On Feb 27, 4:07*pm, glider wrote:
*The big enclosed trailer was made in the UK and the running gear was
from Germany. * The big Tundra did a good job towing until about 60
MPH when the trailer would start to sway. I'll fix that...easy. I toss
in a couple of bags of crushed rocks in front. No change. Another bag
was added to the front for a total of 120 pounds as I recall.
Amazingly, there was no additional tongue weight. How can that be?
*The sway was just as exciting.
* I'll fix that! Now it has a new single axle and all is well.
*GA

I'd bet there's a lot more to it than just tongue weight.

Tires on both the trailer and towing car matter a lot.
A towing vehicle with soft lateral suspension compliance and/or high
CG. (Think Jeep)
Trailer with a high moment of inertia. (lots of weight in front AND
back.)