I'm pulling this out of a vehicle stability class that I took while studding Mechanical Engineering in college. It's been a few years so if I'm mistaken feel free to correct me.

From what I remember, there are three areas of stability:

Stable, under normal speeds when disturbed the trailer will always follow behind the towing vehicle and any oscillations (bumps in the road) will dissipate.

Critically stable, the vehicle will remain stable under a threshold speed and any oscillations will dissipate. Over this speed the oscillations will increase until you loose control of the vehicle (fishtail).

Unstable, at any speed any oscillations will increase and you will loose control at even the smallest speed.

Warning, if you’re not an engineering nerd you may want to skip this next paragraph:

Stability has to do with where your eigen values are located when plotted on a real vs imaginary graph. For fundamental stability the eigen value must be in the quadrent located above the real axis (x axis) and to the left of the imaginary (y axis). Critical stability occurs when any eigen value is located on the y axis and above the x axis, and fundamental unstability occurs when the eigen value is located to the right of the y axis.


When it comes to trailers this is a direct function of where the center of mass is in relation to the axel of your trailer.

If your center of mass is in front of the axel the trailer will remain stable.

If the center of mass is right at the axel, your trailer will be critically stable, over the threshold velocity it will oscillate out of control. This type of loading should be avoided because there is really no easy way to know what the threshold velocity is. Unless you like finding out the hard way.

The trailer will be unstable if the center of mass is behind the axel. This is unsafe at any speed.

I remember a demonstration we had in class where we had a bicycle with a trailer which had a 2 lb weight in it. Just putting this little amount of weight behind the axel was enough to throw someone off the bike at a walking pace.

It's super easy to know what kind of loading you have. If the tongue of the trailer is on the ground before you hitch up and take off you'll be fine, just make sure the load can’t shift. If the tongue is balanced evenly or up in the air, you're screwed.

Of course the easiest way to make a trailer unstable is to simply put your car in reverse.

I’ll put my pocket protector away now.

Adam