Originally Posted by
Andrew R Stewart
Yes the upper pulley is so tight against the underside of the 32T cog that it is trapped by the chain from moving sideways. As mentioned the large cog capacity for this der is 27T IIRC.
A few solutions- You could mount the rear der on a Wolf Tooth or similar device that effectively lowers the der's mounting point. You could install a 9 speed MtB rear der that has a greater capacity. You could go back to the 27T max for the Tiagra der and instead mount a inner ring smaller then the 30T that is the OEM size. I run 26T grannies on a few of my otherwise lightweight bikes (for the Finger Lakes and my old body).
My vote is both the MtB rear der and the smaller granny ring. This way you get both the larger rear cog capacity as well as the greater chain wrapping capacity that a MtB der has. BTW I also run a smaller middle and large ring, then is the typical stock spec. on all my bikes. 26-36-46 is on a number of my bikes. Andy
I would probably go back to the 27T (or maybe a 28T if one is made in the appropriate configuration). 28T is pushing things a little, but as I've discovered on my 105 RD it's workable. And then in front do as suggested, moving from a 30t small ring to a 26T or 28T. The problem here will be a matter of whether the FD can handle the tooth-difference between the smallest ring and largest. On my bike I have 30/39/50 up front, and I don't think I could go smaller than 30 without moving down to a 48T big ring. But the OP may find his FD can handle this just fine. Usually the manufacturer's specs would list the FD capacity.