So I'm getting ready to do brakes/rotors on my VW and this question comes to mind.

I see brake calipers in just about every position from 9 o'clock to 4 o'clock. Never on the lower side. Front and back wheels may be in the same position or completely opposite. Is there a preferred/better location? Was it the whim of the car designer?
Inquiring minds want to know.

I think there are a lot of factors that go into it. There may be suspension bits that interfere with the calipers in a particular position, for instance. (Notice how the calipers are often on the opposite side of the knuckle than the steering arm!) In some cases, weight distribution may be a factor. There may be cooling concerns; I bet that leading calipers are a little easier to cool (or to run cooling ducts to!) than trailing calipers.

My guess is you don't see calipers on top or bottom because it would be too easy to get air bubbles trapped in them.

--DD

I no longer recall clearly, but back in 1974 I purchased a Suzuki MC with dual front disc brakes. The calipers were placed in front of the forks. There were compelling arguments made that braking and steering would be improved if they were located behind the forks. So I made the change. If I recall it had something to do with the direction of a torque reaction.