I have a Cannondale CAAD 10 disc frame and a pair of BB7s that are 160mm rotors.
They do not fit because (I think) I need post mount spacers.
What size rotor does the frame expect, and hence, what size spacers do I need?
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The frame comes stock with 160mm rotor in front and a 140mm rotor in rear and uses a spacer for the front post mount only.
From this link, it looks like you need the 20p spacer, which Avid says is OEM only. However the spacing dimensions are 12.5mm top and 7.5mm bottom. You can probably find spacers of those dimensions to mount it without the OEM spacer.
I have the same bike with a 160mm rotor in front and I can confirm the dimensions are 12.5 top and 7.5 bottom and it looks like spacers would work as well.
Here is what the spacer setup generally looks like - 160mm with OEM spacer on front, and 140mm mounted with no spacer on rear.