So yesterday I took my friend's trailer up and gathered firewood. This is a great custom built 3,500 lb axle trailer with full size tires. I've used it for years with no problems - until yesterday. I loaded it up with firewood about 18" high from front to back. When I went much over 60 mph the thing would start to fishtail severely. It was scary. After 2 events I got off the freeway and stayed under 50 mph. I swear I had the thing loaded with 60% of the weight in front of the trailer axle - maybe not. Also, the hitch height is just right so the trailer sits level.
This is this is the first time firewood cutting that I didn't fill the bed of the truck with wood. I just put the lighter stuff like my chainsaws, tools, chairs, lunch, etc. Going up to the mountains I whipped down the freeway sometimes over 80 mph with the trailer empty and it behaved very well. On the way back the trailer was heavy but the truck was relatively light in the bed.
Question: Does having a light rear end on the truck and a heavy trailer affect fishtailing or it is pretty much the load distribution on the trailer?
Thanks for your advice.