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Back Sprockets Break Loose Under Load
#1
I have a Shimano Hyper 7005 Series bicycle that has an issue in the back wheel. Under load (simple acceleration), the back sprockets seem to break loose.

The chain is not jumping, the gear cluster spins with the chain, but the cluster doesn't seem to catch the wheel. Sorry if my terminology is wrong; I don't know that much about maintenance (hence my question online). It's so bad, I can spin by hand to get it to break loose.

And guidance toward diagnosing and repairing would be greatly appreciated.

THANKS!!!
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#2
It could be that the pawls in the cassette freehub are gummed up a bit and not engaging the ratchet. Try flushing the freehub through with some thin oil, something like WD40, once it's clear and working properly, lay the wheel on its side, let it drain and apply some thicker oil. Try not to flush out the grease from the wheel bearing; it may be best to remove the cassette so that you can see what you're doing.

Take a look at this: http://sheldonbrown.com/k7.html

And this: http://www.parktool.com/blog/repair-help/cassette-and-freewheel-removal
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#3
could also be stripped threads on the hub. if a newish bike, complain to your supplier and ask for repair under warrenty
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