Did you ask for compatible parts or did you ask for a 28T cassette? Exactly when a shop should protect a paying customer from their (the customer's) requests is a slippery slope. Generally as long as the slope isn't a safety one then many shops will, at best, suggest that the results will be a "B" fit if the specs are being exceeded by a small amount.

But again, is it the shops responsibility to look out for (and refuse to sell the parts that can create) the worst case that a customer's choices might result in, when safety isn't at stake? This is all together another topic. Andy.

Usually, you can fudge a little bit WRT the max cog size.
1T larger shouldn't be a problem in most cases. Sometimes, you may be able to do 3T, but much depends on the specific bike and RDER hangar geometry etc.

Screw in the B screw a bit.

Your main problem will be chain wrap capacity.
Size the chain to big:big +1 inch.
See if you can live with it. You'll probably have a bit of chain "sag" with the small ring and 2-3 smallest cogs.
Since you shouldn't really be using that combination, you may be OK.
Try it and find out.

They might have looked at the spec sheet, and saw a 5800 which has a specified largest cog of 28T.
They may have also seen the 36T smallest ring, which you changed to a 34T, resulting in 2T greater wrap capacity being needed.

Garbage in, Garbage out.

Like I said in another post, I am a little miffed at the bike shop that sold me these parts - they should have known better as they did look at the spec sheet of my bike's components. With the limitations you stated on the 5600, I am leaning towards getting either an 5700 or 5800 to replace the 5600. Will either or both of these work? The 5800 seems to have a bit better capacity.

Can a rear DR be replaced without breaking the chain again? This has been altogether way more work than I was expecting.

Cheers,
Larry

I'm pretty good with things that have chains and sprockets, not so good with electronics. Things that are so simple to most folks, like posting a photo on line, seem like way more work than I was expecting. The good news is that, if you stick it out, you will learn the process and it will be lots easier the next time.

1. The first thing that I would do would be to find and dial in the "b" screw about a turn. The b screw is at the back of the derailleur and screwing it in will pull the derailleur back a small amount. My bet is that, by itself, will fix your problem.

2. If, and only if, that doesn't fix it, look to your chain length. Always size the chain yourself. You have no way of knowing how it was sized previously so adding 3 links was almost almost surely more than is necessary.

3. Replacing your rear derailleur is a desperation move. You don't need to do that. The one that you have will work fine.

I agree, with the caveats I've already mentioned, with Retro Grouch.

But to answer Larry's last post- The 5800 is an 11 speed spec der and thus has a different cable pull ratio then the 9 or 10 speed spec ders have. So it won't place nice with your Tiagra shifters. Getting a longer cage der (on the road line this is called a medium cage) will get more capacity and chain length options then a simple B screw adjustment can achieve. One can remove the der pulleys from the cage to install a der and not need to open the chain. But installing a connecting link is a great way of dealing with reattaching a chain and lets one remove it in the future for the best cleaning methods. Andy.

Probably the same kind of reasoning that has folks wanting to put 120psi into a tire rated for 70 and asking if it is OK.

Thanks again, All good info to work with.

Cheers,
Larry

Thank you all for all the tips!!! I tried in vain to fix it with the B-Screw so just tried playing with the chain on all the various positions and figured out the dang chain was in fact too long. I knocked out a couple inches and smooth shifting. Only a minor rub on the small ring on FD and fixed that with the limit screw. You guys are the best!

Cheers,
Larry