New Derailleur

[Peter Salt]

This is a really messy thread for what is a simple question.

In the current setup, your friend needs a rear mech with 33T capacity that covers up to a 28T sprocket. The Ultegra R8000 SS (short cage) meets those requirements (and also future proofs the setup in case he wanted to change to a 11-30 cassette). That's it.

Other things to note: If the friend wants to go above an 11-30 in the future the short cage won't do. So might be a good idea to go with the mid cage (GS) depending on how likely that scenario is - even if it's not technically needed now. On the other hand, the mid cage should not be used with anything below an 11-28.

Use this to calculate the chain length.

 

[Legs]

Other things to note: If the friend wants to go above an 11-30 in the future the short cage won't do. So might be a good idea to go with the mid cage (GS) depending on how likely that scenario is - even if it's not technically needed now. On the other hand, the mid cage should not be used with anything below an 11-28.

So if a mid-cage mech is okay for, say, riding in 53x21 as a middle gear on a wide-ratio cassette, why is is not okay for riding in 53x21 on an 11-23 cassette? Surely the chain tension is only a function of how big the sprockets and chainrings are, chain length etc. and it shouldn't matter whether the larger sprockets exist?

FWIW I run a 53/39 and 13-25 on my Zwifting bike with a mid-cage mech and it's fine.

 

[Dogtrousers]

So if a mid-cage mech is okay for, say, riding in 53x21 as a middle gear on a wide-ratio cassette, why is is not okay for riding in 53x21 on an 11-23 cassette? Surely the chain tension is only a function of how big the sprockets and chainrings are, chain length etc. and it shouldn't matter whether the larger sprockets exist?

FWIW I run a 53/39 and 13-25 on my Zwifting bike with a mid-cage mech and it's fine.

It matters because the chain will need to be long enough to go around those larger sprockets (and the big ring). So the mech needs to be able to take in the slack if/when you move to, or towards, small-small combination.

 

[Sittingduck]

Succinct answer alert:

SS

 

[Legs]

It matters because the chain will need to be long enough to go around those larger sprockets (and the big ring). So the mech needs to be able to take in the slack if/when you move to, or towards, small-small combination.

I understand that, but, consider this thought experiment:

• It's okay to run a mid-cage mech with 50/34 and 11-32 cassette, for some length of chain.
• It's therefore okay to run the same mid-cage mech with 50/34 and 11/32 with the same chain, with the lateral movement of the mech adjusted to allow only access to the smaller sprockets, say between the 11 and the 23.
• It would therefore be okay to use the same mech and chain length with an 11-23 cassette, surely?

 

[Dogtrousers]

I understand that, but, consider this thought experiment:

• It's okay to run a mid-cage mech with 50/34 and 11-32 cassette, for some length of chain.
• It's therefore okay to run the same mid-cage mech with 50/34 and 11/32 with the same chain, with the lateral movement of the mech adjusted to allow only access to the smaller sprockets, say between the 11 and the 23.
• It would therefore be okay to use the same mech and chain length with an 11-23 cassette, surely?

I've just backtracked up the thread to see how this this originated. I think I agree with you. Provided that the mech does indeed have at least the 37T capacity needed for 50/34 & 11-32.

My brain hurts

 

[Legs]

My brain hurts

Mine too!

The logical conclusion of my line of thought is that mid- and long-cage mechs can be used for close-ratio gears. This would probably lead to sub-optimal chain tension and shifting speed, though, which is why short-cage mechs are better in this case, hence what @Peter Salt said upthread:

the mid cage should not be used with anything below an 11-28.

 

[Peter Salt]

Mine too!

The logical conclusion of my line of thought is that mid- and long-cage mechs can be used for close-ratio gears. This would probably lead to sub-optimal chain tension and shifting speed, though, which is why short-cage mechs are better in this case, hence what @Peter Salt said upthread:

Dead right. I was quoting the manufacturer recommendations in this case but do have experience of seeing and feeling what a wrong size cage does with the wrong cassette. Chain tension is probably the biggest issue with semi-regular drops no matter how well the rest is adjusted.