Long cage or medium cage??

[Steve H]

Have slowly been replacing the drive train on my mountain bike. Next up is a replacement rear derailleur. It's 8 speed. How do I know whether I need a long cage or a medium cage derailleur. Is there some magic formulae or is it simply a personal preference thing?

Now if the answer is simply replace on a like for like basis, how do I figure out what I've got already?

 

[RecordAceFromNew]

This should help answering all your questions assuming you have Shimano equipment, for Campag try this.

If you are likely to want more range or lower gears in the future get a long cage - it can do everything a medium cage can while the opposite is not the case. The main "disadvantages" if any of a long cage vs medium is higher chance of being struck by rocks/roots and that shifts might be a teeny weeny bit less crisp.

Oh usually they also weigh two extra grams!