Excellent video demonstrating the importance of proper TW

Not too long ago, I came up on a fellow towing a small trailer with a lawn tractor aboard. We were on I40 just outside of Asheville. The trailer started swaying as I approached, so I backed off. He would hit the brakes, the sway would stop, then he'd speed up and the sway would start again. I finally got a chance to get around the doodlehead. As I passed, I saw the problem. Almost all of the mower was behind the axle.
If he didn't have a negative tongue weight, it sure had to be close.