Quote Originally Posted by emily_in_nc View Post
Hi,

Tech. question here. My Trek Pilot currently has a 12-25 cassette (10 speed) in back with Ultegra derailleur. It's a triple in front (I think it's 30/42/52 but haven't actually counted). In lieu of springing for a new road bike right now, I would really like to change out the cassette for a 12-28, which I assume would be pretty easy. However, I'd love to go even lower (12-30) if my derailleur could handle it. I'm pretty sure it's a medium cage simply because the bike has a triple crank (and from its appearance).

What's the upper limit on what rear cassette I can put on with my current setup without having to go to a long-cage derailleur?

Thanks very much in advance!

According to the Shimano specs for the Ultegra 10-speed rear derailleur (aka Ultegra 6700), your bike probably has the GS shifter (designed for road triples). The maximum capacity is 39 teeth and the maximum rear cog (which is kind of dependent on the B-screw adjustment) is ~28 teeth.

Using the capacity spec, you could use cassette with 17 to 18 tooth difference between the largest cog and the smallest cog (max rear tooth difference = total capacity – front tooth difference).

Since you want lower gearing, using a 12 to 29 (or 12 to 30) cogset would be within the limits. Shimano does make a 12-30 10-speed cassette. This would give you one step lower gearing using the “30 tooth front x 30 tooth rear” combo. This ought to be low enough for road riding.

If you do an internet search, you’ll find that some people have successfully exceeded Shimano’s capacity spec.

P.S. If you decide to replace you current cassette, you should also get a new chain.