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I have been riding my Trek Fuel Ex 8 for a few years now, and as I have improved, I have been attacking more challenging terrain.

I have started to notice a problem in my rear mech - the link arm screw that holds the mech to the dropout seems to slip and cause the whole derailleur to swing forward until the cogs are sandwiching the chain against the sprocket. Now I'm no doctor, but I can easily say that sort of cramping is bad.

On the trail, where this mainly happens, I have been able to repair the system by pulling the whole mech back as far as it will go (there seems to be some sort of physical block or stop in place, so I don't think I'm going too far) and tightening the screw that is allowing the slippage. Now that seems like problem solved, right? My question is this.

Firstly, is that the correct procedure to reposition and fix the mech, or am I doing something wrong?

Secondly, how tight must this screw be? Wrenching it as hard as I sensibly and safely can with a park tool only holds for a while, and then the problem repeats. Also, how would you measure tightness? (I feel like there is an obvious answer there that I should know already...)

Finally, given the problem occurs on the trail, and specifically I think on landings and rough patches, is there something in my riding style that could help prevent damage. Advice is welcome, but I'm not an absolute beginner.

Thank you for any help on this

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There are a few variations of this, but generally there's a single bolt and some sort of "tang" on the derailer that intersects with a pin on the hanger, to keep the derailer from rotating forward. It sounds like maybe the bolt is worn or some such, causing it to work loose. You'll probably have to use "Loctite" on the bolt to keep it from slipping . (Be sure to use "removable" Loctite.) –  Daniel R Hicks Oct 7 '13 at 15:06
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I have started to notice a problem in my rear mech - the link arm screw that holds the mech to the dropout seems to slip and cause the whole derailleur to swing forward until the cogs are sandwiching the chain against the sprocket. Now I'm no doctor, but I can easily say that sort of cramping is bad.

Strange that this would be where things are coming loose. My recommendation is to remove the rear wheel and chain and make sure nothing else is coming loose either. If it's just the connection to the hanger, check the threads on both the derailleur and the hanger as one or the other may have damage or contaminants. Clean and lube/locktite the bolt and re-install the derailleur. If you continue to have problems, I'd have a mechanic check it out.

Secondly, how tight must this screw be? Wrenching it as hard as I sensibly and safely can with a park tool only holds for a while, and then the problem repeats. Also, how would you measure tightness? (I feel like there is an obvious answer there that I should know already...)

Park Tool recommends 8-10 Nm or 70-86 lb-in. You can easily measure this with a torque wrench, you can get a cheap one from a hardware store or go for a more expensive one if you want something that lasts.

Finally, given the problem occurs on the trail, and specifically I think on landings and rough patches, is there something in my riding style that could help prevent damage. Advice is welcome, but I'm not an absolute beginner.

Just trying to stay smooth is about the only advice you need. And don't land to flat! That puts a lot of stress on everything.

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Check everything, Loc-tite, 8-10Nm, torque Wrench, don't land flat. Got it! Thanks for the advice! –  Saxman Oct 8 '13 at 2:11
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