Rear derailleur issues
 

I'm currently having issues with my Sram Force rear derailleur. I'm running a 53/39 crank along with a 11-27 cassette. When I'm in the 39 up front and start to work my way down the cassette towards the 11 in back the chain falls off the little wheel on the derailleur that's supposed to guide it. When I back pedal it comes back on. Does anyone know what the problem might be or have a solution?

http://bikeforums.net/attachment.php...hmentid=254468

I am assuming you understand that the small-small combo is not advisable, but this still should not occur even in that combo, and I also am assuming that the problem extends beyond that combo, as you said it occurs as you go "toward the 11 in back"

Let's look at what the drive train is doing. The chain is traveling from the chainwheel toward the derailleur pulley as you pedal. The angle between the two is increasing as you move outward on the cassette, and the derailleur is wrapping more chain and therefore the pulley assembly is pivoting. If the chain does not unship when closer to frame centerline then either the alignment of the pulley cage is changing as it rotates or the angle between small cogs and chainwheel is too extreme, so that the chain does not feed on properly. It of course corrects with backpedaling because it is always being fed chain in a straight line from the upper pulley.

If this is new behavior the odds are very high that either the derailleur or the derailleur hanger has been damaged such that the cage is no longer parallel to the bike's vertical centerline. The cage's angle would then change as it rotates. The other cause would be poor chainline - that is the entire freewheel being too far out or chainwheel too far in. that would only occur in a crash that bent the rear triangle or an overhaul that changed either hub or chainwheel distance from the center of the bike. The latter is much less likely to have occurred without you mentioning it and is also less likely to cause the problem without other issues as well.

Correct, it happens in probably about the 5-6 smallest cogs. The problem began happening after I replaced a chain that broke. I almost wonder if maybe the derailleur might have bent when the chain snapped?

Do you think that there's a way to fix this or should I just take it in to the LBS and have them take a look?

Shift into a gear combination that makes your derailleur arm point straight down and look at it from the back. If the derailleur arm seems to point toward the back tire something, probably the derailleur hanger, is bent.

Sorry after all that I posted above I should have made it clear that you need to take it to a shop and a competent mechanic, who can determine what the problem is and correct it. Don't bother checking the alignment yourself, as the hanger is most commonly bent in, which would NOT cause the chain to unship to the inside, and in addition as you know the derailleur can look/seem OK in one gear combination and not so in others. The hanger could have been twisted, however, as could the derailleur when the chain broke. A hanger can be realigned or replaced. It's best just to replace the derailleur if that's the problem - usually too much labor to attempt to straighten out a twisted derailleur, with no guarantee of success. The shop should first check the derailleur hanger, then the derailleur itself. It's actually quite easy for most mechanics to tell if there is a twisting type issue by just pivoting the derailleur back and seeing if the angle of the pulley assembly changes.

BTW for you and others who post here: It is critical that we have full information, especially what else happened or was done when a problem began, if we are to help you efficiently. Diagnosis is a form of detective work, and we need all of the evidence available, even if you aren't sure it's relevent.