New chainrings or new crankset?
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  1. #1
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    New chainrings or new crankset?

    I have a Bontrager Race Lite GXP triple with a bad small ring and questionable middle ring. The set is 30/39/52. The rest of my components are Ultegra with a 10 speed cassette. I believe it's all Ultegra 6800, but am not sure how to tell. The cassette is 6800. The bike has ~4K miles on it.

    I have no idea why the small chain ring is bad given that it's probably the least used. My chain won't stay on it when I climb. I live on a steep street and need the small ring to get to the main street to ride.

    So the question is: should I replace those two chain rings with Bonty rings? Shimano rings? One mechanic told me Salsa would fit fine and wear better. Given that two rings will be $50-60 and that I can get a 105 crankset for $100, is just going with Shimano a better option?

    Some history - feel free to skip this paragraph: One bike mechanic told me he wasn't sure what the issue was but wants to replace the crank set, rear cassette, and chain. This sounds like overkill to me. The chain and rear cassette look perfectly fine. Another guy told me the Bonty stuff doesn't match well with Shimano and that I need to replace it or it'll never be right. I don't get either as the bike shifted fine previously. A third one told me the small chain ring is toast and the middle one is iffy. This confirms what I notice when riding. He has 30 years of experience and is the one I believe. He also told me to change out those two chain rings and be done. He didn't have the chain rights in stock, but recommended I go with Salsa or Shimano chain rings.
    Last edited by Dario; 06-07-2015 at 09:36 AM.

  2. #2
    wim
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    Quote Originally Posted by Dario View Post
    My chain won't stay on it when I climb.
    What actually happens to the chain--does it fall off towards the frame and land on the bottom bracket shell? Or does it skate over the teeth of small ring, meaning not actually falling off, but not engaging the teeth either?

    Worn rings generally don't cause the chain to fall off, and 4 K miles is nothing for a chain ring, even a small one. I'm a bit puzzled by the diagnosis of your trusted mechanic. My first check would be for "autoshifting" coupled with a slightly misadjusted limit screw, meaning something other than your shifter moving the front derailleur when you put a lot of force on the pedal.

  3. #3
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    Is it even possible to wear out a chainring in 4K? Not disputing it is wore out, just hard to figure. Are you sure it only has 4K on it?
    Maybe if you only ride in it and always up some 20% grade, IMO.
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  4. #4
    wim
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    Quote Originally Posted by duriel View Post
    Is it even possible to wear out a chainring in 4K?
    It's possible, but it would take some doing. Same chain, much cross-chaining, lots of rain rides, no cleaning, improper chain lube regimen.

    People often pronounce chainrings "worn" as soon as they don't look brand new any more, so I always take that with a grain of salt. Below a truly worn chain ring: pointy teeth, asymmetrical space between the teeth ("shark fin"), material displaced. And even that one didn't cause a chain to not stay on if "not stay on" means falls off and away from the ring.
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    Last edited by wim; 06-07-2015 at 03:18 PM.

  5. #5
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    If the chain has 4,000 miles on it, that is probably your issue. Start there.

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    Quote Originally Posted by Blue CheeseHead View Post
    If the chain has 4,000 miles on it, that is probably your issue. Start there.
    Already replaced the chain a couple of months ago. Didn't help. And that was a guess.

  7. #7
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    Quote Originally Posted by wim View Post
    What actually happens to the chain--does it fall off towards the frame and land on the bottom bracket shell? Or does it skate over the teeth of small ring, meaning not actually falling off, but not engaging the teeth either?
    It jumps off to the bottom bracket shell. It's not slipping off the teeth - it's jumping off when I ride hard uphill.

    Quote Originally Posted by wim View Post
    Worn rings generally don't cause the chain to fall off, and 4 K miles is nothing for a chain ring, even a small one. I'm a bit puzzled by the diagnosis of your trusted mechanic. My first check would be for "autoshifting" coupled with a slightly misadjusted limit screw, meaning something other than your shifter moving the front derailleur when you put a lot of force on the pedal.
    I get that. However, the trusted guy told me the small chain ring looks bad. I can try to take a pic and post if you want. It could be a limit screw issue, but I checked this and tested it and a local shop did as well. And if that were the case, why would it only happen when climbing.

    And I'm with you on the movement. I'm wondering of there's more flex than previously when I pedal hard uphill, perhaps indicating a from issue or something else.

  8. #8
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    I've had a lot of rain rides. But the rest is pretty standard. I did go for one ride when the chain was dry - bike sat for a year and during my prep I just forgot to lube the chain. It got pretty sticky during the ride. But I cut that short. Hard to believe one 5 mile ride without adequate lube would have caused the problem.

  9. #9
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    No, one 5 mile ride with a squeaky chain would not cause problems.

    Look at the small chain ring and see if the teeth look worn. Is the chain worn? Dirty?
    Small chain rings for triples are inexpensive if they use the standard 74mm BCD.

    Did you check the frame to make sure it's not cracked somewhere and flexing?

  10. #10
    wim
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    Quote Originally Posted by Dario View Post
    It could be a limit screw issue, but I checked this and tested it and a local shop did as well. And if that were the case, why would it only happen when climbing.
    When you climb hard, you put forces into the entire bike that can't be duplicated on a repair stand or on a ride around the parking lot. Those forces bend and torque things--move them in some way. This means that a supposedly perfect limit screw adjustment may not be so perfect when you climb.

    What often happens is this: When in the smallest front and largest rear, people don't want the chain the rub the inner plate of the front derailleur cage. So they adjust the front derailleur low limit screw for plenty of cage clearance when in that smallest front-largest rear combo. But when you climb, it's entirely possible that this "plenty" becomes "too much," with "too much" throwing the chain off.

    Try this: first, adjust the front derailleur low limit screw for zero clearance. In other words, make the chain just begin to rub on the front derailleur inner plate when in the smallest front-largest rear.

    Take a screw driver and ride your bike uphill somewhere in that smallest front-largest rear combo. Every so often, get off the bike and turn the front derailleur low limit screw 1/4 of a turn counter-clockwise until the rub just stops. (Some people even prefer the slightest bit of rub (a very light tinkle) when climbing really hard in that combo because it tells them that the chain can't possibly come off to the inside.)

    Long-winded, but it's worth a try. Cheap and quick, and might solve the issue. Of course, all of his assumes that there's no play in the bottom bracket, the ring isn't bent or has a bent tooth. Nothing will work right with a wobbly crank or a bent ("whacked") tooth.
    Last edited by wim; 06-07-2015 at 04:01 PM.

  11. #11
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    Good idea. Will give it a try.

  12. #12
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    Chain is relatively new and looks great. Looked at the frame and didn't see anything. Doesn't mean it's not there. The shop is convinced it's not the frame. But something has to be different. I've done a ton of hills with this bike and never had problems.

    Will try the limit screw first (see other message).

  13. #13
    wim
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    Quote Originally Posted by Dario View Post
    Good idea. Will give it a try.
    Not saying you will, but readjusting the limit screw for less clearance could slacken the front derailleur shift cable a bit (noticeable in that smallest front-largest rear combo). If this happens, just restore the tension with the adjuster. If you don't have an adjuster, loosen the cable clamp screw, pull the slack out of the cable with your fingers and tighten the screw again.

    There's no need for that cable to be guitar-string tight, just "no noticeable slack" is what you're after. This is because you want the derailleur in the smallest front position to rest on the low-limit screw, not hang on the cable.
    Last edited by wim; 06-08-2015 at 04:29 AM.

  14. #14
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    Understand. Thanks for the tip.

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    Quote Originally Posted by wim View Post
    People often pronounce chainrings "worn" as soon as they don't look brand new any more, so I always take that with a grain of salt. Below a truly worn chain ring: pointy teeth, asymmetrical space between the teeth ("shark fin"), material displaced. And even that one didn't cause a chain to not stay on if "not stay on" means falls off and away from the ring.
    I start getting chain-suck long before my chain rings ever look like that. That's not the image of a worn ring, that's what happens when a bicycle is stolen by a ninja guild.

  16. #16
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    If you left a worn/stretched chain on too long, it could have easily ruined your chainrings, and cassette. I'd just go with a 105 crank and be done.
    Tis the season for all of us not hard enough to play to belittle those not hard enough to win. We are a funny lot. - dave @ November Bicycles

  17. #17
    wim
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    Quote Originally Posted by headloss View Post
    I start getting chain-suck long before my chain rings ever look like that.
    Probably because you're replacing your chains when they need to be replaced. With a 10,000-mile, stretched-beyond-belief chain you wouldn't get chain-suck, even with that ninja ring. :-)

  18. #18
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    Quote Originally Posted by Dario View Post
    Chain is relatively new and looks great. .
    You can't see the wear by looking at it. You need to measure the wear.

  19. #19
    tjc
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    I'm not sure the problem is in your chain rings either, but if you go that route, rather than buy new crankset and bottom bracket... http://forums.roadbikereview.com/com...et-340935.html

    My problem was the chain was actually slipping (not dropping off ring) when I stood up, ended up replacing all three rings (plus chain & cassette), but that was my fault for letting it go for far too long >10k miles.

  20. #20
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    The odd thing is I never had an overly worn chain - I check it every few months. And my cassette looks great. I'll try to get a pick of the small chain ring if I can.

  21. #21
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    Yeah, you should make sure everything's adjusted right. I'd start with the front derailer. Check and see if the bottom bracket is loose. What about the chain ring bolts -- are they tight? I know, obvious stuff, but its cheap and easy to check. If it's bad, buy a small and medium chain ring. I can't believe you have managed to wear out a small chain ring in 4000 miles, but WTF, they're cheap. Maybe the Trek house brand really is that bad. Seems hard to believe.

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    Quote Originally Posted by pmf View Post
    Yeah, you should make sure everything's adjusted right. I'd start with the front derailer. Check and see if the bottom bracket is loose. What about the chain ring bolts -- are they tight? I know, obvious stuff, but its cheap and easy to check. If it's bad, buy a small and medium chain ring. I can't believe you have managed to wear out a small chain ring in 4000 miles, but WTF, they're cheap. Maybe the Trek house brand really is that bad. Seems hard to believe.
    Thanks. All the obvious stuff has been checked. I checked it and then a local shop checked it. I had a KMC link in the chain - can't recall why. After looking at everything the shop told me the KMC link was likely the issue and replaced it with a shimano link and pin. Their next option was to replace the entire drive train. I can check again, but I'm pretty sure the basics are covered.

  23. #23
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    The KMC link, assuming it was for the proper speed, would not have caused the problem. That statement by the shop brings their competence into question.

  24. #24
    pmf
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    Quote Originally Posted by Blue CheeseHead View Post
    The KMC link, assuming it was for the proper speed, would not have caused the problem. That statement by the shop brings their competence into question.
    I'll second that. If it was the wrong size, that would be immediately obvious. I hate those PITA Shimano pins. Its a lot easier to screw one of those up than it is to screw a link up. How do you screw a link up?

  25. #25
    tlg
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    Quote Originally Posted by Dario View Post
    The odd thing is I never had an overly worn chain - I check it every few months.
    How exactly are you checking your chain?

    Have you or your shop actually measured it? I'm with the others who don't believe you wore out your rings in 4k miles. That would require an extremely worn chain. Any competent shop would've measured the chain and known instantly it was the cause.
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