Hello there,
I'm designing my first frame by studying the geometry of my existing frames. My question is about seatstay-chainstay angles (and therefore also rear dropout angles).

In looking at dropouts, it appears there are few common angles, none of which match the angles of those on my current bikes. Can someone explain this? Without building their own dropouts, do builders generally need to design around the available dropout angles? Or are there adjustable ones?

Any help in clearing this up for me would be greatly appreciated.