tongue weight seems high for 13' Burro - Fiberglass RV


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Old 06-29-2020, 08:43 PM   #1
gwc
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tongue weight seems high for 13' Burro

An '85 Burro, 13' with a tongue weight of 154 lbs.



Total trailer weight unloaded should be less than 1200 lbs. if original specs & other testimonials on this sight, people who have properly weighed their trailer, are accurate. Have even seen pictures of others picking up the tongue & pushing/pulling the trailer around. Some even claim a 1200 lb. or less with accessories such as tv's, fridge, air C. etc.



My situation;

. have removed the front bunk
. have removed the front propane tank
. new hatch covers eliminating the heavy press board stuff
. removed some heavy click flooring, replacing with sheet of vinyl
. lightened the table by 7 lbs
. removed the propane stove/ replaced with portable propane
. no bathroom option
Tongue weight of 145 lbs. does include
. rear water tank with 5 gal. h2o
. front potti with 1 gal h2o
. 3 way Dometic fridge
. 1 front 24 series LA battery

. tongue is weight at level height, same as TV hitch height with digital

scale



I understand factory weights are optimistic, that weight distribution can change T.weight and tongue needs to be 10%-15% of trailer weight. Was expecting a T weight of about 120 lbs. or maybe even less with all reductions that were made. 154 lbs would indicate to me an unloaded, no options trailer weight designed at 1500 lbs. There's no way it could be that much unloaded? I was up to 165 lbs with propane tank & bunk as it probably came from the factory.



Yes, the only way to know is to take it to a scale station, which I will do at some point, but something doesn't seem right compared to similar postings.
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Old 06-30-2020, 08:36 AM   #2
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My very basic 13’ Scamp (no fridge, but does have front bunks with G24 battery and 1 LP tank up front) carries around 200# on the tongue. Unloaded I still cannot lift the tongue unless I remove the battery and LP tank, but I can move it around by hand with the help of a trailer dolly.

In short, sounds okay to me. Burros are perhaps a bit lighter than Scamps but not as light as some claim.

Do take it to a scale. Then you’ll know.
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Old 06-30-2020, 10:10 AM   #3
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Thanks much for you're input. I now realize after making the post, I am carrying a wheel jack on front of tongue. That could easily be a 20# item. Maybe I can relocate it back a bit & get under the 150 limit?
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Old 06-30-2020, 10:22 AM   #4
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Thanks much for you're input. I now realize after making the post, I am carrying a wheel jack on front of tongue. That could easily be a 20# item. Maybe I can relocate it back a bit & get under the 150 limit?
Is that "under the limit" a 150 lb. tongue weight limit for your tow vehicle? Or the 10-15 percent of total weight as tongue weight guideline?

It isn't only the weight in front of the axle, tongue weight is also influenced by absence of weight behind the axle. Empty under seat storage or removed spare tire from back shifts the weight toward the front more.

I can pick up my tongue even with battery and 20# propane tank. I just loudly declare "Hercules Unchained!"..... and have wife stand on back bumper
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Old 06-30-2020, 11:24 AM   #5
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My situation;
. new hatch covers eliminating the heavy press board stuff
. removed some heavy click flooring, replacing with sheet of vinyl
. lightened the table by 7 lbs
Was expecting a T weight of about 120 lbs. or maybe even less with all reductions that were made.
It looks like some of the things you've removed or lightened might have actually increased the tongue weight a bit as any weight removed from behind the axle will actually increase the trailer's tongue weight.

Think of it as a balance, like a teeter-totter. Any item located a certain distance behind the axle will have exactly the opposite effect on the tongue weight as something of the same weight located the same distance in front of the axle.

Lightening the table would increase the tongue weight a bit, while lightening the load on the axle. The lighter flooring probably had little effect on the tongue weight because it's located both in front of and behind the axle.

Together, the axle and the tongue support all of the weight. An object located midway between the axle and the coupler will increase both the axle weight and the tongue weight by half the weight of the object.

Similarly, an object located 3/4 of the distance between the axle and coupler will add 3/4 of it's weight to the nearer support, and 1/4 to the more distant support. So, pound-for-pound, the propane tank was probably one of the most effective things removed.
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Old 06-30-2020, 12:43 PM   #6
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absolutely right Mike. Most of my modifications affected total weight. Guess that will allow me to carry more water in the tank which will get tongue weight to where I need it for a Rav4. Pretty sure the total weight, even when traveling will be close or under my 1500 lb limit.


Going to look into moving back the wheel jack a bit which might change 5# at the tongue.


Thanks for the input
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Old 06-30-2020, 12:45 PM   #7
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Ah yes. Wife on the back bumper trick. Can I leave her there when heading out to a campsite?
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Old 06-30-2020, 01:04 PM   #8
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tongue weight seems high for 13' Burro

I can just hear her saying, “Wouldn’t the tongue weight situation be better with you on the back?”

You would not dare argue that one!
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Old 06-30-2020, 01:19 PM   #9
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...maybe just paint it red, add a tailboard and a siren, and see if you attract a crowd.
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Old 06-30-2020, 04:06 PM   #10
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tongue weight seems high for 13' Burro

Quote:
Originally Posted by gwc View Post
Going to look into moving back the wheel jack a bit which might change 5# at the tongue.
I doubt moving the tongue jack a few inches will make a measurable difference in the tongue weight.
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Old 07-01-2020, 08:21 AM   #11
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Originally Posted by gwc View Post
…..


Going to look into moving back the wheel jack a bit which might change 5# at the tongue.

…..
Have to consider the weight location in relationship to the axle. If the whole 20# was moved back to the axle you would lose the whole 20# from tongue weight. So at the least I would expect one would need to move 1/4 of the distance from tongue to the axle to lose 1/4 of the hitch weight or 5#

Although I would expect that the shift isn't perfectly linear. A foot movement at the tongue or axle would cause less weight difference than a shift of same distance at mid point between the two. But that is just an idea. A balance beam scale has marks at equal distances so maybe it is the same amount of change all the way.

Although the slides are different weights and the marks are different for different units of weight.
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Old 07-01-2020, 08:26 AM   #12
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As a side note. Weight at the extreme front or rear creates less stable towing.

In theory you could put weight on tongue and rear bumper to get correct weight distribution and hitch weight but it would not be stable.

Weight closer to the axle is more stable.

At one time there was a YouTube video of a model trailer with weights showing the effect of repositioning the weight further toward the front and rear.
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Old 07-01-2020, 08:51 AM   #13
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Roger, I experienced exactly what you’re talking about. At one time I carried 2 bikes on the back, counterbalanced by the weight of our 10x10 canopy and a 6 gallon jug of water at the front of the trailer for around 12% tongue weight. I did not have any stability issues, however...

When a part in my rack broke on the eve of a trip, I found a way to carry all the heavy stuff over the axle (shown below), with just enough forward weight to maintain 12% tongue weight. It was like trading a lumbering SUV for a mid-engine sports car. The trailer tracked and handled so much better, felt light on its feet. Never going back.

I think the technical term is polar moment of inertia or something like that... I’ve seen the video you’re thinking of, but haven’t been able to find it recently. It’s a variation of the tongue weight video.
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Old 07-01-2020, 11:09 AM   #14
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Good point. Tongue weight equal to 10-15% of total trailer weight is recommended. I’ve found around 12% is ideal for my 13’ Scamp.

Until you know what your loaded trailer weighs, you won’t know what your tongue weight should be.
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Old 07-01-2020, 12:35 PM   #15
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Question

Quote:
Originally Posted by RogerDat View Post
A balance beam scale has marks at equal distances so maybe it is the same amount of change all the way.
It is. While it can be a bit tricky, figuring the impact of loading on tongue weight can be pretty straightforward if you are careful to keep track of a couple of things.

The key is to consider the axle as zero with distances measured toward the coupler being positive, and distances measured behind the axle being negative. You can measure in whatever units you like; feet, metres, inches, whatever. So long as it's the same unit, it will cancel out.

Say the coupler is located 8 ft in front of the axle and you set a 20 lb box of rocks 4 ft in front of the axle. The 4 ft and 8 ft distances will be both calculated with positive signs because they are both located in front of the axle.

This equates to (20 lbs x 4 ft) / 8 ft = 80 ft-lbs / 8 ft = 10 lbs.

The numbers in the parentheses are multiplied first. Then the initial result of 80 is divided by the number 8 located after the slash symbol (/). The lengths of ft / ft cancel out and the result is that the box of rocks increases the tongue weight by 10 lbs.

Now, the additional 20 lbs of payload has to be accounted for in the net changes to the loads at the coupler and the axle.

That's 20 lbs of additional payload minus a positive 10 lbs of the additional payload which is carried on the coupler equals an additional 10 lbs to be carried on the axle: 20 lbs - 10 lbs = 10 lbs, the portion of the additional payload which is carried by the axle.

~~~~~~~~~~~~

Figuring the effect of locating the box 4 ft behind of the axle requires treating the 4 ft distance as a negative number.

(20 lbs x -4 ft) / 8 ft = -80 ft-lbs / 8 ft = -10 lbs.

Here, the result is now a decrease in the tongue weight carried on the ball. Because the box of rocks was located 4 feet behind the axle, the 4 ft was multiplied as a negative number.

In this case the box of rocks will also increase the weight on the axle by 30 lbs. Why 30 lbs? Because 20 lbs additional payload weight in the trailer minus a negative 10 lbs on the coupler = 30 lbs.

That's 20 lbs - -10 lbs = 30 lbs. (Two negative signs cancel and effectively become a positive, so subtracting a negative number is the same as adding a positive number.)

~~~~~~~~~~~~

Finally, for dual axles, you can use the midpoint between the axles as zero. Because trailers are designed to start with weight on the tongue, the actual location will be slightly forward of the midpoint. However, the midpoint will be very close to the actual balancing point.
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Old 07-01-2020, 04:40 PM   #16
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Originally Posted by RogerDat View Post
As a side note. Weight at the extreme front or rear creates less stable towing.

In theory you could put weight on tongue and rear bumper to get correct weight distribution and hitch weight but it would not be stable.

Weight closer to the axle is more stable.

At one time there was a YouTube video of a model trailer with weights showing the effect of repositioning the weight further toward the front and rear.
Something like this?

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Old 07-01-2020, 06:04 PM   #17
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tongue weight seems high for 13' Burro

That’s the general idea. My eyes weren’t too good viewing on the small screen, but it looked like there were more weights further from the axle in the back. That could point to tongue weigh being a contributing factor. He used the term “yaw inertia.” I looked that up and got lost in a jumble of related terms: angular inertia, yaw angular inertia, rotational inertia, yaw moment of inertia... Help!

I’m talking about the resistance of the trailer to a change in angular velocity about the axle, as happens during a turn. That’s different from turning centered on the ball (sway). I think.

What says the engineer?
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Old 07-01-2020, 07:55 PM   #18
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What says the engineer?
First of all, I got a head start on forgetting anything I ever might have known many years ago. So, it might be slim pickin's for good answers here.

I say that your observations concerning the effects of shifting the masses of the bikes and canopy from fore and aft to locations near the axle are right on the mark. You are describing just what is to be expected under these circumstances, and that's what's really important.

I am able to do a few simple things with statics, the forces acting on a body at rest, like the earlier post on tongue weight. However, what you observed involves dynamics, the forces on a body in motion. There's a lot more going on and as you saw by your dip into the internet, it can quickly become a bit much to take in.

I say we go with our cousins across the pond there in the video and refer to what I suspect you primarily observed as a change in the "yaw inertia". From Wikipedia:

Quote:
The yaw angular inertia tends to keep the direction the car is pointing changing at a constant rate. This makes it slower to swerve or go into a tight curve, and it also makes it slower to turn straight again.
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Well, the effects on our "caravans" are similar; having the mass of your bikes and the canopy further from the yaw axis tends to resist the initiation of a swerve, but then in turn the mass resists recovery once a swerve has begun.

Hold a heavy jug of water at arm's length and spin around. Then try to do it again while holding the jug close to your chest. You will observe the practical effects of the mass being located first further, and then closer to the center of rotation.

You can also see in the diagram how the masses of the bike and canopy can have the same effects on rotation around the pitch axis, and perhaps even a bit on roll. The pitch axis involves motion that can lead to the oscillation referred to as porpoising. So, there's a lot that happened when you shifted those masses closer to the yaw and pitch axes.

Going back to yaw, the first swerve and it's subsequent reversal can in turn become sway, oscillating like a pendulum suspended from the tow ball which is moving through space at 60 miles an hour. Cue up the Eric Idle.

With all this going on, along with the flexing of the trailer's tires and the motions of the suspension system to boot, we have quickly moved beyond my ability to do any justice to the subject.

But the important part is just knowing the practical part that weight is best placed low and central to the axle, so I think we're all forewarned and forearmed by your experience and observations here.
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Old 07-06-2020, 08:30 AM   #19
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That is a pretty slick arrangement on stowing the bikes.


Hitch height can have a large impact. Surprisingly large, not in a good surprise way. On our trip to the Algonac rally our tow vehicle had to go into the shop. Loaner was the larger Ford SUV (Expidition) but it had the less common 1 inch draw bar hitch so I couldn't find anything available on short notice low enough to get the 13' Scamp level.


Found at any speed over 50 the little Scamp started swaying at 55 it was yanking the heavy SUV around. Was a most "interesting" drive.


Conversely my 6 x 10 utility trailer is taller than the scamp and when I started towing with a different Escape with the hitch higher the drop that puts the Scamp level is low for the utility trailer. A little nose down can be good but too much creates a poor tow, not swaying but tends to be nose diving on bumps and doesn't track as smooth in my opinion.
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Old 07-09-2020, 08:21 PM   #20
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Got the trailer weighed!!

Amazing what the right search words can discover. Found a local recycling business with certified scale....only $5 !


So my original post concerning a 13' Burro (1985) that I had attempted to lighten up resulted in 1100 lb. weight. That's with the 3 way fridge & no propane tank or front bunk, 6 gal. of h2o & a porta potti.


Thanks for all the return info on tongue weight. Sure to help avoid future errors in strategy.
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