Hello,

For the last few months I have been using a new Shimano 13-25 (10 speed) cassette with a first gen Sram Force derailleur. I was previously using a 12-25 cassette, and shifting was fine. Since first installing the 13-25 cassette, there has been a delay shifting into the outermost cog, which was annoying but not a big deal.
More recently, I installed a new chain (same brand as before, KMC) and now suddenly the bike refuses to shift into the outermost (13t) cog. No amount of fiddling with limit screws and cable tension fixes the problem. The derailleur itself moves over the outermost (13t) cog and beyond, but the chain just seems to get hung up on the adjacent (14t) cog and stays there . The bike shifts fine otherwise.

I have no idea why a new chain exacerbates the problem to the point that I can't even shift into the 13t cog. It seems to me there are two potential reasons for the problem, though maybe there are other potential causes.

(1) Either, Sram derailleurs are not designed to work with a 25-13 cassette since SRAM doesn't make such as cassette

or

(2) my derailleur is just worn out. My rear derailleur is far from new. Is it possible that the springs in the derailleur are so worn that it can't clear the outermost cog when it is a 13t. Its worth noting here that I tried a 12-25 cassette with the new chain and it shifts into the outermost position perfectly.

Any ideas what is the most likely problem here?

If you tried to fiddle with limit screws and cable tension and it didn't help, the next logical step is to fiddle with the B-screw. Screw it in a bit so that the RD hangs further away from the axle. Have you tried that?

Yeah, I hadn't thought of that. You might be onto something there. I'll give it a try and shout back.

did you use the spacer behind the cassette?

Well, you nailed it. I put the 3-25 cassette back on, gave the b-screw a few turns and it works. Thanks! All these years doing my own wrenching and I can't believe I didn't think of that. There's always something to learn.

Never. I've never understood the point, nor have I ever gotten one with any of the many 10 speed cassettes I have ever bought. Is there something I am missing out on here.
In any case, hamster solved my problem.

with shimano 10 spd cassettes not using the spacer will leave the cogs loose when tightened down. sram does not use the spacer. i think tiagra 10 sp cassettes do not use the spacer either

I seem to have had plenty of space to tighten the cog down on any hub I have ever used so apparently I don't need it?

try to twist the cogs behind the lock ring and high gear it will be really obvious if they are loose. if the cogs move independently then you need a spacer but shimano calls for the spacer by design.

Sluggish shifts to the smallest cogs can also be fixed by finding a way to increase tension on the derailleur's return spring.

Some Shimano derailleurs have a two-stage preload adjustment, but lacking that you can wedge something under the end of the spring, or use some fine wire to bind coils together if it's a tension spring.