Hi - over the past week I've noticed my bike doing something funny... every once in a while, especially when I fairly suddenly increase the amount of torque I'm applying to my pedal, my chain jumps to a different cog on my cassette. I think it always jumps to a lower # of teeth cog, but I'm not positive about that.

Any ideas what could cause this?

Thanks!

check that the teeth on your cogs are not worn down and sharp, or pointy- as opposed to dull at the tips of the teeth

Two like causes come to mind.

1- Trim. if the trim is slightly outboard of center, the chain can hang and begin a tentative shift and at high load complete it. Also high loads cause frame deflections which can very slightly change the trim. If you're at the edge, this can be enough to cause a shift. Try adjusting the trim inboard a bit, but not enough that upshifts aren't clean and crisp.

2- wear, you didn't say how many miles, but worn chains and sprockets don't mesh perfectly, and high loads can pull the chain forward on the sprocket which can cause the engagement to hang. You can get a hint of how likely this be measuring the chain for stretch.

So the cogs look OK to me - but my eyes may not be well adjusted for looking. I'll post photos later tonight. I measured 16 links of chain and it was somewhere around 8.1-8.15" long. Looking at Sheldon Brown's article it sounds like it's time to replace my chain, and possibly time to replace my cassette as well. Thoughts?

Yeah, it's time to replace your chain, but this is the more relevant Sheldon Brown article to your issue: Automatic Upshifting

Like Francis said, your derailleur adjustment is probably off, but lined up enough to stay put during casual riding, and then frame flex from getting out of the saddle and cranking is what allows things to get out of whack enough to autoshift. Does the chain rattle much when you ride?

OK so I've attached photos of my crankset and cassette. What do you all think? The largest chainring is newer than the smaller two since I tend to spend about 80% of my time on the large one.

Thanks for the article on upshifting - sounds like my problem. I'll try adding some tri-flo to the shift cable to see if that helps.

Nobody can judge rear cog wear by eye unless it's extreme. If you put on a new chain and then certain cogs skip then they are worn. It is of course impossible to judge wear on the middle ring, as you still have the chain on it, but the other two do not have the typical very pointy teeth, let alone the "shark's fin" profile that indicate wear.

It MAY be time to replace a chain, and then again it may not. Also, the chain may not be the cause f your problem, and replacing the chain MAY call for a new cassette and again it may not. There are people who spend money on what they MAY need, but I'm not one of them.

If you've decided to replace the chain based on wear. it's strictly up to you. But if you're replacing the chain because it MAY solve your skipping problem, you may be throwing money away unnecessarily.

I mentioned 2 likely causes, one would involving replacing parts, but checking and adjusting trim is FREE and involves only a minute of your time. Since you don't know the exact cause, I think it makes sense to rule out the free one before spending any dough. But I'm kind of tight with repair dough.

I completely understand your point - but my understanding is that a stretched chain will wear out your crankset and cassette - so once one is stretched it should be replaced quickly so that you don't have to replace anything else. is that not right?

To be clear about your suggestion for the change - I would just need to turn the adjustment knob that is on my brifter a teeny bit to attempt your tweak?

Yes, the conventional wisdom is that if a chain is stretched about 1/2%, you should replace it before it causes excess wear on the cassette. OTOH if it's stretched much more than that, especially more than 1% it's too late and you might as well leave it on.

You want to adjust the trim to move the RD inboard a hair. You do that by turning the trim adjuster (on the RD, cable stop, or levers) outward, (unscrew direction) so as to tighten the cable. Try this first and see if it solves the problem, then make a decision about the chain as a separate issue.