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New Trek FX 7.2 Disc, Front derailleur rubbing chain...

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New Trek FX 7.2 Disc, Front derailleur rubbing chain...

Old 09-15-13, 06:10 PM
  #1  
cynergy
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New Trek FX 7.2 Disc, Front derailleur rubbing chain...

Hi All,

I have a new Trek FX 7.2 Disc that I bought as a winter/bad weather commuter. I picked it up today and rode it around a few miles this afternoon and evening.

The bike comes with a Shimano 8 speed Altus shifter, a Shimano M191 front derailleur, and a Shimano Alivio rear derailleur.


I noticed this evening that when my I have the chain on the largest front chain ring and on the third largest or second largest sprocket, the chain rubs against the front derailleur. On my other bikes that have a triple crank set, I am usually able to use at least the third and second largest sprockets with the largest chain ring and not have the chain rub the derailleur. I understand if the largest chain ring and largest sprocket combo do not work, but I thought I could at least get the largest chain ring and second largest sprocket combo to work.

Any ideas on what the problem may be, or is this normal for this FD setup? I adjusted the Low and High limits on the FD (per the Park Tool website) but I am wondering if the derailleur was installed a little too low.

If I raise the FD up a little, do you think I would be able to get enough clearance to clear the FD for the second and third largest sprockets?

Thanks!
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Old 09-15-13, 07:13 PM
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Make sure that the derailleur is parallel to the chain rings. Rather than merely adjusting the limit screws I would suggest that you follow the entire Park procedure from the beginning without skipping any steps.

Having your derailleur higher than optimum to enable use of gear combinations which are duplicated more efficiently (less cross-chained) with another chain ring/cog combination will degrade the crispness of your shifting elsewhere for little real benefit.
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Old 09-15-13, 07:17 PM
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just bring it back to the shop
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Old 09-15-13, 07:28 PM
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To check for proper height put tension on your front derailleur shifter so that the Front derailleur cage is lined up with the large chainring and you should be about 2mm above the teeth. That won't be the problem though, instead you may want to ensure that the derailleur is parallel with your chainrings. If that doesn't correct the problem it is between your limits and cable tension. With componentry of that level you're probably going to have to deal with some rubbing but most likely whoever assembled the bike just rushed it. If you can't get it and it keeps bothering you your shop will adjust it for you, guarenteed.
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Old 09-15-13, 09:22 PM
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From a Shimano Techdoc.

8 Speed gears combinations they don't recommend using.



The straighter the chainline is the less rubbing there will be.
Attached Images
File Type: jpeg
Shimano 8 Gear Drivetrain.jpeg (40.1 KB, 18 views)

Last edited by cobba; 09-15-13 at 09:46 PM.
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Old 09-17-13, 02:17 PM
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cynergy
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Thanks All for the replies. I think cobba's image from the Techdoc capture what is happening.

The derailleurs shift smoothly and are very responsive and I am happy with the setup. The derailleur height looks good and it's parallel to the chain. I will just keep riding it - it runs really well and I have lots of other gear combos that I can use instead of the big ring and the two largest sprockets.

Thanks again!
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