derailleur.

[Sshields32]

My rear derailleur isnt work properly. Its a Sram x-9
My brother and I set the high limit and low limit screws perfectly, and it runs fine except that it skips the 8th gear. Any clues as to whats going on?

[jonKranked]

did you check your cable tension? that affects your shifting far more than the limit screws.

[DownCycles_Stef]

Assuming the hanger isn't bent, your limit screws are correctly set and you have the correct tension on the cable, check your B-Tension screw. On SRAM derailleurs there should ~ 5mm of space between the upper pulley and big ring on the cassette when your in the lowest gear. Turning the B-Tension screw clockwise will draw the pulley away from the cassette.

If I had to take a guess, whenever there is only one gear that's giving you trouble it's usually either a slightly bent hanger or a B-Tension screw that's not set properly. There's also and outside possibility that you could have a bent tooth on your cassette, but thats really uncommon (and if you do go to check, keep in mind that one or two teeth will always appear slightly bent on each gear because they are ramped and angled to aid shifting).

[Sshields32]

Well, its not a bent hanger, because i have never crashed on this bike, well, not to badly. It must be the b-tension screw. Ill check it tomorrow, but i know we adjusted that too, but im not sure how far away it was set from the cassette.

[DownCycles_Stef]

Keep in mind that even a small crash can tweak a hanger. It doesn't need to be all that far off to cause an issue. Just eye it up and give it a little tug by grabbing the derailleur if needed. Remember that you want it to be square to the cassette, not the frame.