Per spec, 27T is the largest cog a 5500 (9-speed 105) RD will take:
https://www.bike-components.de/bedie.../rd-5500.2.pdf
Since Shimano is conservative in their ratings, I would imagine it could take at least a 28T, but it'll depend on the bike. If I were in your shoes, I'd be considering a triple.
__________________
Originally Posted by
chandltp
There's no such thing as too far.. just lack of time
Originally Posted by noglider
People in this forum are not typical.
RUSA #7498
Last edited by ThermionicScott; 10-07-14 at 09:11 AM.