Why crowned pulleys and bandsaw wheels?

I had it saved as a link in my bookmarks.

I have a 13" wide belt sander that will not keep track with a convexed crown, and even though theory says a concave crown fails, it did not......
The "original" China copy had the idler pulley a smooth cylinder, no crowning, and the motor driven pulley that contacts the wood work piece a convex crown. Well that didn't work, as it made the board have a dish concave scallop along it's length, and depending on where you fed the board, it would remove so much material, it would be burned by the belt.
So I pulled the whole thing apart and machined the pulley in contact with the wood board (sanding belt in between) to be a perfect cylinder.
I used duct tape on the idler pulley to make the crown convexed originally. But whenever the belt got warm, it would take off from center position, and the sanding belt would grind the sides of the machine internals. I muttled with it this way for years. Then last year I stripped off all the duct tape, and tried it with a convex crown. That has worked reliably.
There were a few other serious manufacturing design/manufacturing defects. The power feed belt they used, that had a drive roller and idler, however the idler support brackets were machine in the incorrect location, and it was at an angle to the table, essentially twisting the boards being fed in. The root cause of machining the hole in the wrong location, looks like a casting pattern that didn't leave enough material to put the hole where it needed to be, so they just located it .1 inch higher. There were several importers of the 13x32 belt sander. I got mine from Woodworkers supply, with the Wood Tec name brand. But Harbor freight imported them and others.