True, to a point. The critical point is within a compatible model series. No Shimano road derailleur prior to 2012 model year (mid-2011 release) is compatible with larger than 28T. Longer cages were always available, but they were for use with triple-ring cranks. 4600 Tiagra, released in '11 for '12 model year was the first model to be available from launch with 32T compatibility. 5700-A/5701, a second-year update to that 105 series, followed soon after, and 3500 Sora & 6700-A/6701 Ultegra update the following year, and 2400 Claris the year after that. Dura-Ace finally got 30T compatibility with the current R9100 series, released around 18 months ago.RobertL wrote:From the information that Duck has provided in the past I believe that there is never a problem with the upper jockey wheel clearing the 30t cassette, because the upper body part of the short cage and medium cage RDs are the same. You just need to adjust the B screw to suit.
The problem is whether the short cage RD can carry enough chain in total to allow you to access the big-big combination. That is down to the specifics of the bike (chainstay length) and the length of the RD hanger.
Even if it doesn't quite work, the worst outcome is not being able to crosschain to the big-big combination. Everything else will be fine.
Hanger length doesn't really have a lot to do with it, beyond possibly extending the tolerance for a model that may not officially be compatible with a particular sprocket size. When the B-screw is properly adjusted the clearance between cassette & derailleur should be about the same regardless of hanger length. What does matter, and which by extension is where the chainstay length comes into play, is the difference between the front rings. Compact & mid-compact, which have a 16T difference, need the derailleur to hold more chain than a tighter ring pairing, such as a traditional 53/39 (14T difference) or CX-type 36-46, with a mere 10T difference. The actual chainring size isn't relevant at all, because you just cut the chain to suit.
Chainstay length dictates how much tension (using the small ring/small sprocket method of chain measurement) you need to pull on the derailleur from its fully contracted state to reach an appropriate join without any chain slack. Short stays are not necessarily better.
And yes, if it does all go wrong, the only gear you'll lose is big ring/big sprocket, but you should always aim to avoid that combination anyway, regardless of what gear combination you have, because that's the most damaging to your drivetrain.