Originally Posted by
Mikebike12
I did. I bought one! Really nice and all mechanical. Now I have a gearing question. This is my second bike and I'd like to make the lowest gear the same as on my primary bike. My primary has a 46/33T front and 10-36T rear for my lowest gear. My question is, how can I tell how big a rear sprocket I can get on the back with the stock rear derailuer? The 2012 Cannondale 5 Synapse Carbon (I know, I got the BB30, what ya gonna do?) comes with 50/34 and an 11-28. I don't want to change the front sprockets now, but want to know if I can do something like an 11-36 on the back, It has a 105 rear derailuer. I don't know how to tell what the rear can handle. Any assistance would be great! Thanks!
So you passed on BB90 bike because you heard those were problematic, and instead bought a Cannondale with BB30? That's ironic.
If you have the short cage, that RD5700 derailleur is supposed to max out at 28t low cog. The medium cage will go to 30t. Those numbers aren't exact - some frames have longer hangers that allow for bigger cogs. This generation did not use big cassettes - there was a triple crank available instead. Only certain current derailleurs will work with your shifters. Claris will get you to at least 32t, for instance.
If you need to go to 36, I think your best option is the Deore M592 mtb derailleur. It is also on the old road cable pull ratio so it will work with your shifters.