I installed a new (to me) drivetrain on my road bike. It's an Ultegra 53/39 crank, Rival Front Derailleur, Rival rear derailleur, Shimano 10-speed cassette (12-25) and an Ultegra chain.

The chain is (obviously) for 10-speed road bikes and when I fit it to the big ring on the crank and the largest cog on the cassette, the fit was just as it should be. However, as I shift down the cassette, the rear derailleur doesn't pick up the slack and the chain sags. I installed this thing myself and I'm wondering what I did wrong or what I need to adjust to get it to take up the slack.

FWIW, I had the derailleur running temporarily on a Shimano 9-sp cassette with no problems (except for the screwed-up shift increments, of course) before I bought the 10-speed.

Thanks for any light you might be able to shed on my situation.

Highly unlikely that you could use a brand new chain without removing any links. My preferred method of determining proper chain length is to put the chain on the large chainring and large cog (without being threaded through the rear derailleur) and add a pair of links (one inner one outer).

Did you fit the chain as received, or did you shorten it to the length that you bike needs?

There is a how too guide on the Park Tools website https://www.parktool.com/blog/repair-...-length-sizing

If you need to shorten the chain, you will need to use another Shimano Rivit to connect the chain again.

So its largest cog to largest cog, pulled tight, plus one rivet(or was it a plate? that wasnt very clear), plus one powerlink?

This method goes big to big outside the RD. Add a full link (inner and outer plates). The powerlink would take the place of the outer link.

yeah, I wonder if he ran it through the RD. easy thing to forget.

MKIV987: Make sure you have the chain threaded through the rear derailleur properly. Here's a link to a picture: https://bike198.com/how-to-install-an...ar-derailleur/

Yeah, I initially ran it through the RD when I did the "big ring - big cog" test. I'll do it over tonight and hopefully that'll solve my problems. Thanks guys.

OK so I ran it around the big ring and largest cog, then added two links. Everything works fine when I'm in the smaller (more difficult) cogs, but once I'm about halfway up my cassette, I get this really annoying chain skip that seems to get worse as I move up to larger (easier) cogs. Chain and cassette are both brand new. Any idea what I should check for? Thanks again for the help.

You may need to tweek cable tension (and possibly limit screws) to adjust to the new cassette.