What makes a towed trailer want to sway?
Do fifth wheel systems (on pickups, etc) experience sway?
Peace,
mangeorge
Generally improper weight distribution makes trailers sway, specifically having too much weight on the front of the trailer. High crosswinds or excessive speed can also make a trailer sway.
Fifth wheels can still sway.
One thing that can cause it is too much weight behind the trailer’s wheels. We were towing our sailboat once with the engine on the engine mount, and it got scary. As soon as we took the engine off and put it inside the boat near the bow, problem was solved. I can’t talk to fifth wheel trailers, but I know this is how you fix the issue on a standard trailer-ball trailer connection.
Besides weight distribution tire inflation and shock absorbers can be major factors. If all factors are just a little off problem can be very bad.
The effect of trailer side forces on the towing vehicle has a lot to do with the “lever arm”: the distance from the hitch pivot point to the towing vehicle’s rear axle. A big advantage of a fifth wheel arrangement is that this pivot point is more or less over the rear axle, which has the effect of seriously reducing sway.
IME - which comprises about 200k miles of towing - problems are much more often due to too little weight at the hitch.
If you’re referring to fishtailing, as others mentioned too much weight to the back of the trailer.
In general, the tips to avoiding “snaking” (trailer sway) include:
checking the nose-weight (and adjusting load to suit)
ensuring all tyres are correctly pressurised
using a suitable “anti-snake” device
Travelling at a moderate speed.
In the UK, people use either a stabilizer bar or a friction-style coupling head stabilizer (my personal choice). Works a charm.
Si
Sway is determined mainly by the center of gravity.
While being towed, a trailer *attempts *to pivot about its center of gravity. It is “allowed” (if I can misuse that term) to pivot only about its wheels. The tendency for sway is determined by how close together those two points are. Notice how large semi-trucks have the trailer essentially suspended between the hitch in front, and the wheels in back. They’ve almost maximized the distance between CG and wheels. That’s why there’s very little sway in those trucks (and at 80K lbs, it would be disastrous).
Non 5th wheels (bumper pull*) setups typically have a small tongue weight. A 6000 lb travel trailer will likely only have about 800 lbs on the hitch. This indicates that it is almost balanced atop its wheels (ie. CG is near the wheels), allowing it to rotate about that point easily. Fifth-wheel setups will have a far larger tongue (pin) weight, often exceeding 17-18% of the trailer. This is due to the CG being well forward of the wheels, suspending it (so to speak) like the larger rigs. This is why they are more stable.
I’ve been trailering RVs in various configurations for decades, and have learned the difference thru expericence. In fact, some setups can only be done safely with a 5th-wheel configuration. Here’s a pic of my setup. Trying this with a “bumper pull” would probably get me on the news.
*It’s not really on the bumper, but they’re referred to that way in RV circles
Is that set-up legal in all states?
Can’t speak for the US, but in Canada, it’s legal in Alberta, but not in BC. Don’t know about the other provinces, either.
Cause #1: too little tongue weight.
10-15% of the tow package’s weight should rest on the hitch.
80-90% of sway issues are addressed by getting the tongue weight squared away.
The facts above are not wrong, but except for **Pullin **do do little to address the why.
It is very hard to explain the dynamics with words, but the hitching point on the tug is a large factor.
When the hitching point is well behind the rear axle (bumper pull) the trailer tends to steer the tug a bit in a direction that exacerbates sway. The further back the hitch, the worse this is. If the trailer’s CG is behind the axle, this tends to steer the trailer in a way that exacerbates sway. Combine the two, and you have a recipe for disaster, especially when braking, as the trailer pushing the tug is also destabilizing.
For maximum stability:
-Lots of tongue weight. As much as the tug can handle. If the front wheels become light, weight equalizing hitches can help, as this still keeps the tongue-heavy dynamic properties.
-Friction devices at the hitch allow the trailer to steer the tug slightly in a direction that reduces sway. They can help a lot, but should not be used if the rig is unstable without them, as they can fail, or you can forget to attach or set the friction properly.
-Hitch over rear axle, or even slightly ahead. 5th wheel or gooseneck, though there are a couple proprietary systems that place a “virtual” hitch at the rear axle while still physically coupling the trailer at the bumper.
-Brakes on trailer.