RH-550 Shimano Hub problem...

remove the rear wheel and rotate the cassette by hand. If it sticks , the freewheel-mechanism has failed.

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hold the axle and rotate the wheel , if it sticks , the axle bearings have failed.
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Lionfish said:

Hello everyone. I have a 4 month old RH-550 Shimano wheel with a 9spd 105 cassette. When I got to work today I noticed the hub was louder then normal & when i spin the back tire it "catches" & moves the cranks at the same spot with every revolution. I removed the wheel & slightly feel the cassette budge at the same spot. Any ideas?

The freehub body probably just needs to be flushed or the hub bearings need cleaning/adjusting. I'd bet on the freehub.

http://www.parktool.com/repair/readhowto.asp?id=118

See the Park tool website for advice on how to do this. It is pretty easy. You can actually flush some spray lube into it without removing the freehub from the hub--aim it at the seam just under where the lockring threads are and spin as you spray. If it still sticks, then you probably need to remove it and give it a throrough solvent flush.

Four month old shimano would be a warranty job so let the shop worry about it.
It sounds like a fault in the freehub.