The only empirical data I’ve found on trailer stability is Darling, Tilley, and Gao, published in 2009. Highly recommend reading, although you’ll need some UK → US translation. (Estate car → station wagon, nose mass → tongue weight, tyre → tire). You probably already knew that last one.
They found the most important factors are in order:
- Speed (slower is more stable)
- “Trailer yaw inertia”
- Tongue weight; they recommend 6 to 8 %.
- Axle position
Trailer yaw inertia. Moment of inertia in the horizontal plane. It is complicated to calculate, but the more weight further away from the axle, the higher it is.
A greater distance from the hitch to the trailer axle is more stable, but the axle position is fixed for a given trailer unless you’re going to rebuild the trailer.
A couple of factors not studied by Darling are tow vehicle (TV) mass and axle-to-tow hitch distance (“overhang”). A bigger tow vehicle and shorter overhang are better, according to Aussie journalist Robert Pepper, which I find credible.
Tongue weight
There are lots of statements about tongue weight on the interwebs.
- 6 to 8 percent (Darling).
- 5 to 10 percent (Pepper, around 3:45)
- 9 to 15 percent, MrTruck.com
- 9 to 11 percent, Toyota Motor Corporation
- 10 to 15 percent, etrailer
So what’s a trailer tower to do? Simple. Read your tow vehicle’s owner’s manual. If you don’t have / can’t find a paper one, it’s 2022. It’s most likely online.
