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chain falls off bottom cog of derailleur

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chain falls off bottom cog of derailleur

Old 06-29-08, 08:30 AM
  #1  
Wissahickon
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chain falls off bottom cog of derailleur

Good Morning,

I recently replaced me rear derailleur on my MTB. I went the cheap route and got a deore for about $20. I took it for a ride in the park Wissahickon near where I live. It seems that going down bumpy hills, most especially if I shift at all, causes it to slip off the bottom cog of the derailleur. It slips toward the spokes, because it isn't caged on that side. I can still ride it like that, but I don't since I am sure it's not good for the bike.

I've considered a couple of ideas, but I want to see if anyone else has a good solution to this problem.

Thanks in Advance
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Old 06-29-08, 09:12 AM
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onbike 1939
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The "Low" limit screw which limits the amount of inward travel of the RD needs to be adjusted. See below
http://www.parktool.com/repair/readhowto.asp?id=64

Last edited by RonH; 06-29-08 at 10:17 AM.
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Old 06-29-08, 09:57 AM
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It's probably not the low limit screw.

1. Shift your bike into a gear combination that makes the derailleur arm point straight down.
2. Prop your bike against something and look at the derailleur from the back. The arm should point straight down. My guess is your's will point toward the rear wheel.
3. If you have a 6 or 7-speed rear cassette, grab the derailleur with your hand and tweak it back into line. You'll probably be able to eyeball it accurately enough.
4. If you have a 9-speed, take your bike to a shop and have them realign the derailleur hanger with a gauge. 9-speeds are picky about having near-perfect derailleur hanger alignment.
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Old 06-29-08, 12:51 PM
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Wissahickon
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thanks!

thanks for your comments.

I would be surprised if it is the screw, because it happens regardless of what gear I am in, and because bouncing is the primary cause of dislocation (shifting, I think, just increases the chance). Hmm, unless, does the low limit screw effect the resistance of the spring, or just the distance the RD can travel?

I had wondered about the angle, I will check this out. Interestingly, you mentioned 6, 7, and 9 but mine happens to be an 8... We shall see...

I was surprised nobody has mentioned chain length. I have the same chain as with my previous derailleur, but it seemed to me that if the chain was shorter it would be less likely to bounce off...

Unfortunately it's hard to simulate the bouncing of the trail, so it may be awhile before I can check it out. I'll let you all know what works.
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