Quote:
Originally Posted by M_Six
Threw the chain while shifting the front from small to big tonight. Aside from getting my fingers all greasy, it went back on with no problem and I continued on my way. Think I need to have the LBS look at it?
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It does happen at times Mark. But honestly, i think i've thrown a chain 2 times over 8000 miles, and one of those times was on a maiden voyage for a new bike. For me i think of shifting a bike like manual shifting a car and "let off the gas" a bit to make the shift.
In general, i do that because it's easier on the drive train and i likely will get more miles out of it before it needs changing. That means chain, cassette, hub and while not as much, the chain rings. But depending on what gears your in, changing gear under power will cause the chain to slap around, possibly making it get off track. Next time you fiddle with your chain, twist it a little bit to see just how much side to side / twisting flex it has. It's designed this way because the more gears we have in the back, the more out of alignment they will be with the front chainrings. So when it slaps, it can move around quite a bit.
But the most likely cause for trowing the chain off the front while changing from small to big, assuming your outer limit is correctly adjusted on the front derailleur, is being in the outer most gear in the back while changing from small to big in the front. The chain is at it's most slack when you're using small in front and small in back. Changing the front causes the most wiggle in the chain. Lots of wiggle while being slack often results in a thrown chain.
When i've run out of rear gears while in the small ring, i downshift a few gears before going into the big ring. This helps keep the chain more taunt during the front ring changes.
You can inspect your front derailleur's outer limit yourself. Shift it in the big ring and then while off the bike move the front derailleur lever as if you're trying to shift into the big ring. If your derailleur cage makes contact with your chain (while already in the big ring) you may need to have it adjusted inward by 1mm or less. If it pushes against the chain the adjustment needed is likely much more.
If the front derailleur doesn't touch the chain in the test above, try to be in the middle of the cassette when switching from small to big. In most situations, when you'e changing from small to big, you'll need to downshift the rear to match your previous cadence anyway.
If you're in your big ring and the middle of the cassette while riding and it looks like your chain bounces around, you may need a link taken out of it. I'm not sure if factory bikes come with the chain already installed or if the LBS has to install the chain. Either way, this is the least likely cause.
I ride and maintain the wife's bikes and she complains about throwing her chain from time to time. (it used to be much more often) But when i ride with her, i see (and hear) her shifting under full power often. I also see her shifting from small to big while in her 11t or 12t gear in the rear. Her issue is clearly user error. After explaining these things to her several times, her chain throwing has decreased significantly. But i'm sure she just forgets about it when doing intervals and what not. I've seen her press the lap button on her computer to start an interval and immediately lay down some power only to realize she's in the wrong gear and change while under power. Old habits are hard to break.