Why does my chain keep jumping?

whenever i apply a little pressure to the pedals, the chain jumps around on the gears. i have alot of experience with bike mechanics, but this boggles me. i straightened the deurailler hanger as best i can as well as adjusted the rear deurailler to perfect alignment, (shimano Deore 9spd) and i know the cassette and chain cant be worn. I bought them a couple months ago and have since taken good care of them and never ridden through wet or muddy conditions. my chain almost ripped apart when i rode it today. could worn chainring teeth cause this much trouble? they do look a bit worn, (i heard they are shot when they look like shark fins, which they resemble VERY slightly). if thats the case, is it cheaper to buy a new set of cranks or just chainrings? i currently have Shimano STX (owned only a few months) with a square taper XTR bottom bracket. please help! I go through withdrawl from not riding my bike!!

Why does my chain keep jumping?
It's one of three problems:





1. Excessive chain wear


2. Excessive sprocket wear


3. Frame flex





Since you say you've recently replaced your chain and cassette, that should rule them out. The other thing that could be happening is that your frame is flexing, causing your derailleur to shift in and out of gear as you pedal. Is the chain jumping the sprocket or is it shifting from high to low to high?





If it's shifting, it's frame flex. If it's jumping, either your chain or sprocket or both are worn.
Reply:Where is your chain jumping? If your chain and cassette are new, and your rear reurailler is straight and properly adjusted, your chain rings are likely bad. Your cranks should be fine, you can just replace the worn chain-ring.
Reply:So you have "alot" of experience with bike mechanics? Meaning that you know people at bike shops?





STX was an 8-speed system and hasn't been made for 10 years, but you've only had it a few months? Square taper BBs were replaced on mountain bikes starting in 1999 to go with the ISIS standard, which has changed again to external bearing cranksets two years ago. You mix 8 and 9 speed components and want me to believe you know how to PERFECTLY adjust a derailleur?





You didn't mention the last time you replaced the cables and housing, or if you know the difference between brake housing and shifter housing. If you use brake housing on the shifters, the bike will NEVER shift properly or stay in gear.





So you have extremely old components, most likely used and worn out? Sounds like you mixed old 8-speed cranks/BB with a 9-speed derailleur (you didn't mention if you were running an 8 or 9 speed cogset and/or chain), or have the wrong housing for your derailleurs. If you really knew A LOT about bicycles, you would understand that your parts are not compatible with each other.





Take it to a shop and let an experienced bicycle mechanic look at it.
Reply:Check the limit screws on the derailleur
Reply:Check the ENTIRE drivetrain. I have found that something like that is usually a stretched chain or rear hub (check the palls). The chain may be new, but new stuff can be defective too. STX is not that great. If you ride alot, I would upgrade at least to XT. XTR, of course, is the best.



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