Newbie here again. I just had a new chain and cassette installed at my LBS. I just noticed when I had it on the stand that when in the small chain ring and the smallest cog the chain sags down and actually rest on the chain stay. I don't ride here so never noticed it riding. Could the LBS mechanic sized the chain too long? The new cassette is an 11-28. It replaced a 12-30. I don't know enough to know if that makes a difference. Thanks guys for sharing your knowledge.

Modern derailleurs apply very little chain tension in an effort to minimize drag. At rest the both the upper an lower loops will sag. I'm not talking slack, jut normal sag under the chain's weight (look at any clothesline when no clothes are hung).

However, whenever you ride and apply the lest force to the pedal, the upper loop will straighten.

One problem on bikes with small inner rings is the height of the chain over the stay. Hold the rear wheel from turning and press on a pedal, and you;ll see that even when straight, a chain running on small sprocket isn't very high above the chainstay. This is why many mtn bikes have chainstays rerouted away from the chain, either bending down or run very high above on the way to the rear dropout.

Odds are that that the chain will slap the chainstay if you coast on bumpy roads or trails, so consider a chainstay protector to deal with it. These can be bought or improvised for very little cost.

This often happens on tandems, especially older ones, when the RD doesn't have enough chain wrap capacity to accommodate the wide gear ranges. In your case, the switch from the 12T to 11T small cog may be partially responsible, but also the chain is probably a bit too long. If its a double chainring, then even more likely the chain is too long.

Did you happen to notice if you did or did not have this issue before the swap?

And, even though you probably shouldn't ride in this cross chain gear, when you pedal in this configuration (when the chain is under tension) the chain is lifted up off the chainstay... you're not "chain sawing" through your frame while you ride.

Consider taking it back to your LBS and ask them, I guessing they would address the problem without charge. .

If you're really concerned that the chain might be long, it's easy enough to confirm either way.

Shift to small/small, and wind the RD cage clockwise. If you can do so enough to pull up the slack, then the chain is short enough. If you can't pull out the slack before hitting the RD's cage stop, then it's a bit long.

In any case, shortening the chain carries it's own risk, which is far more serious than a slight long chain. Most mechanics measure chains by the big/big+1 method, which yields the shortest safe length. So, shift to big/big and check how much chain you have to spare before cutting.

In any case, you just paid a professional to do a job. If you're uncomfortable or concerned with the results you should bring it back and have him check his work and explain how and why it's correct. Part of what you pay a pro for t=is the peace of mind that it's done right. If you don't have that, you're letting yourself be shortchanged.

I like to use as much chain as possible on my bikes, considering that I paid for all the links, whether I use them or not. Sometimes that makes for a loose chain in the small-small combination, but that's no loss as one shouldn't use that combination anyway. Perhaps your tech sized the chain a little long in case you switched back to a 12-30 cassette in the future.

Thanks guys. I don't normally ride small/small anyway so that's why I noticed it on the stand. I typically ride in the 14t cog in the small chain ring. Any thing else and I shift to the large chain ring. Going to take it back to the LBS anyway for them to check it out. I'm a rookie so have a lot to learn but really am enjoying it.

If you are within specs for the capacity of the rear derailleur with the 30 tooth cassette, then the small-small method above will allow you to safely change back if you find you want to. If you do big-big with the 28 and then go back to the 30, you risk putting your derailleur into the spokes if you shift into big-big.
The chain should not droop in small-small, unless you are exceeding the specs for wrap. That may be a reasonable thing to do, but you would be well advised not to pedal in combinations like that, as it can skip (IME).

Yes, the chain is too long. The chain should not have so much slack as to drag on the chain stay. There are numerous articles in this Forum and the Internet describing how to determine the correct chain length. Plus, I'd think twice about going back to that bike shop for service -- it doesn't get more basic than installing a chain properly.