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Old 02-07-14, 11:16 AM   #1
DavidTony
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intermittent chain rub on front derailleur

Hello everyone!

I am basically an amateur in regards to bike mechanics and repair, and have come across a slight issue. I recently started winter commuting on a 2013 Trek 820. I feel comfortable in terms of overall preventative maintenance due to the salt, but while going over my bike, I noticed that the chain rubs for a short duration at the same point, every revolution of the crank.

I have some ideas in mind, but given my lack of experience with the mechanical repair, I feel more comfortable asking those who have significant more experience than me. What could the issue be, and how could I go about fixing it?

If you need anymore information in order to help, please ask and I will do my best to provide the necessary details, including pictures.

Your assistance is graciously appreciated!
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Old 02-07-14, 12:56 PM   #2
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Rubbing when it is in which chainring up front? Which cogs out back?

Could be: Front derailleur inner limit screw (small ring, big cog), FD outer limit screw (big ring, small cog), cable tension (middle ring, big or small cog but not both). YouTube videos about adjusting front derailleurs would go a long way toward informing you... Also check the Park Tools website, under "repair help" tab.

820 also does not use top-shelf components. On price point components, engineering tolerances are usually higher, so you could have runout issue at the crank which means that perfect shifting adjustment might not alleviate rubbing, but which would also be cause for defective part replacement under warranty if you are the original owner of the bike and it is less than a year since you bought it.
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Old 02-07-14, 01:49 PM   #3
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I would check that the drive side crank is tight, that could cause a slight wobble in the chain ring and eventually damage the crank. I would also sight along the chain ring as it turns. I've seen a few that were bent.
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Old 02-07-14, 02:25 PM   #4
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Make sure chainring bolts are secure. Determine if chainrings are bent. Make sure the crank arm is not hitting derailleur.
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Old 02-07-14, 02:36 PM   #5
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Some cheaper chainrings that are stamped don't even come straight from the factory. Neither do the crank spiders.

Most of the time a shop mechanic will tune a front derailer to compensate for this which will cause chain rub in some of the cross chain gears and tell you that you aren't supposed to use those gears anyways. However it is possible to true a spider and chainrings with some effort and a wrench. Of course you should check for more obvious sources of wobble first.
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Old 02-07-14, 02:47 PM   #6
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Maybe improvise something like this tool: http://www.bicycletool.com/chainringstraightener.aspx

I have used a pair of long-nosed locking pliers to tweak a slightly bent chain ring. http://www.amazon.com/Irwin-Tools-14...ose+vise+grips
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Old 02-08-14, 06:06 AM   #7
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http://sheldonbrown.com/straighten-chw.html
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Old 02-08-14, 06:29 AM   #8
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Once you've confirmed everything is tight and as straight as possible, the "fix" is to shift the derailleur slightly as you ride so it doesn't rub. The term is to "trim" it. Most chains will rub the front derailleur unless you trim a bit as you shift the rear up & down.
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Old 02-08-14, 12:59 PM   #9
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^^ this ^^ .. advantage: Friction FD shifting .. STI is not offering that option.

Last edited by fietsbob; 02-08-14 at 02:52 PM.
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Old 02-08-14, 01:55 PM   #10
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It can be a bent/or losses chainring or a twist in the chain which you replace the links or the whole chain all together .
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Old 02-08-14, 02:14 PM   #11
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So far I've seen everything mentioned, except for the chain, but that is certainly a possibility as well. Start with the most simple steps first and work towards more complicated solutions as you need too. Something like this:

First check the derailleur adjustment screws to see if you can fine tune the position for any offending gear combination(s).
Next check for tightness of the drive side crank and chainring bolts.
If this is fine then check the alignment of the chainring and spider to make sure they are true, you can do this by placing your bicycle upside down and looking down on the components as you slowly rotate them. You will see a wobble if anything is out of true here.
If that's not an issue, check the crank arm as you look down on it while it is aligned parallel to the ground to see if it's bent/straight or if it rubs the derailleur.
If not, slowly rotate the crank and watch the chains as it approaches the chainring's teeth to see if any of the links flop, try to pop off, etc. I put this step last because it takes longest to check, the solution is very easy.
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Old 02-10-14, 09:55 AM   #12
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Thanks for the responses! After going through it, everything seems nice and tight. I don't think it is the chain, as I believe a portion of the middle (2) chainring is bent, for every every rotation of the crank, the rub occurs at the same point: from about 9 o'clock to about 11 o'clock on the crank, the chain slightly rubs the derailleur. BTW, it appears to occur only when the front is set on 2. (currently it is at 2 and 3), and normally I try avoid any dramatic gear combinations while riding. I will look into the possibilities the next couple days, and let you all know.

Once again, all responses are greatly appreciated! Buen dia!
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Old 02-10-14, 12:11 PM   #13
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Rear triangle and BB may be flexing under power .. pedal with less stomping , more spinning .
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