chain droping from big ring

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devinci
Posts: 3039
Joined: Sat Jan 03, 2009 9:43 pm
Location: Canada

by devinci

I have a chain droping issue since a few weeks now

Bike is NEW (2 months of use), full sram XO, crankset is a SRM and chainrings have about 3000km of riding, mostly road riding. Chain is BRAND NEW.

When I go downhill on the trails and hit some consecutives bumps or some 2-3 feet deep pumping section at speed the chain will sometimes drop from the big ring onto the small ring.

I sucks because it happened a lot in last 2 races and makes me loose time.

What could be the problem? I thought my old chain could be too stretched out so I changed it but its not better. Are my chainrings already dead? They saw a lot of road km in the dry and I rode maybe twice in slightly wet conditions...

thanks

scott_racer
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Joined: Sat Sep 23, 2006 11:30 am
Location: Melbourne, Australia

by scott_racer

Getting the FD position just right is critical. With your bike on the workstand, slowly turn the cranks and try and push the chain off towards the small ring - if it fits down between the big ring and inner FD plate then you need to turn the limit screws so that it sits further out.

BUT, make sure the chain can't fit between the big ring and outer FD plate and fall off the outside either. You may find your cage has spread a bit, in which case get some pliers and gently bend the outer side of the cage in a bit.

SRAM FDs have a lot of room at the top where the chain can bounce up, which never helps with dropping chains. Check out this mod which may help you too: http://www.cyclingnews.com/features/pho ... ite/185584
Start flat out ... then pick the pace up halfway

by Weenie


TheRookie
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Location: Midlands, United Kingdom

by TheRookie

scott_racer wrote:Getting the FD position just right is critical. With your bike on the workstand, slowly turn the cranks and try and push the chain off towards the small ring - if it fits down between the big ring and inner FD plate then you need to turn the limit screws so that it sits further out.

You what, that will be cable tension, unless the high limit screw is preventing it moving further, the low screw certainly has nothing to do with it!
Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

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devinci
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by devinci

What do you precisely suggest TheRookie?

I thought scott racer advice was very sound...

mattr
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Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Just moving the limit screw will do nothing, unless you adjust the cable pull as well. Unless it's badly set up in the first place.

If you've not adjusted the cable since you got the bike, half a turn on the barrel adjuster might be worth trying as a quick fix.

TheRookie
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by TheRookie

devinci wrote:What do you precisely suggest TheRookie?

I thought scott racer advice was very sound...

Matt said it for me....you ever set up a front mech
Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

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devinci
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Location: Canada

by devinci

thanks guys

I dont think I have readjusted the cable since I got the bike. The limit screws are properly adjusted, I think. I have a little chain rub when on 26/36 gear combo though.

I will tighten the cable and see what happend

mattr
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Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

If you have rub in the 26/36 it's probably time to get it tweaked properly. Tightening the cable might make that worse, or it might need the inner limit screw backing off half a turn. Can't tell without looking at it.

Any decent bike mechanic/fettler should be able to set it up, rub and drop free. in 5 minutes or so.

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devinci
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Joined: Sat Jan 03, 2009 9:43 pm
Location: Canada

by devinci

I'd just like to be able to set it up properly myself

I am getting rub in the 26/36 AND rub in the 39/11-12-13

If you can point me out which screw I should turn and which way I should turn it, it would be useful

thank you

mattr
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Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Have a look on the park tools website.

Possibly in combination with the instructions!

02GF74
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Joined: Thu Jan 13, 2011 4:04 pm
Location: Sunny UK

by 02GF74

devinci wrote:When I go downhill on the trails and hit some consecutives bumps or some 2-3 feet deep pumping section at speed the chain will sometimes drop from the big ring onto the small ring.



there are two ways for the chain to shift ffrom the one ring to another - either the front mech pushes the chain over or the chain is thrown off by vibration.

if you are able to shift the chain onto the big ring and then ride without the chain rubbing on the front mech, then it does not sound like maladjuster front mech.

If there is insufficient tension on the chain, then it is susceptible to being thrown off - or it may get thrown of by the front mech when the frame flexing <-- highly unlikely.

Assuming the front mech is set up correctly, to address chain tension check that the chain is of the correct length - put it on biggest front ring and biggest rear sprocket and add two links. Then verify that the rear mech is of sufficent length - a short cage will not give enough tension if you have large range of gears. Also check that hte spring in hte rear mech has good tension. Lastly verify you are not using extreme gear combinations that result in low tension i.e. small ring with smallest sprocket - not the case here since you aree in the largest sprocket.

If this is occurse when you are pedalling, check the teeth on the chaing rings and the chain itselft for stiff or damaged links.

If all that fails, then the only solution is to buy a new bike.

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devinci
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Location: Canada

by devinci

the park tool website is very handy, thanks for that mattr

heigh and angle looks good.

Rear derailleur is new, I dont know how it could lack tension. Should I reset the B screw correctly?

Now I will have a look at the limit screws after next week end's race. I tightened the cable a little bit for now. See what happen. I will report back. Thanks for your input guys

planB
Posts: 19
Joined: Wed Nov 02, 2011 5:34 pm

by planB

I used SRAM XX for three seasons and had the same problem. Particularly while riding fast on technical terrain in the big/big gear or similar. My front derailleur was set up correctly and I even did the mod linked to above. Nothing worked. Until I replaced the big chainring. Problem solved -- for a couple months at most... The rings always shifted well and looked basically okay but as soon as the chain started frequently falling off I knew it was time to replace. Totally sucks.

I think a Type 2 rear derailleur would have also minimized the problem but they weren't commonly available back then and I refused to believe that my top-drawer groupset could have this deficiency...

I've since replaced everything with XTR (including the superb Shadow Plus rear derailleur.)

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Getter
Posts: 877
Joined: Sat Nov 14, 2009 6:30 am
Location: So Cal

by Getter

I'm having the same issue with my X7 FD. The chain will randomly fall off the big ring onto the small. I have the XT shadow Plus RD. I have a new big ring on the way. I hope that will fix the issue...knowing my luck...it probably won't.

by Weenie


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devinci
Posts: 3039
Joined: Sat Jan 03, 2009 9:43 pm
Location: Canada

by devinci

Getter

update us when you try the new rings, please.

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