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Old 06-05-2023, 07:35 AM   #1
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Optimal tongue weight and trailer swaying.

I'm about to hit the road pulling a 1979 Trillium 4500 with a 2003 Subaru Outback, 6 cylinder, 3 litre. Does anyone know what the optimal tongue weight should be at the hitching ball? Is there an Optimal tongue weight? The trailer is 11 feet and the tongue is about 3 feet.

Last summer, the first time out, I noticed the trailer sways severely sometimes at higher speeds- very scary. I'd slow down immediately. What causes this?. Is it related to the tongue weight? or the trailer weight?
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Old 06-05-2023, 11:43 AM   #2
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The usual number for required tongue weight is 10% of the total weight of the trailer. So, if your trailer weighs 2000 lbs., there should be 200 lbs. on the tongue.

What causes trailer sway? I have a theory. My job is selling industrial instrumentation. One of the things I sell is called a vortex flow meter. A bluff, (blunt) body is placed in the flow. A series of vortices rolls off either side of the bluff body. This causes an oscillating force that oscillates at a rate that is perfectly proportional to the velocity of the flowing fluid, (gas or liquid).
Click image for larger version

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That picture is animated if you click on it.

It is my theory that the front of the trailer acts as a bluff body that sets off the low frequency sway. Insufficient tongue weight amplifies this sway.

I usually tow with a travel van that is slightly taller than my 4500. At one point I felt like the trailer was acting weird. I stopped at a truck weigh scale and found that I only had about 20 lbs. on the tongue. But I never experienced any sway. My theory for this is that the trailer was in the wind shadow, (my term) of the van. When it strayed out of this shadow, the only force on it just pushed it back behind the van.
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Old 06-05-2023, 02:33 PM   #3
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There are a lot of factors that contribute to trailer sway. High center of gravity, soft suspension or tires, weight behind the axle, or on the back of the trailer, short tongue vs width of the trailer, are a few. A lightweight or softly sprung tow vehicle with light duty tires can contribute as well.

Try to address the factors you do have control of first, such as tire pressure and weight distribution, and see if you can fix it. These might involve removing weight from the back of the trailer, moving heavy cargo forward, traveling with rearward tanks empty and getting load range E tires. See if others with the same trailer model have the same problems and find out what they did to solve it. Some trailers are not stable and others are just barely stable, while some are extremely stable.

You can also add a friction sway control device available from etrailer and other places. There are also electronic sway controllers. But try to make it stable before using these as a bandaid fix. Don't expect it to always be rock solid behind your tow vehicle. Responding to a crosswind, for instance is not a problem, but it should not get into a repetitive motion that builds on itself.

If it ever does begin to get out of control, reach down and apply the trailer brakes manually at the controller. This will fix it. Practice how to do this and use the procedure to test your brakes each time you start out on a trip.
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Old 06-05-2023, 05:09 PM   #4
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Recommended tongue weight is 10-15% of total trailer weight. Most smaller molded trailers end up in the lower half of that range. I've found about 12% works great for my 13' Scamp.

A Trillium 4500 typically weighs 2000# or a bit more fully loaded, which means you should probably have 200-225# on the tongue. Weigh your loaded trailer to be sure.

That puts you right up against 200# tongue weight rating of your Outback. You might even need to run a bit over to get a stable tow. That means you'll need to eliminate any excess cargo in the back of your vehicle.

Sway is more likely when you're going faster, descending a grade, driving in gusty crosswinds, or encountering heavy truck traffic. Slow down, and then slow down more as driving conditions deteriorate. Add a friction anti-sway bar, and know how to engage trailer brakes manually as John describes.
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Old 06-06-2023, 03:10 AM   #5
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David- interesting... and thanks. Love the animated picture...hypnotic. Where did you ever get that? Did you create it yourself?
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Old 06-06-2023, 03:40 AM   #6
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Jon- thanks for your reply. I have to say getting replies is nice, appreciated, interesting and HELPFULL...
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Old 06-06-2023, 08:34 AM   #7
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Recommended weight depends where you are. In Europe it is 6 to 9 percent. In North America it’s 10 to 15 percent. We are right at about 10 percent with a slightly bigger trailer than yours.

Rear cargo racks on the back of a trailer can cause sway. So can soft vehicle suspensions. So can speed. In most of Europe the max towing speed is 80 kmh. In North America it varies province to province and state to state. We keep it at or under 90 kmh and it’s pretty solid.

Good luck and safe travels.
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Old 06-06-2023, 11:00 AM   #8
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I realize that people log into these forums from all over the world, but the OP lives in North America, so I didn't feel it was relevant to bring in towing practices elsewhere. It causes confusion and fuels conspiracy theories.

I could also have mentioned that even in North America, boat trailers typically tow with less than 10% tongue weight. Boat trailers have very different geometry and weight distribution than travel trailers.
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Old 06-06-2023, 03:46 PM   #9
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I towed a utility trailer with my
Series II Land Rover with a negative tongue weight and the trailer started to sway at 20 mph.
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Old 06-06-2023, 07:53 PM   #10
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In your case (having felt the trailer get squirrelly) it is really important to weigh the trailer, and check the trailer's tongue weight, when you're loaded as usual for camping. (Keep in mind where the water tanks are and possible effects of having them empty or full, since being in front of or behind the axle will make a big difference... water is 8.3 lb per gallon!) Make sure your Subaru's tires are inflated to the max cold pressure marked on the sidewall, to help provide sidewall stiffness.



Small mods that could help:

* lower profile tires (less sidewall, less side-to-side squirm possible)
* light truck tires, if available in that size (they have stiffer sidewalls to begin with)
* shorten the overhang distance a couple inches by drilling a new hole in the ball mount or the receiver
* add a sway bar (I'd definitely want this in your situation since you probably aren't allowed an anti-sway, weight distribution hitch)
* change the load to add a few lbs to the hitch, if your Subaru can have more (not sure if you're at the limit already)
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Old 06-07-2023, 04:35 AM   #11
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Quote:
Originally Posted by Jon in AZ View Post
I realize that people log into these forums from all over the world, but the OP lives in North America, so I didn't feel it was relevant to bring in towing practices elsewhere. It causes confusion and fuels conspiracy theories.

I could also have mentioned that even in North America, boat trailers typically tow with less than 10% tongue weight. Boat trailers have very different geometry and weight distribution than travel trailers.
We used to live in one of Minnesota's lake country areas and took I-94 almost ever day. We'd see flipped boats on the Interstate quite often. With their long tongues with wheels located further towards the rear the 10-15% is needed to travel at Interstate speeds. The faster the speeds the more important it is to have more weight on the tongue. That's why Europe has a 80 kph limit. The physics in Europe aren't any different, it just they have sensible speeds. Even California has a 55 mph max speed limit with trailers, for good reason. Tongue weight AND speed are key.

My father farmed and I took over the farm for over 20 years until I became a single parent and he had a stroke the same year. He had a agricultural engineering degree from the U of M and drilled into me the physics involved to keep the tongue heavy and/or drive slow. I shoot for 12% or greater and a max speed of 62 mph, except in cities where too slow is dangerous.

Enjoy,

Perry
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Old 06-07-2023, 08:44 AM   #12
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Quote:
Originally Posted by Vic Berger View Post
David- interesting... and thanks. Love the animated picture...hypnotic. Where did you ever get that? Did you create it yourself?
Wikipedia

https://en.wikipedia.org/wiki/K%C3%A..._vortex_street
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Old 06-07-2023, 09:40 AM   #13
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Quote:
Originally Posted by Perryb67 View Post
We used to live in one of Minnesota's lake country areas and took I-94 almost ever day. We'd see flipped boats on the Interstate quite often. With their long tongues with wheels located further towards the rear the 10-15% is needed to travel at Interstate speeds.
Thanks for the clarification about boat trailers. No personal experience, just going by what I’ve read and observed. In my region they seem to tow much faster than travel trailers, and I’ve yet to see a boat upside down. But around here they’re mostly low-profile ski boats, often inboards with the engines in the center. That seems likely be a pretty stable set-up.
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Old 06-09-2023, 07:02 AM   #14
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Thanks Perry for your reply. All the responses I got are appreciated. I was completely in the dark previously regarding this issue. Now I have a good comprehension - tongue weight and speed. It makes perfect sense.
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Old 06-09-2023, 08:26 AM   #15
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I’ll add one more principle, not as critical as tongue weight and speed, but helpful. Other things equal, towing stability improves as you center weight over the trailer axle.

People do carry gear on the ends of their trailers (bikes on the back and tongue boxes on the front, for example). While it can work okay if properly balanced for 10-15% tongue weight, the best and most stable tow results from concentrating weight as much as possible inside the trailer low and near the axle.

I discovered that when a critical part on our rear bike rack broke on the eve of a trip, and I macgyvered a way to secure the bikes inside over the axle. With bikes on the back, offset by heavy gear at the front of the cabin, the trailer was stable enough but always felt a bit lumbering, as if it was resisting following the tow vehicle. With the bikes inside and weight centered it tracked effortlessly and felt much more confident, even though the tongue weight was actually a bit less (still within the 10-15% window).

Best way to describe it is to imagine picking up a 5# dumbbell, raising it in the air with one hand, and twisting it back and forth quickly. Then try the same with a 25# dumbbell.

It’s not practical to pile all your gear in the middle of your trailer of course, but the more you can center your load, the more stable your tow.
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Old 06-09-2023, 03:55 PM   #16
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Jon, What you're describing is a change to angular momentum.
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Old 06-10-2023, 05:55 AM   #17
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Didn’t want to get too technical but yes, the goal is a low (rotational) moment of inertia.
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Old 06-10-2023, 07:54 AM   #18
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Thanks Jon. I appreciate all the interesting details.
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Old 06-10-2023, 10:20 AM   #19
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Trailer sway

Trailer sway is caused by a combination of two issues one issue which everybody has addressed is the percentage of weight on the tongue compared to the total weight of the trailer and that’s 10 to 12% the other issue is trailer level if the tongue of the trailer is above level. The trailer will swear if the trailer tongue is level or below level, it will not sway.
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Old 06-10-2023, 11:25 AM   #20
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Originally Posted by ehoepner View Post
The trailer will swear if the trailer tongue is level or below level, it will not sway.
The trailers must not like it if it makes them swear.
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