Originally Posted by TheRCF
I understand the reason why I would have to change - just trying to make sure it is always the case with these specific gear rings. I'm still getting conflicting reports. One shop agrees with you (and I suspect you are both right), but a friend says he talked to another repairman who says it may not be necessary. Since this particular repairman is arguably the best around, that is worth noting. When I get a chance, I hope to take my bike for him to see for sure.
My suspicion is that my friend either didn't pass on accurate info about my bike to him or he misunderstood what he was told! So while I wait for a chance to go there myself, I thought I'd do some basic checking with people here.
Thanks.
To understand why you need a compact crank to go to a 34 tooth small ring, just go to the bike shop and hold 34 tooth ring next to your crank. Or hold a 39 tooth 130 BCD ring next to the 34 tooth. It is easy to understand once you see it. The bolt holes in the 130 BCD ring interfere with where the teeth are on the 34. You don't need to take anyone's word for it, take a look. Even a 38 with 130 BCD is pretty close.
The guy who says it may not be necessary may be thinking about swapping the cassette to get similar gear ratios.