shifting problem

Question
I just bought a new Giant Sedona. When shifting from large chainring to middle chainring, the L chainring occasionaly will not let go and pulls the chain up and jams between the chainring and rear tire fork tearing up the paint in the process.( L chain ring is very close to fork, 1/8 inch maybe.) Bike shop is baffled. Chainrings seem fine but we're going to replace them anyhow. Have you ever experienced such a problem and what are your thoughts?

Answer
Mark,

I am going to give you the best estimation of what's going on that I can, even though this is definitely a situation where I need to see the problem to tell definitively.  

Usually when the chainrings on the front have a problem with wanting to pick up the chain and "suck" it in-between the frame and the chainring, the problem is a result of excessive wear or flaws in the shape of the chainring and it is called "chain suck."  In your case, I can't see your chainrings, but I can rule out wear as a problem, considering the rings are new.  That leaves two possibilities: 1) chainring flaws - such as a burr or bump left in the metal that catches the chain and keeps it from exiting cleanly at the bottom.  That problem will be cured by new chainrings from the shop; and, 2) Chain line problems.  Chain line problems come from a chain (most often a new chain) being asked to twist at too great an angle from the plane that the front ring is on to the plane that the back gear is on.  Strong displacement from front big ring to back large gear will accentuate this.  

So - Is the problem worst when you are on the big ring front and the big gears in the back?  That would tend to suggest a chain line problem.  I don't know if your bike shop has considered this, but if the chain line is too far off straight, the bottom bracket may not be the right size or adjusted correctly.  What you would want to do is adjust the bottom bracket to the left (as you sit on the bike and look down) in order to straighten the chain line.  Of course, this will bring the big chainring closer to the chain stay (one side of the fork at the back is called the chain stay), but you don't really need a ton of clearance between the chainring and the chain stay.  Correct chain line is more important than clearance.

Another problem that I just thought of and am sure the bike shop has checked is the adjustment of the front derailleur.  When it shifts into the big ring, does it pass about 2mm above the chainring teeth?  Too much may be a problem.  Is the plane of the derailleur parallel to the plane of the chainring.  That could also be a problem.

To sum it up, it's hard to tell without actually looking at the bike. It could be a defect, it could be a chain line problem, or it could be a derailleur alignment problem.  The best way for you to diagnose a bit yourself is to put the bike up somewhere you can spin the chain and shift.  As you shift through the gears, watch what the chain does.  Does it hang up on something on the chainring? Does it come off the chainring at a severe angle? Does the derailleur swing parallel?  Very often you can use your mechanical sense to determine what's going on.

Hopefully, this is helpful.  Write me back with additional details if you need more help.

Graeme