2009 Trek 1.7 issues

[chambo34]

Recently got back into road riding after a few years off. I've a 2009 Trek 1.7 ,it's fitted with an 11/25 rear which coupled with a 34/50 front makes for hard going on some Derbyshire hills. So short of it what's the capacity for the 105 short cage rear deraillieur, is it 27 or 28t?

 

[chambo34]

note to anyone I bit the bullet and I've been running a 11-28t on the rear now for well over a year and it works no problem.