AMoney

I will likely need to get a new derailleur for my road bike. Right now, it has an 11-30 cassette with a short cage derailleur. I would like to get a middle cage derailleur because in the future I would like to get an 11-32 or 11-34 cassette. I was wondering if I need to lengthen my chain if the only immediate change I'm making is switching from a short to a middle cage derailleur? Thanks

terrymorse 

Assuming the chain was sized correctly, you don't need to lengthen it for a different derailleur, although you may need a longer chain when switching to a wider cassette.

The Shimano chain sizing instructions for wide-range cassettes is to set the chain on the big-big combination, without running the chain through the derailleur. Then add one set of links. No mention of the type of derailleur (short or long cage).

Polaris OBark

This is a great summary

Iride01 

You will need a new chain eventually anyhow, so why not just get a chain to have on hand if you find out the length of the new DR cage does make your existing chain a less than appropriate length?

DaveSSS 

I use the chain length formula at parktool.com.

​​​​​​If 4T is added, 1 inch of additional chain length is needed.

AMoney

Thanks for the link! It is good to know that changing the derailleur doesn't require a longer chain. I replaced the derailleur and sized up to an 11-32 cassette. I previously had three extra links so I was able to size up without having to lengthen or replace the chain. I don't want to replace the chain yet because it is waxed. The process of removing the factory grease was time consuming and my waxed drivetrain is still very new (less than 1,000 miles).