socaltony

Question: What is the difference between a long and a short rear derailluer?? I currently have a short one, but see other riders with long. Any thoughts???

Sheldon Brown

Long-cage derailers can take up more chain slack when shifting, so they are required for wide-range gearing.

If your bike has close or medium-range gearing, you can get by with a short-cage derailer, and it will be a couple of grams lighter.

Sheldon "It Depends" Brown

slagjumper

The longer one can take up more chain slack. That is the difference between the largest front and smallest rear dictates the length. Better to use one that is short as possible, since they shift more responsively. Check out the Shimano web site and you will find two numbers for most of their rear derailleurs. Max. Front Difference is the largest – smallest front ring. Total Capacity is the difference between the largest front and smallest rear.

https://bike.shimano.com/catalog/cycl...=1199672965738

Max. Front Difference: 14T
Total Capacity: 29T

socaltony

"Wide range"??? I have a double and 9 speed rear.

slagjumper

Double or tripple does not matter. Only the differnece.

socaltony

Thanks, I'll check out the site.

slagjumper

I guess there are other things that you might want to pay attention to if you are near the extremes. From the link above here is more example data based on that RD.

Max. Front Difference 16T
Total Capacity 29T
Max. Rear Sprocket 27T
Min. Rear Sprocket 11T

well biked

The way you figure your max. chain wrap requirement is:
large chainring minus small chainring plus largest cog minus smallest cog.

Example: 53t -39t +27t-12t=29t.
Shimano short cage "road" rear derailleurs have a max. chainwrap capacity of 29t, so for this scenario a short cage would work fine no matter what chainring/cog combo you use.

I expect a short cage rear derailleur will work fine for your nine speed double unless you're using a very wide ratio "mountain" cassette.

socaltony

Thanks!!! Thats exactly the info I was looking for. It's been a question at the back of my mind for some time now. Thats exactly the set up I have.

IK_biker

slagjumper,

I see you have been on this forum for a while.
I thought it's time for you to learn porper terminology, perhaps reconsider what is "Total capacity", and aim to give correct answers to people who don't know any better than ask question on bikeforums.net.

Your statement:
is wrong.

Total capacity of a rear derailleur is the sum of the front range plus the rear range of the bicycle drivetrain - or merely the total chain wrap the derailleur can handle.
What Shimano chose to publish as a "Total capacity" value at the link you provided is irrelevant.

Cheers!

slagjumper

Thanks. The formula provided by well biked is correct. The data was an example.