The 920 should happen in a week. I bought a 12-28 cassette for pavement and hard pack trails. from the 11-36 it comes with.
So the question is. Do I need a shorter chain with the tighter cassette?

No.

Brad

I'm in the process of buying a Trek 920, changing it to brake/shifter levers. I bought a 12-28 cassette to replace the 11-36 it comes with so when I do pavement and hard pack trails.
So the big question is! Do I need a shorter chain for the 12-28? The chainrings are 28-42.

Are you changing the rear derailleur?

If you're keeping the rear derailleur, then there is probalby no pressing need to shorten the chain.

What happens when you cross chain, small-small? 28-12? Is there adequate tension to keep the chain from dragging back across itself?

Shortening it, however, isn't a big deal, and you could do it now, or later when you next replace the chain. Any plans to change chainrings?

We have merged your cross-posted threads into one thread.

Please do not post the same question across different sections of the forum.

Likely not. The situation where the chain may be too long is in the small ring and smallest cog. The difference between the smallest cogs on your two cassettes is one tooth. The chain only wraps 1/2 way around the cog so the difference in chain wrap is 1/2 tooth or ~1/4", which isn't much. You can only change chain length in two tooth (1") increments.

Assuming your chain is now long enough to allow big-big (it is, isn't it?) you could shorten the chain by (36-28)/2=4 half-links and still safely be able to shift into the new big-big. However, as above, unless your chain is very slack in the new small-small there is no compelling reason to do so.

As mentioned, you can, but there's really no reason.
Leaving as is, allows the ability to switch back to your 36T cog if desired without having to add links back.
You might also consider you have 2" of extra chain to "share the wear".

I take my hybrid on vacation to much hillier terrain.
I leave my chain a bit extra long so I can swap on my "mountain" cassette and simply not have to bother with chain length when I do.

Thanks!!