Originally Posted by JamieGeek
Vehicle sway when pulling a trailer is directly related to the lever arm between the hitch and the rear axle--a longer arm and you get more sway.
I agree that's an important factor, but there are other variables that also affect stability. Trailer weight (actually mass) distribution being a very important one.
Many conventional trailers get by with 10% on hitch, while 5th wheels can easily go to 25% or higher. In my opinion this gets back to trailer design versus truck capability.
If you look at trailer I posted picture of above, it appears the center of gravity is far enough forward to result in a fairly high percent of weight on hitch. I think that makes a huge difference in high-speed stability.
I have a friend who installed a bike carrier and 4 bikes on a trailer's rear bumper (approximately 100-pounds total) and that was enough to make the small trailer become unstable at higher speeds. Before that it had not been an issue.