Fitting 18" wheels to a 2018 XC...

[DeltaNu1142]

Hello, V90 forum members - this is my first post, and I'm hoping to get some info for my neighbor. She has a 2018 V90 XC that originally came fitted with 21" wheels. She is blowing tires at a rate of at least one per year from potholes and rough northeastern roads and is interested in moving down a size or two. Can she get away with 18" wheels? The calipers are most certainly the limiting factor, but from what I can tell the XCs of that year came with wheels as small as 18". As long as the brakes remained the same across the line, 18" wheels should fit... right? Tell me if I'm wrong. I don't want to, ahem, steer her wrong.

Thank you!