[suss6052]

The 13's are 12.6 front and 14's are 13.1 front right? So if we use this on a 14 it would reduce our rotor size?

No.

Every single Focus ST built prior to 3/30/2014 and that includes at least the first half of 2014 Model Year Production has the 12.6" Rotor. Only the cars built on or after 3/31/2014 have the 13.1" front rotors. So unless your car was built after this point you're sticking with the stock rotor size.

 

[suss6052]

Mine is a MY 13 so its not an issue for me but are you able to switch rotor size like that without doing anything to the calipers? It's only half an inch diameter difference but would that affect things?

You would need to change the brackets to re-orient the caliper correctly if you had a 14 with the bigger rotors. I would not recommend downgrading to a 12.6" rotor if the car had a 13.1" front rotor set up though.

 

[suss6052]

I remember reading that the 12.6 rotors are actually the better rotor. They have more bite. The only reason for the 13.1 rotors was because of complaints of noise and dust from the 12.6 ones. Bigger doesn't always mean better.

The pads are more aggressive on the 12.6" rotor package allegedly and dust more, but a larger rotor means greater thermal mass, so if you put the aggressive highly dusting pads back on or a more track focused aftermarket pad it should still function better.