I have a 10-speed Rival rear derailleur and just installed a Shimano 105 11-28 cassette this weekend running an Ultegra chain. When I shift up to the 28-tooth sprocket, there's so little clearance between it and the RD pulley that the chain jump and skips.

Was Rival not meant to take a 28-tooth top-end? It seems pretty odd that this would happen.

Rival rd accommodates 28t. My Rival rd mates perfectly with the SRAM 1070 cassette (shifting from 25t to 28t). No problems using stock cables that came with shifters and yokozunas. Using a 1071 chain.

Did you try the usual barrel adjuster tweaks? B screw? Worn chain?

Adjust the b screw to get the proper clearance of 6mm between the largest sprocket and the upper pulley wheel. Rival RD can handle up to 28 teeth.

Check your chain length.

Edit: beaten to the punch by others..

Make sure you've adjusted the b-limit screw on the RD. If you've got a short cage RD, it may not technically be able to take a 28-tooth cog, but it should be close enough where it would work fine. I've got an Ultegra 6600 rear RD that's supposed to only work with up to 27-tooth cogs and got it working with a 28-tooth cog by properly adjusting the b-limit screw. One tooth beyond the stated max capacity should be fine even for a short cage RD if it's properly adjusted.

SRAM Rival RDs come in 2 sizes. The mid cage is designed to accomodate 32. The regular/short cage is designed for up to 28 teeth.

Good call, very important.