Mountain bike chain skipping... problem please help

Question

i just bought a gt outpost about a week ago. and its great but im already have a problem. when im in either 1st or 2nd gear in the front and 8th geat in the back the chain skips when i peddle hard. it only does it for that for those 2 gear combinations. the bike shifts fine. nice and smooth and with no hesitation. but it skips when its in 8th gear. iv tried adjusting the rear derailur, lubing the chain, checking it for stiff links and i even brought it back to the dealer. but it still does it. this just started happening. please help i really want to fix this.

Answer 1

maybe your chain is too big for your derailur, try a thinner chain

Answer 2

If you mean the smallest (hardest to pedal) gear in back when you say 8th gear, then chances are that you're doing what's called "cross-chaining".   That's where the angle of the chain from front-to-rear is too much....it needs to be lined up better so that it runs quietly and stays on track.  This is a problem with almost every bike no matter how expensive or how well-tuned it is.   There's a chance that maybe your derailleur and cable tension aren't set up the right way, but it's probably just cross-chaining.   Sometimes you can run that gear ok when pedaling easy, but when you lay the hammer down the bike will flex just a tiny bit and cause that skipping.  

The only solution is to learn how to shift properly....and I don't mean that as an insult.  When you want a harder gear and you're already in the middle chainring, ready to shift to a smaller (harder) gear in back and close to 7 or 8, then you need to instead shift up to the 3 gear in front (big chainring) and shift back 2 or 3 easier gears in the rear.  It depends on the gearing, but 2 or 3 gears will get you close to the gear you were already in with 2/6 or whatever.  

If I could show you a gear chart you'd understand why....it's just about ratios and gear inches.  But that's the proper way to shift and it'll keep you from throwing your chain, having skipping problems, or wearing out your drivetrain metal prematurely.   Also, the same problem works in reverse....if you're trying to shift to an easier gear you need to keep an eye on the cross-chain thing, too.  It's easy once you get the hang of it.

Really, it's probably not the bike itself.  There is a tiny chance that your derailleur hanger could be bent if you crashed or the bike fell over on that side, but when that happens it usually affects the gears mid-range or low-range and works ok on the harder gears.  You could ask your shop to check it for you and double-check the derailleur adjustement (maybe at a different shop), but it's probably just cross-chaining.  

Hope this helps...if you need more help on it just post up again.   :o)