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  1. #1
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    What is causing my chainrub?

    Hello all, new to this forum, and this is my first post. I'm also new to cycling as well. I recently purchased a Trek 7500FX. So far the bike only has about 75 miles on it. One thing that is driving me crazy is that when I'm on the outside chainring, the chain rubs on the outer side of the front derailleur cage, REGARDLESS OF WHAT GEAR I'M IN ON THE REAR CASSETTE. I thought it was a simple case of just adjusting the front derailleur. When I adjust it, everything is fine when I hand turn the crank, but as soon as I start riding the chain rubs. I notice that when I pedal, the chainring flexes outward slightly, but just enough to induce rubbing. Is this normal? Am I just supposed to adjust the derailleur to compensate? I can't adjust the derailleur screw anymore, it is all the way up. I've read a lot of forums where people say they can't get the Deore front derailleur properly adjusted and they just scrap it and get something better. I'm also worried that maybe the Bontrager Select crankset/chainrings just aren't that good and tend to flex. So what do you folks think: a) front derailleur just needs more adjusting, b) front derailleur just sucks and needs to be swapped out for something better, c) crankset/chainrings are just not good enough, d) none of the above

    I've read a lot of the articles out there on derailleur adjustment(at least all the ones that people in forums always say to go and read), and I've played with the derailleur quite a bit. I also asked Trek if there was any known issues with the Bontrager Select crankset that they use. They haven't gotten back to me yet.

    Thanks

  2. #2
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by Theresadog
    Hello all, new to this forum, and this is my first post. I'm also new to cycling as well. I recently purchased a Trek 7500FX. So far the bike only has about 75 miles on it. One thing that is driving me crazy is that when I'm on the outside chainring, the chain rubs on the outer side of the front derailleur cage, REGARDLESS OF WHAT GEAR I'M IN ON THE REAR CASSETTE. I thought it was a simple case of just adjusting the front derailleur. When I adjust it, everything is fine when I hand turn the crank, but as soon as I start riding the chain rubs. I notice that when I pedal, the chainring flexes outward slightly, but just enough to induce rubbing. Is this normal? Am I just supposed to adjust the derailleur to compensate? I can't adjust the derailleur screw anymore, it is all the way up. I've read a lot of forums where people say they can't get the Deore front derailleur properly adjusted and they just scrap it and get something better. I'm also worried that maybe the Bontrager Select crankset/chainrings just aren't that good and tend to flex. So what do you folks think: a) front derailleur just needs more adjusting, b) front derailleur just sucks and needs to be swapped out for something better, c) crankset/chainrings are just not good enough, d) none of the above

    I've read a lot of the articles out there on derailleur adjustment(at least all the ones that people in forums always say to go and read), and I've played with the derailleur quite a bit. I also asked Trek if there was any known issues with the Bontrager Select crankset that they use. They haven't gotten back to me yet.

    Thanks
    Considering the low mileage, I don't think that the adjustment screw or the outer limit is your problem. Your cable has probably stretched and needs to be tightened a little.
    Stuart Black
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  3. #3
    Senior Member sydney's Avatar
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    That won't do a thing if it shifts cleanly to the big ring, but it could screw up the shift to the small ring.

  4. #4
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    Quote Originally Posted by cyccommute
    Considering the low mileage, I don't think that the adjustment screw or the outer limit is your problem. Your cable has probably stretched and needs to be tightened a little.
    Ahh, this brings up another question. Exactly how tight is tight? Most instructions just say pull the cable taut and clamp it down. Well that's pretty subjective if you ask me. Should I pull on it as hard as I can?

  5. #5
    Senior Member sydney's Avatar
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    Quote Originally Posted by Theresadog
    Hello all, new to this forum, and this is my first post. I'm also new to cycling as well. I recently purchased a Trek 7500FX. So far the bike only has about 75 miles on it. One thing that is driving me crazy is that when I'm on the outside chainring, the chain rubs on the outer side of the front derailleur cage, REGARDLESS OF WHAT GEAR I'M IN ON THE REAR CASSETTE. I thought it was a simple case of just adjusting the front derailleur. When I adjust it, everything is fine when I hand turn the crank, but as soon as I start riding the chain rubs. I notice that when I pedal, the chainring flexes outward slightly, but just enough to induce rubbing. Is this normal? Am I just supposed to adjust the derailleur to compensate? I can't adjust the derailleur screw anymore, it is all the way up. I've read a lot of forums where people say they can't get the Deore front derailleur properly adjusted and they just scrap it and get something better. I'm also worried that maybe the Bontrager Select crankset/chainrings just aren't that good and tend to flex. So what do you folks think: a) front derailleur just needs more adjusting, b) front derailleur just sucks and needs to be swapped out for something better, c) crankset/chainrings are just not good enough, d) none of the above

    I've read a lot of the articles out there on derailleur adjustment(at least all the ones that people in forums always say to go and read), and I've played with the derailleur quite a bit. I also asked Trek if there was any known issues with the Bontrager Select crankset that they use. They haven't gotten back to me yet.

    Thanks
    Folks like to blame FDs for their inability to adjust one. Considering the bike only has 75 miles on it,I'd take it back to where you got it and have them sort it out. I don't think the crank/chainrings are the problem either unless something is bent or otherwise out of spec.

  6. #6
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    Aside from the rub, I have no other shifting problems. Everything is crisp and quick.

  7. #7
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    Quote Originally Posted by sydney
    Folds like to blame FDs for their inability to adjust one. Considering the bike only has 75 miles on it,I'd take it back to where you got it and have them sort it out. I don't think the crank/chainrings are the problem either unless something is bent or otherwise out of spec.
    What about the part where I said that the chainring seems to flex outward slightly under load...is that normal?

  8. #8
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by sydney
    That won't do a thing if it shifts cleanly to the big ring, but it could screw up the shift to the small ring.
    If he has enough slack in the cable, the bike will shift to the big ring just fine but will move back just a little after pressure is released from the shifter and allow the chain to rub. A small adjustment (at most a couple of turns on the derailer cable adjuster) will take up the stretch and keep the derailer in place without screwing up the shift to the small ring.
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
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    Days of Wineless Roads. Bed and Breakfasting along the KATY
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  9. #9
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by Theresadog
    What about the part where I said that the chainring seems to flex outward slightly under load...is that normal?
    Probably. Bikes have a small amount of flex in them. Some have a lot, some have a little but they all have some flexibility. It's inherrent in the design. Some bikes will even ghost shift if you stand up and really wail on them.
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
    Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
    Days of Wineless Roads. Bed and Breakfasting along the KATY
    Twisting Down the Alley. Misadventures in tornado alley.
    An Good Ol' Fashion Appalachian Butt Whoopin'.

  10. #10
    Senior Member sydney's Avatar
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    Quote Originally Posted by cyccommute
    [b]If he has enough slack in the cable, the bike will shift to the big ring just fine but will move back just a little after pressure is released from the shifter and allow the chain to rub. [b] A small adjustment (at most a couple of turns on the derailer cable adjuster) will take up the stretch and keep the derailer in place without screwing up the shift to the small ring.
    Well actually, if you take a look at any shimano front setup,that is exactly how they work when adjusted correctly.The FD does overshift slightly to get the chain to the big ring and then the FD falls back slightly to the left, when pressure on the shifter is released. To cure his problem of shifting properly to the big ring but then apparently falling back enough to rub would normally be cured by loosening the hi limit screw slightly.He said he has no more adjuatment left. If he had too much slack in the cable a shift to the big ring would be impossible.

  11. #11
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by sydney
    Well actually, if you take a look at any shimano front setup,that is exactly how they work when adjusted correctly.The FD does overshift slightly to get the chain to the big ring and then the FD falls back slightly to the left, when pressure on the shifter is released. To cure his problem of shifting properly to the big ring but then apparently falling back enough to rub would normally be cured by loosening the hi limit screw slightly.He said he has no more adjuatment left. If he had too much slack in the cable a shift to the big ring would be impossible.
    So your suggestion would be?

    Cables stretch and need to be adjusted. The cable can stretch enough to allow too much lash in the leftward movement of the derailer and still be tight enough to still be able to shift. It's not an all or nothing kind of thing. He's already messed with the limit screws and nothing happened. If he takes it back to the shop (which you should probably do anyway, Theresadog) they are most likely going to adjust the cable anyway.
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
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    Days of Wineless Roads. Bed and Breakfasting along the KATY
    Twisting Down the Alley. Misadventures in tornado alley.
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  12. #12
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    Quote Originally Posted by cyccommute
    So your suggestion would be?

    Cables stretch and need to be adjusted. The cable can stretch enough to allow too much lash in the leftward movement of the derailer and still be tight enough to still be able to shift. It's not an all or nothing kind of thing. He's already messed with the limit screws and nothing happened. If he takes it back to the shop (which you should probably do anyway, Theresadog) they are most likely going to adjust the cable anyway.
    Well I tried adjusting the derailleur again and this time I pulled the cable very very tight. It's still rubbing though. I wonder if by chance the derailler itself isn't installed correctly...like maybe it just needs to be rotated clockwise ever so slightly...

  13. #13
    Senior Member sydney's Avatar
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    Try the repair section ar www.parktool.com. Start at the beginning with loose cable and proper position, and leave out no steps.

  14. #14
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    Does the chain rub through a full 360 degree rotation or just through part of a crank rotation?

    Is there any looseness in the bottom bracket spindle or wobble in the big chainring?

    Be sure that the shifter is in its lowest position with all of the cable pulled out before attaching the cable to the derailleur.

    Al

  15. #15
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by Theresadog
    Well I tried adjusting the derailleur again and this time I pulled the cable very very tight. It's still rubbing though. I wonder if by chance the derailler itself isn't installed correctly...like maybe it just needs to be rotated clockwise ever so slightly...
    I said a little tightening of the cable might be in order not a lot . The derailer should be fairly parallel with the chainrings. Sounds like it may have slipped because someone didn't tighten it enough. A trip to the shop may be in order, especially considering the newness of the bike. Make 'em fix it while you wait but be nice about it. Everybody makes mistakes at times.
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
    Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
    Days of Wineless Roads. Bed and Breakfasting along the KATY
    Twisting Down the Alley. Misadventures in tornado alley.
    An Good Ol' Fashion Appalachian Butt Whoopin'.

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