I have a trek 3900 and like the questions says i will be riding along and all of a sudden my chain feels like it goes loose and changes gears, and i loose all power when i pedel for a second. what can i do to fix this since i wanna ride again tomarrow.
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sounds like you need to grease your chain and derailleurs.
What your bike is doing is called autoshifiting. You can fix this by adjusting the rear derailleur. Whenever you want to learn how to do anything on your bike, start with the Sheldon Brown web pages (RIP, Sheldon). The derailleur adjustment page is http://sheldonbrown.com/derailer-adjustment.html The special condition on autoshifting is http://sheldonbrown.com/autoshift.html The Park tools web site is also pretty excellent.
HTH
Sounds like your bike is auto shifting to a lower gear. Check to see if the rear derailleur is lose or bent. If it is OK the cable doubtless stretched which is normal. You can try to adjust it by turning the barrel adjuster on the derailleur. Keep track of how many turns you go it so you can return to the original position if need be. Turn it 1/2 turn at a time in a counter clockwise direction and try it.
Here is a excellent derailleur adjustment video.
http://bicycletutor.com/adjust-rear-derailleur/
very nearly definately cable tension on the rear derailleur, spend 20mins playing around with the barrel adjuster to tune your gears perfectly. Once you get some practice at it you can tune your gears in about 30seconds or even whilst your riding!
One more thought that hasn’t been not compulsory is that you force have a stiff link in your chain. Lean your bike against something and pedal backwards at a snail’s pace. If you see the chain popping at a certain spot, you will need to lube your chain or use a chain tool to free up the link. If your bike is a few years ancient or if you ride it in muddy/dirty circumstances a lot, it could also be that your chain needs to be replaced.
Inspect your sprockets and chain. If they are worn, you can adjust your derailleurs all day and still have a conundrum. If they don’t check out to be overly worn, adjusting the derailleurs and chain must do it. Swap the chain if you reflect it is iffy. They are inexpensive and can be worn/stretched out well before the sprockets.