?

    In november I bought a new 2013 Epic Comp Carbon. On my first ride out I noticed that when in the small front ring the chain would gnarl and grab at the big ring when I put the boots to it. I brought it in and the mechanic adjusted it and it seemed better in the parking lot. (don't know what he did)

    Forward to the next day on a trail I notice it still does it but only when I'm in the 3 small on the back. (I know small/small isn't a good idea to ride in but it should still work)
    Being under such pressure when the chain grabs it has really worn the big ring significantly. (10 or so rides on it. I try not to have it in those gears but sometimes forget.
    I've been back to the bike store twice and keep getting "our mechanic isn't in, we'll call you" 2 months later I'm wondering how I can fix this myself.
    It's not (as far as I can see) a derailleur adjustment. It seems to grab from being flexed. It only does so under strong load like mashing.(which I do a lot)

    Any advice would be greatly appreciated.
    Some pics.
    EDIT to add
    CRANKSET 38t/24t
    Custom SRAM S-1250, 7050 alloy arms, 10-speed XC double, GXP spindle, S: 170mm, Others: 175mm



    Last edited by chuckeieio; 12-30-2013 at 12:56 PM.
    GTA
    Ontario

  2. #2
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    Reputation: Saul Lumikko's Avatar
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    I would not expect the geartrain to work properly when mashing while cross-chained. The first remedy would be to switch to the larger chainring as soon as you can, preferably before you're at the middle of the cassette.

    However, the chain should not grab the large chainring even when cross-chained. What size are the chainrings?

  3. #3
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    I would play with the low limit adjustment a bit. When In the small front chainring and small rear cogs, the front derialleur should just barely rub the chain (or miss it by less than 1mm).
    If it still grabs after that adjustment, your drivetrain just might not like the small/small combo. It really is a bad idea to ride this way since it puts lots of strain on the chain and drivetrain.
    A roller chainguide could also help bandaid the situation. But it really would be a bandaid and not a fix. The real fix is to not run the small/small combo and adjust the FD low limit setting to very close tolerances.

  4. #4
    Picture Unrelated
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    It's sort of a weird gear combo but it should still work properly. How's your chain length? Can you adjust the chainline (move BB spacers or something similar) on that bike?
    Don't you hate it when a sentence doesn't end the way you think it octopus?

  5. #5
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    I don't think there is any fix for this other than don't run small - small gears. I would think that when the chainline becomes extreme the chain crosses the big chainring and gets picked up by the teeth. You might be able to put a spacer on the crank to move everything over but that's backwards to what most people do - makes the chainline worse.
    A smaller big ring would help as it would have a smaller diameter.

  6. #6
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    Thanks for your responses.
    The rings are 38/24
    It's not just the small/ small, it's the smallest 3 I can't use. The small small I don't even have to be mashing for it to grab, any uphill pedalling does it. I understand that crossing the chain isn't a good way to ride the majority of time with as it accelerates wear, however I've never owned a bike where I haven't had functional use of all my gears. If I happen to be in one of these gears I want it to work without ripping apart my ring and bringing me to a stop. This can't be normal.

    I don't think adjusting the FD would help as it wouldn't flex with the BB to deflect the chain.
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    Ontario

  7. #7
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    Time to talk to the owner of the shop about the runaround you are getting or find a different shop.

  8. #8
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    Sounds like the High and low adjustments need to be tuned or maybe it isn't installed correctly. If you don't know how to adjust your derailleurs it would be worth your time learning as it really isn't that hard. This is a good starting place.

    Derailer Adjustment

    If you aren't up for the task, I would take it back and ask more questions this time and maybe a different mechanic as a decent one should be able to fix it.

    All that said you really shouldn't be riding cross chained even though I am guilty of it sometimes myself. Technically it shouldn't be giving you the issues that it is.


    Off topic but is that the way your cables are supposed to be routed? Mine would be destroyed in no time if they were routed under my bottom bracket.
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  9. #9
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    Think of it this way:

    It's a chainline issue, plain and simple. Doubles are optimized to be ridden in the big ring most of the time, and in the small ring only when you need granny gears for climbing. Think about it, on a standard triple, you've got a 44/32/22 gearing. 24/38 is a bigger jump than 24/32 and that 38 ring is essentially in the same spot as the middle ring on a triple.

    Put the bike in a workstand and check the chainline as you work through the gears. Since this only occurs when you're mashing, flex is certainly playing a role, but even without the flex, clearance is going to be super tight. You could try a smaller big ring - say a 36t or maybe 34t, which would give you some more clearance to work with. But if you actually use the big gears you'd lose by going smaller here, that's not a good fix. I suppose you could try a larger small ring, too, but then you'd lose gearing at the low end, which for the way I ride, is more important. The only other way to address it is to change your shifting.

  10. #10
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    NateHawk is right on the money: it's about the chainline.

    When the chain is running from one chainring to a cog and back without rubbing on the derailler or being off-cog in the back, adjusting the limit screws and/or cable tension is an exercise in futility. You'll only mess up initial settings, which most likely are correct already. It's not about your derailleurs or shifters: you could run the chain from the smallest cog to the granny gear even without any shifting parts installed and it would still hit the large chainring on the way.

    A very long shot is that the crankset is off-center to the left. This is easy to check: is the clearance between the crank arm and chainstay the same on both sides?

    It's not an on/off rule that small/small is the bad combo but small/second smallest is totally OK. There's a straight chainline and then there's gradually worse and worse chainline to a point where it's a really big "NO". If you only use the granny gear for climbing and spend most of your time in the big ring and around centre of the cassette, the service life of your chain will be better. If you find that gearing to be too tall, you can switch to a smaller big ring.

  11. #11
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    I'd expect to be able to (and can, on my bike with a similar drivetrain) use a small/smallish combo like that.

    For me, this is often a cleanliness issue. I wipe down my chain with a dry rag after every ride and try to relubricate when it starts getting noisy.

    It wouldn't hurt to check chain length. One of the problems with the small/small combo is that the tension in the lower part of the loop the chain makes isn't that high. So it's easier for it to ride back up the inner ring. If your chain is longer than it needs to be, removing a link may help.

    Since nobody else has commented, your big ring looks normal to me. The finish on the inside face gets trashed in no time, IME.

    FWIW, if cross-compatibility isn't too bad, my SRAM crank will get Shimano rings when the existing ones wear out. Probably something smaller than a 38, that's too big for me and my usual trails. Which is another thing to think about - if your 38 ring is too big for you, you do have options and making your large ring a more useful size is actually a pretty nice improvement to a drivetrain. My road bike has 34/46, for example, and I really like that pairing. Nothing to prove here and the stock 50, let alone a 52 or 53, was too big. I'm not hugely fond of the current 2x MTB cranks because instead of dropping the big ring like some of the old-school doubles, they compromise between a useless chainring and my favorite chainring, resulting in something less good.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  12. #12
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    I'd think about two things. I'd hang out and watch and learn as the tech checks and adjusts it.
    I'd test ride another example of your bike or a model with the size setup to see if you notice a difference.
    Use the dealer where you bought the bike.

  13. #13
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    I can't even imagine wearing the big ring while you are in the small ring in 10 rides!! It seems that for this to happen your big ring ramps would need to be gone already (worn off) because they'd be in the way. It's hard to know what shape your big ring really is in because there is paint wear and real wear and they are hard to tell apart in the images.

    As others have pointed out what you are doing is not healthy for the drivetrain: small/small is the worst combo there is. Nonetheless, it should work and showing wear in 10 rides is not normal. Maybe unless you are unusually big and strong?

    I use a SRAM/Truvativ 2x10 with 39/26 front and love it. It's virtually noise-free in all 20 gear combos. I rarely find myself in small/small but mash in large/large all the time which is also cross chained. If I had noisiness in the front while in the small ring the first thing I'd do is ease some front derailleur tension to see if that helps.

    But I suspect there is something wrong beyond adjustments. Saul mentioned off-center cranks. That could be. Also could be a loose crank with a little bit of play. Maybe remove the crank. Verify the bb is tight. Re-grease the crank splines. Tighten and verify smooth operation without play. Or better yet, have your shop do this.

  14. #14
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    As previously stated, chainline and flex seem to be the culprit here, definitely not derailleur limit adjustments. Does it rub in small/small combo with no pressure (on a stand)? If not then it's either frame or crank flex, or both.

    Double cranksets are already oriented farther outboard than center so I kind of doubt you'll need to move it that way any more. The large difference between the small and big ring make the small/smaller combos difficult to manage.

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