brett jerk

I wish that I could better explain this problem, but here's what happens:
I'll be mashing on my mountain bike (this virtually always occurs when I'm going uphill or putting extra force on my bike) and all the sudden their will be no resistance (usually on the downstroke) which has caused me to wipe out and come close to wiping out a few times. Immediately after doing this the chain will resist again, without any adjustments (if I can keep from wiping out, I can keep going)

what might be the cause of this/how can i fix it? I'd post relevant details, but its just a ****ty old trek 800 frame with a bunch of swapped in parts as old ones broke (their virtually all cheapo)

thanks!

frankenmike 

Sound like either a slipping freewheel or missing/worn tooth.

barba

How many miles are on your chain and cassette? It sounds like they may be worn.

brett jerk

the chain and casette haven't been ridden very much, few hundred miles at most (i replaced them and then got a new bike so I haven't put much on them)
although the riding I have done has been in the snow and salted roads, so they probably took a decent beating (I didn't really know how to care for a bike at the time).

barba

Measure the chain just to be certain.

joshandlauri

i just had this happen to me and found several missing teeth on the freewheel, it only skipped when taking off and under a load (hills)

brett jerk

the freewheel is a pretty decent shimano casette and I've switched gears all over the place (i leave my bike on one gear for the most part) so it may actually be the chain? this never really occurred to me. what should i measure? how should I match this measurement up?

Br1an1127

It sounds like you may have a worn chainlink or crank tooth. If thats not the case you may need to adjust your front derailleaur becuase you may be overshifting of the crankset

Cannondaler

If you are not feeling any grinding or popping in the pedals when this happens it could be the freehub body, especially if the chain and cassette aren't that old and there aren't any obviously missing teeth. If it is just completely letting go as if you are pedaling air this would be my guess. If you have another wheel that you could try using the same cassette you are using now you could test this.

barba

Each complete link should measure exactly one inch. The standard method is to measure 12 links worth against a ruler. It should be one foot on the nose for a new chain. If the final rivet is more than 1/16th of an inch past the mark (at the center of the rivet), the chain should be replaced. If the final rivet is more than an 1/8th of an inch past the mark, there is a good chance that the teeth of the freewheel/cassette is worn as well and should be replaced.

Oh, and a new chain on a worn cassette can skip badly as well, as cassettes and chains wear together. People often buy a new chain and then are confused why their shifting suddenly gets worse.

brett jerk

sounds like its finally time to convert my crappy mountain bike to a single-speed! (new crankset/freewheel)
thanks everyone for your help