bykylist

Hello, I tried a search before posting, but only found a similar question for FD.

The full SP:

My chain was slipping from the rear cassette in mid-range gears, until eventually the chain broke.

I got a new chain from LBS which claimed to be ok for 6,7,8 or 9 gear cassettes. Applied the chain acording to manufacturer's instructions, it was worse.

Eventually got the tools to replace the rear cassette (as advised in other forum) with one which was not brand new, but had seen less than 50 miles.

Same problem. I've also tried the chain which was used for the 2nd (new) cassette, but to no avail.

If this is an elementary problem, please call me an idiot, and point me in the right direction.

If, on the other hand, more accurate information is needed for a diagnosis, please specify, and I'll find it all out!

My bike is a 27 speed trek 4500 (similar to attached pic, but older), alu MTB, used 90% on-road, 10% off.
I do 20 - 50 miles per week, and need to be back on the road ASAP!
(my lil bro's bike is just not fast enough!)

So please reply ASAP, and make a sad bykylist happy again!

Retro Grouch 

There are several possibilities:

The very first thing that I would do is to check the alignment of your derailleur hanger. The accuracy required for a 9-speed cassette requires a special gauge which bike shops have. My bet is that's it.

Second would be a careful re-tuneing of the rear derailleur.

In your case, assuming you got the chain length right, I doubt you are experiencing problems with chain or cassette wear issues because the stuff you are using right now is pretty new.

BCRider

There's more to go with the good stuff that RG gave above as well as pictures if you go to www.parktool.com/repair . Hover your mouse over the rear derrailleur on the bike picture until the flag says "derrailleur" and click. From what you've posted you're either doing or missing something very basic or you have a bent hanger as Retro mentioned.

cobba

9 speed chains are narrower than the 6, 7, 8 speed chain.
9 speed chains will work on 6, 7, 8 speed cassettes/clusters.
6, 7, 8 speed chains won't work on a 9 speed cluster.

FBinNY 

9s chain on 9s bike, right?

If you have a wider chain like a 7s or 8s it might not be settling properly in the cassette. This is easy to test for. There should be a gap between the chain on a sprocket and the side of the next higher sprocket. Slip a dollar bill into the gap, then use a pencil to push the chain over to trap it, and see if it pulls out easily. If not your chain might be too wide.

One other possibility not mentioned by the others is that the RD might be too high and not leading the chain onto the sprockets. There should always be about 1" of chain between the upper pulley and the sprocket. If it's too close the sprocket can push the pulley through the chain causing shipping or in extreme cases pushing back hard enough to break the hanger.

Also take a minute to check for stiff links. Backpedal while pushing the lower pulley forward to slacken the lower loop of the chain. The chain should spool off smoothly and each link should straighten instantly as it clears the pulley. If not lube the chain, and check again.

Otherwise, correct new chain, new cassette with straight hanger, and correct trim shouldn't skip.

BTW- make sure it's skipping in the back and not on the chainrings. Chainring skips are routinely misdiagnosed as a cassette skips because things happen so fast, and both feel the same.