Rear cassette/Derailer Problem

benhben

My Moda Stretto seems to have constant issues with the rear gear alignment. A new cassette was fitted to the bike taking the rear cassette from a 11-25 to a 12-27. Now, the gears will not stay in the lowest two gears (big cogs). They jump around and grind like mad. The bike has been to two bike shops now with this issue, who set the bike up so all seems to be working. Yet as soon as the bike is ridden the gear problem returns and the chain jumps off the sprocket to a lower gear. Once the problem has appeared again under load, it seems to pull all other gears out of alignment and cause problems shifting with all gears.

All cables have been replaced, the cassette was installed by a professional mechanic at the LBS.

My patience is fast running out with this bike. It has been in and out of the bike shop 5 times now and they still havent fixed the problem.

Any ideas what might cause this problem which only seems to appear under load and also cause two bike stores (Edge Cycleworks and Leisurelakes) to fail to repair the issue 5 times?

desweller

B-screw adjustment?

lef

If the chain was a little short in the first place and you've put a larger cassette on then there may need to be a new and correctly sized chain put on.

If you are cross chaining (big ring, big cog) on a 27 it will probably just be that, you're asking too much of the chain.

desweller

Another potential side effect of an undersized chain is a broken chainstay.

Setting chain length

JamesB

A new cassette was fitted

and a new chain too, or is old one being used ?

smidsy

What is teh capacity of the rear mech - i.e can it actually tale a 27??