I replaced the rear shifter cable and now the chain won’t stay on the largest 2 cogs. 10 speed SRAM Rival. I can push it up there with the shifter lever and it stays while I hold the lever in place, but as soon as I let go the chain drops down to the 3rd largest cog. I’ve been messing with the barrel adjuster and limit adjusters ad nauseam but nothing seems to work. Any ideas what I could be missing?
Try re-doing the cable tension as it may be a whole gear, or more, too loose.
Start by screwing the barrel adjuster in almost all the way. Shift the rd into the hardest gear in the back so that the cable is at its most slack. Then loosen the bolt holding the cable and pull the cable tight by hand, re-tighten the bolt to hold it. Then try tuning again, backing out the barrel adjuster to increase tension more if needed.
I would guess a bent mech hanger is the main culprit with cassette wear contributing to exactly where on the cassette the slippage occurs. It is really easy to bend/twist a hanger and once this happens you lose perfect indexing of the gears. The good news is that on the vast majority of bikes this is an easy thing for a bike shop to fix. You can always try to straighten it yourself but its one of the those jobs that is 100 times easier to do with the right tool vs trying to bodge at home.
Sounds like your cable is way too slack and you are a good one or two sprockets out in your indexing. Unless something weird is going on where a cable housing is buckling or bending when it is under the greatest tension, or the housing is being pulled out of its usual routing.
As SBRcanuck says, best thing is to start from scratch with the indexing. Lots of guides on the internet about how to do this but I like to start with the cable tension a little too high with the barrel adjuster let out a fair bit (turned anti-clockwise), so that the chain sits on the 2nd smallest sprocket when the rear shifter is clicked all the way down (cable too tight to let it shift down). Then I gradually let in the barrel adjuster (turn clockwise) until the chain just falls onto the smallest sprocket while turning the pedals, and then tweak it from there. I usually find a further 1/4 to 1/2 turn clockwise gets me right into the sweet spot. Check by shifting up and down through the full cassette.
You might want to leave some further wriggle room with the barrel adjuster ideally close to fully in, because as the cable stretches you might need to adjust for more tension by winding that barrel adjuster back out a bit.
Good luck.
Cheers, Rich.
If you are in the smallest cog and shift up 1 click, does the chain move up a cog? If not, the cable is too loose and should be an easy fix. If it shifts up with no issues, then I’d think the hanger is likely bent (may not look bent but a small tweak can throw it off). I’d suggest getting a new hanger to eliminate that as being the issue, they are cheap and a backup is good to have.