Why flat belts need crowned pulleys.

[tonyfoale]

I have a bandsaw with 12" wheels, commercially made (pretty old Australian manufacture) both wheels are perfectly flat, how ever, the top wheel can be 'canted' out of vertical to control the blade position on the wheels would this be your "misalignement" albeit adjustable?

Yes, exactly.
A misaligned wheel or pulley presents an angle to the belt which is like one side of a crowned pulley. Many bandsaws, mine included ( Homemade vertical bandsaw. ) have a tracking adjustment as well as crowned wheels. This serves two purposes, the crowning keeps the blade on the machine and the adjustment allows you to place the blade where you want on the wheels.

I only have the top wheel crowned, and I use the adjustment to place the part of the blade just behind the teeth, over the top of the crown. This means that the tightest part of the blade is toward the front and this gives the best directional stability. I explained that with a post on this forum and a video, but I cannot find either right now.

Blade front set to crown.