Why is my trailer more stable at 60-65 than 50-55?

Question for you physics majors or truckers...

Why is my tow vehicle and trailer more stable at 60-65 when it gets all nervous and sway happy at 50-55? Seems more speed would make the swaying worse but it is quite the opposite. Any good explanations out there?

Fwiw, I'm pulling a 7,000 lb trailer (26 ft) with weight distribution hitch and sway control bar. Tow vehicle is 2010 Navigator with tow package.

 

Harmonics.

 

Also, possibly tire and wheel balances could be off slightly causing gyration. When a wheel is out of balance it may shake or shudder within a range of speeds (rotations) but that effect dissipates at lower or higher speed.

 

^^^^^This, start easy and check your tire pressures, but sounds like a wheel weight or balance issue.

 

It's also possible that the aereodynamics are better at one speed than another.

CW

 

...which when off only slighly results in very noticeable vibrations due do the manifestation of...

 

Tongue weight is going negative for some reason; probably due to aerodynamic forces. When tongue weight is negative trailer will start to fishtail but if you apply acceleration it will overcome the fishtail. I would check into that possibility.

 

Which you can check with a handy harmonica.

 

Thanks for all the replies.

I'm thinking it's a combination of aerodynamics and possibly the "harmonics" theory.

The tire pressures are good and wheel balance seems to be fine - I know the feeling of an out of balance tire/wheel and this ain't it.

During the 1,000 miles of towing I did this past week, I noticed that it wasn't always so much about the speed I was traveling but also the wind speed/direction played a big role.

Maybe the best idea is to just buy a truck and forget about it!