I've always looked at these dinner plate cassettes as a way to increase the high end gearing.
Follow me on this logic. if i run a 30T in the front, and 11-42 in the back, i can install a 32T in the front
and 11-46 in the back. 30/42=.714, 32/46=.695, gained a little mechanical advantage there, but
30/11=2.7 32/11=2.9 gained 6% on the top end.
I don't do TTs on the mtb, so how often am I in the little cogs? Finding myself unable to
climb comfortably in the 32T chainring scenario, would mean I'd go to the 30T chainring
with the 11-46 - giving even more mechanical advantage at the low end, at the cost of
the high end - which actually might come into use now.
Other advantages of the smaller chainring:
- a bit more clearance
- Upgrade cost is less (there is a lower limit before changing cranks to accept smaller chainrings)
- rear der is used in its designed range
I think i'm 30-10/46 on the tallboy, 30-11/42 on the fatbike (which is 9 sp, with a extension link - total cost of upgrade less than $50) -
since wheel diameter on the fatbike is smaller, the mechanical advantage of the cassette doesn't need to be as large.