Swaying Scamp 16' - HELP - Page 4 - Fiberglass RV


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Old 08-12-2013, 04:17 PM   #43
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Originally Posted by Brian B-P View Post
I would say instead that measuring tongue weight and comparing it to axle weight is just a way of judging centre of mass position, regardless of the (polar moment of) inertia.
This is a better way to say it, but my point is that the benefit of tongue weight is not the downward force on the ball, but rather the assurance that the center of mass of the trailer is not rearward of the axle. Also, in the process of moving the center of mass rearward, one would likely also change the yaw inertia by having weight at the extreme rear of the trailer (like the bikes).

But you are right, the center of mass and yaw inertia are different characteristics.
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Old 08-12-2013, 05:50 PM   #44
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Just a thought but... when you put passengers and gear in the TV I'm guessing it rides lower which might pull the tungue down with the hitch giving the trailer a more nose down posture.
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Old 08-12-2013, 11:11 PM   #45
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If you dropped the trailer tires to 50 lbs, it sounds like they were over-inflated to start.
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Old 08-13-2013, 01:18 AM   #46
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For the benefit of others reading this thread sometime in the future, the reason that bikes on the back contribute to sway is because they act to increase the trailer's angular (yaw) inertia (about the vertical axis). Sway is when the trailer / tow vehicle system goes into resonance about this axis.[/URL]
Tom,
That is interesting what you are explaining about yaw inertia. Not being an engineer I can only observe why these trailers act the way they do. It seems that when weight is added at the rear (like bikes) that you then have to keep the trailer balanced by shifting weight forward or adding weight to the front. This provides proper tongue weight, but changes the yaw frequency. I would think that by moving the weight to the polar ends that the yaw frequency would slow down. The slower pivoting around the vertical axis is probably why the sway takes over and gets amplified. I imagine if say you moved the bikes inside closer to the axle, and moved the propane tanks closer to the axle inside achieving the same fore aft balance that the yaw frequency would go higher, but maybe not travel as far in amplitude and may not perpetuate sustained motion. Just a theory...?
Russ
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Old 08-13-2013, 05:07 AM   #47
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When I first added the cargo carrier to the receiver on the back of my ParkLiner it swayed side to side alot and I felt it...so I took a ratchet strap to strap it to my bumper and the difference was amazing... drastically reduced the sway on the trailer. Perhaps strapping the tires to the bumper while on the rear rack so the bikes cant move around side to side might reduce the trailers sway till you can get a better bike rack for upfront.
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Old 08-13-2013, 08:37 AM   #48
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Tom,
That is interesting what you are explaining about yaw inertia. Not being an engineer I can only observe why these trailers act the way they do. It seems that when weight is added at the rear (like bikes) that you then have to keep the trailer balanced by shifting weight forward or adding weight to the front. This provides proper tongue weight, but changes the yaw frequency. I would think that by moving the weight to the polar ends that the yaw frequency would slow down. The slower pivoting around the vertical axis is probably why the sway takes over and gets amplified. I imagine if say you moved the bikes inside closer to the axle, and moved the propane tanks closer to the axle inside achieving the same fore aft balance that the yaw frequency would go higher, but maybe not travel as far in amplitude and may not perpetuate sustained motion. Just a theory...?
Russ
This sounds about right.
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Old 08-13-2013, 12:09 PM   #49
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Originally Posted by ruscal View Post
It seems that when weight is added at the rear (like bikes) that you then have to keep the trailer balanced by shifting weight forward or adding weight to the front. This provides proper tongue weight, but changes the yaw frequency. I would think that by moving the weight to the polar ends that the yaw frequency would slow down. The slower pivoting around the vertical axis is probably why the sway takes over and gets amplified. I imagine if say you moved the bikes inside closer to the axle, and moved the propane tanks closer to the axle inside achieving the same fore aft balance that the yaw frequency would go higher, but maybe not travel as far in amplitude and may not perpetuate sustained motion.
You have a good handle on it, Russ. You might want to think about damping (energy loss from the oscillation) to complete the picture.
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Old 09-25-2013, 07:52 PM   #50
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I want to summarize what we did to eliminate swaying side to side. On our 4,000 mile trip to Cape Breton Island, Nova Scotia, I

1 replaced the hitch ball with one 1.25" lower
2 made sure to load the tongue inside the Scamp
3 had trailer tires at 50# and rear tires at ~34#
4 left the bikes at home
5 added a Kurt 17200 sway control for safety

In many and varied driving conditions (storms, high winds, traffic, etc) we had no sway issues.
Item 4 probably did the most to help.

I tested moving the weight in the front to over the axle and swaying began to occur.

Hope to see if we can manage the rear bikes in the future. Our bike rack is heavy duty but may wiggle a little. I know the sway bar will help and probably eliminate the swaying.

Thanks again for all the good advice.

Hoping to meet you guys someday,

David
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Old 09-25-2013, 08:45 PM   #51
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Small and large approaches to sway.

We do a lot to tow successfully, some small and other large. Hopefully they contribute to a safe tow.

We towed a Scamp 16 for 2000 miles with the lowest tongue weight on the forum and never experienced sway. After that we added an anti-sway bar because we felt it was prudent, but not because we had experienced sway.

We had another non fiberglass trailer where we put a heavy load on the rear bumper and immediately had sway. That was enough to convince me that sway was not something I ever wanted to see again. Moving that heavy rearward load solved the sway problem. I am convinced that balancing a heavy rearward load with a heavy forward load is not a satisfactory solution.

We learned, as many lessons show, to keep the heavy items near or over the axle. Our pots and pans, dishes, water heater, stove, fridge, can pantry and air conditioner are all near or over the axle. Our clothes, bedding and other fluff items are at the ends of the trailer.

I'm also avoid heavy roof items, one reason I don't like a roof top air conditioner, to me weight belongs low.

Besides having a well balanced trailer, I boost tire pressures on the tow vehicle to stiffen tire sidewalls. In addition I have shortened the ball's distance to the rear axle to a minimum. This helps compensate for a relatively short wheelbase of our tow vehicle, though something that makes sense for any towing situation. I accomplished this by drilling a new hole in the ball's coupling shaft and sliding the coupling shaft further forward towards the front of tow vehicle. This also reduces the see-saw effect of tongue weight on the front axle.

We occasionally fill the trailer's rearward water tank when going into boondock situation, though I've never measured it, I'm sure it further lightens our tongue. In this conditions we restrict ourselves to short distances and slower speeds.

We follow some similar measures with our tow vehicle, keeping heavy weights between the axles.

Additionally we have replaced the trailer's heavy cabinet doors and pressboard table with lighter materials.

Hope this provides some benefit.
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