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Front derailleur throwing chain off big ring

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Front derailleur throwing chain off big ring

Old 06-05-17, 06:49 PM
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Front derailleur throwing chain off big ring

I've checked the FD and the high limit looks to be ok. I've tried to back the barrel adjuster off, but that just makes the chain not want to move up to the big ring. Am I missing anything? Does the low limit have anything to do with it? It's something that has recently started. Nothing has changed on the bike.

The bike is a 2012 Cervelo S2 with Q rings. Front derailleur is a 10sp SRAM Red with the steel cage.
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Old 06-05-17, 06:58 PM
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Probably a Derailleur height or alignment issue. The Q rings might give too much space between the cage and the ring when changing in a certain part of the rotation...

Honestly it's probably just having a SRAM FD. Go shimano.
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Old 06-05-17, 06:58 PM
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Low limit only changes how far in the derailleur can go; i.e., if the chain falls off to the inside, it's the low limit.

Likewise, since it's falling off to the outside, it's usually the high limit screw. It doesn't take much; a quarter turn adjustment of the limit screws is quite a bit.

The only other thing I can think of is that the derailleur has become mis-aligned. i.e., the bike has gotten bounced and jostled around enough that the cage is no longer parallel to the chainrings. You can check, but that would take some pretty serious bouncing.

As you discovered, the cable tension on the front derailleur is to allow the cable to move the derailleur through its full range of motion.
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Old 06-07-17, 05:24 AM
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Originally Posted by I <3 Robots View Post
I've checked the FD and the high limit looks to be ok. I've tried to back the barrel adjuster off, but that just makes the chain not want to move up to the big ring. Am I missing anything? Does the low limit have anything to do with it? It's something that has recently started. Nothing has changed on the bike.

The bike is a 2012 Cervelo S2 with Q rings. Front derailleur is a 10sp SRAM Red with the steel cage.
it's the high limit screw not the low one, also make sure you have a 2mm gap between the FD cage and the sharpest tooth on your big chainring
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Old 06-07-17, 05:36 AM
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don't neglect a problem with your big chain ring. had a folded over tooth that threw the chain repeatedly
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Old 06-07-17, 05:54 AM
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Adjust your FD from scratch thanks to videos on youtube

if you chain won't go back to the big chainring, you gotta check the cable tension as well
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Old 06-07-17, 06:53 AM
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Pretty much all FDs have issues with Q-Rings.

You have to set the FD height to the highest point of the chainring, which means that gap between the ring and the FD cage will grow considerably during the flatter portions of the ring. The chain can easily slip through that gap.
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Old 06-08-17, 03:47 PM
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Check to see if your FD has spread since this can cause thrown chains even if everything is adjusted properly. If that's the issue, you can fix it quickly with appropriate pliers
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Old 06-08-17, 04:17 PM
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Elvo
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The old SRAM red FD has been known to flex and spread. Replace it with one of the yaw FD's
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Old 06-08-17, 04:19 PM
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height, angle, and limits all play a role as pointed out. Start from scratch and move the FD manually by pulling on the cable to eliminate tension issues
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Old 06-09-17, 05:33 PM
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Cool. I'll readjust and check for wear.

Thanks for the replies!
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