Originally Posted by
Al1943
It's not just about cage length and chain wrap. A mountain type Shimano rear derailleur has a different geometry in its upper half that allows for a larger cog, like a 34 or 36.
Yes, and I barely touched on that. It really didn't matter, since he will most likely need a mtb rd for any of the proposed gearing changes to work reliably or even at all.
Originally Posted by
shelbyfv
You might try your triple road rd with a 32t cassette. Sometimes it will work. If you cannot adjust the rd top pulley away from the 32t cog, you know you need a mtb rd. A 34t cog has virtually no chance of working without a mtb rd.
That's a thought, but even if his Tiagra rd can handle a 32t cog on his frame, it's not designed for that amount of chainwrap. And then it results in only a 25" low gear, exactly what he'd get if he kept his current cassette and changed out his inner ring for a 24t but at considerably greater cost.