new cassette do i need new chain length??
 

hi there
i want to install a new 10 speed cassette a 12-27 instead of a 12-25 What i wondered was do i need a new chain ie does the chain need to be longer for the slightly bigger cassette. if i need a new chain length can i lengthen the old chain or do i need a new chain
thanks
leon

probably not. since it is just two teeth i bet you derailleur can take it up. If you just shift smartly than it shouldnt matter at all.

You should be fine.

You are probably fine as mentioned above, but if you want to make sure, put the bike on a rack or upside down and pedal it into the big chainring and then slowly shift into bigger and bigger cogs, maybe sure the rear derailleur arms never straighten completely out. If you get to the largest cog, you're safe. If you can only get to the second largest cog, you shouldn't cross chain while riding.

+1

If your chain is barely long enough for the big-big with the 25, it might just rip your rear derailleur off or your rear wheel out in the big big with the 27.

Unscrewing the b-screw might help a bit, if it's not already unscrewed all the way.

good luck,
cdr

I did just that with a 12-28 cassette. I knew I had exceeded the chainwrap capacity by 3, but I never cross chain. Still, I started from a traffic light and pieces of derailleur flew everywhere and I got to pretend my bike was a scooter all the way home.

The b-screw position won't make any difference as the rear derailleur's position is immaterial if the chain is too short to cover big-big. When the chain is stretched completely tight, the derailleur is in a straight line and effectively isn't there as far as chain length is concerned. That's why sizing a new chains can be done by wrapping it around the large chainring and largest sprocket but NOT through the rear derailleur.

The difference between the minimum chainlength for a 25T cog and a 27T cog is 1/2 link (1/2") which you can't add. So, if the chain won't allow big-big with the 27T cog, the OP will have to add a full link or 1" of chain.

Take the 30s, use the big-big method and size your chain.

Or, even simpler, measure your chain (each link is 0.5 inch) and google to a site with a chain length calculator : you'll need : big chainring nb of teeth, biggest cog nb of teeth, length of chainstay. Only if your chain is too short, you'll have to add some links to it.

Gonna have to agree that operator had the simpler method.
http://sheldonbrown.com/derailer-adjustment.html#chain