My 16'er weight - Fiberglass RV


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Old 04-19-2006, 11:00 PM   #1
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I've weighed my Scamp yesterday to get a weight configuration.

Take a look at my pics in the sig link.

Scamp config: bunk/couch, table/bed, 2 closets, empty water tank, power converter, hand pump sink, 3 burner stove, ac, heater, propane tank (full), battery, spare tire, 1 upper shelf, awning and polls. I had the tongue redone for height adjustability. I had the camper axle on 1 scale, tongue another. Tongue weighed 260#s, axle weight 1560 for a total of 1820#s.
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Old 04-20-2006, 12:38 AM   #2
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I like the tongue mod....looks like your jack crank handle runs pretty close to the propane bottle but that may just be an illusion...getting late here so will look at the other pics tomorrow...good nite...Benny
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Old 04-20-2006, 10:01 AM   #3
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I've weighed my Scamp yesterday to get a weight configuration.

I had the tongue redone for height adjustability. I had the camper axle on 1 scale, tongue another. Tongue weighed 260#s, axle weight 1560 for a total of 1820#s.
I don't think this is quite right. You can't add the tongue weight to axel weight and get total weight. I can move weight around inside the trailer to get a negative tongue weight. By the above method that would subtract from the axel weight.
Example: Tongue weight -20lbs. Axel weight 1820 lbs. Total trailer weight 1800lbs.

Actually I kind of like this. If I add lots of weight to back of the trailer I can reduce the the total weight.

Anyway I think you get the picture.

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Old 04-20-2006, 04:15 PM   #4
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... You can't add the tongue weight to axel weight and get total weight. I can move weight around inside the trailer to get a negative tongue weight. By the above method that would subtract from the axel weight.
Example: Tongue weight -20lbs. Axel weight 1820 lbs. Total trailer weight 1800lbs.

Actually I kind of like this. If I add lots of weight to back of the trailer I can reduce the the total weight.
While negative weight doesn't sound right, it actually works. Let's use Byron's numbers and consider this...
  • some test trailer actually weighs 1800 lbs (if you put it by itself on a scale you read 1800 lbs)
  • centre of mass is behind the axle, so if it isn't hooked up is sits back on its bumper
  • put a jack under the bumper (still no tug) to hold it level, the jack holds some of the weight, and the axle holds the rest (maybe 30lb on the bumper, 1770 on the axle, distribution depends on the distances between bumper, axle, and coupler)
  • push down on the tongue just enough to take the weight off of the jack - that will take 20 lbs downward
  • the axle is supporting the original 1770 lb, plus it is the fulcrum of a lever which is balancing 30 lb of trailer at the bumper against 20 lb (negative) hitch load at the coupler, which means an additional 50 lb on the axle, so...
  • with a tongue load of negative 20 lb, and an axle load of 1820, the net force upward on the trailer to balance it's weight is 1800 lb.
Yes, as long as the hitch is just a simple support point (not a "weight-distribution" hitch system), you can just add the tongue and axle weights.

The answer to Byron's dilemma is that if you move weight around to get a negative tongue weight, that same re-arrangement also increases the axle weight. The total force of all of the supporting points remains the same.

Mike's numbers are 260 (tongue) plus 1560 (axle) equals 1820 (total). If the contents were moved to give -20 lb at the tongue, the axle load would rise to 1840 lb, to keep the same total: the trailer axle would be carrying the entire 1820 lb of trailer, plus 20 lb of the tow vehicle.

And that's your phun with physics for today.
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Old 04-20-2006, 05:19 PM   #5
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While negative weight doesn't sound right, it actually works. Let's use Byron's numbers and consider this...
  • some test trailer actually weighs 1800 lbs (if you put it by itself on a scale you read 1800 lbs)
  • centre of mass is behind the axle, so if it isn't hooked up is sits back on its bumper
  • put a jack under the bumper (still no tug) to hold it level, the jack holds some of the weight, and the axle holds the rest (maybe 30lb on the bumper, 1770 on the axle, distribution depends on the distances between bumper, axle, and coupler)
  • push down on the tongue just enough to take the weight off of the jack - that will take 20 lbs downward
  • the axle is supporting the original 1770 lb, plus it is the fulcrum of a lever which is balancing 30 lb of trailer at the bumper against 20 lb (negative) hitch load at the coupler, which means an additional 50 lb on the axle, so...
  • with a tongue load of negative 20 lb, and an axle load of 1820, the net force upward on the trailer to balance it's weight is 1800 lb.
Yes, as long as the hitch is just a simple support point (not a "weight-distribution" hitch system), you can just add the tongue and axle weights.

The answer to Byron's dilemma is that if you move weight around to get a negative tongue weight, that same re-arrangement also increases the axle weight. The total force of all of the supporting points remains the same.

Mike's numbers are 260 (tongue) plus 1560 (axle) equals 1820 (total). If the contents were moved to give -20 lb at the tongue, the axle load would rise to 1840 lb, to keep the same total: the trailer axle would be carrying the entire 1820 lb of trailer, plus 20 lb of the tow vehicle.

And that's your phun with physics for today.
I think you're missing some force vectors and moments of inertia in your calculations.
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Old 04-20-2006, 06:40 PM   #6
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A little more research -- The force vectors are such that they would have an almost non- detectable effect. Therefore totaling tongue weight and axel weight are pretty darn close. The stuff I was looking at suggests that doing something like that to keep everything level. But since tongue weight is such a small part of overall weight I doubt it would make much difference.

Now where that crow?
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Old 04-20-2006, 11:05 PM   #7
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All well and good guys, but the weigh was performed on a scale system with separated platforms for over-the-road tractor trailers. There were actually 4 scales. I parked my tow vehicle (an XL7) on the front scale (3800#s with 1/3 tank), then the tongue was on the second scale, axle/wheels on the 3rd and nothing on the 4th. I think you would come up with the same weight if I used individual scales at all ground points.

From my understanding for the CAT trailer scales, it doesn't matter where the point of weight is on the scale, the scale measures the weight.

As an engineer, I can understand the 'technical analysis' of the weight computation, but without knowing the cog (center of gravity) of each part in the camper, it's difficult to come up with a truthful fulcrum determination.

However, the information I provided was to help understand the weight of these things. I searched the forums looking for a discussion particular for weights of campers. I found a lot of inquiries on tow vehicle to pull my camper scenarios. Few comments on estimated weights and recently a discussion on the weight of a camper with a long list of items removed with weight info. So I hope this helps those for informational purposes.

Thanks.
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Old 04-21-2006, 09:20 AM   #8
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...However, the information I provided was to help understand the weight of these things. I searched the forums looking for a discussion particular for weights of campers. I found a lot of inquiries on tow vehicle to pull my camper scenarios. Few comments on estimated weights and recently a discussion on the weight of a camper with a long list of items removed with weight info. So I hope this helps those for informational purposes.
I think it does help; thanks, Mike.

Pete can supply an appropriate comment here, but I think this confirms that the 1-7/8" ball on his Scamp 16' is a strangely lightweight choice (by Scamp), given that this starting weight will put a trailer loaded for camping over 2000 lb.

The other thing I noticed is that this is significantly heavier than the Scamp spec, and almost all on the tongue weight. Maybe that makes sense, since they probably are not including either the propane or the battery. That's a good thing to keep in mind when considering manufacturer's spec weights.

I agree that the scales were used as intended and have no problem with the validity of the measurement. Of course, scales intended to weigh truck axles will be limited in precision and accuracy (since they need such a high range), so we should not expect these numbers (or those that any of us have measured) to be correct to the pound. The highway scales here only display in increments of 10 kg (22 lb). Does anyone know the tolerance of these or any other truck scales?
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Old 04-21-2006, 06:47 PM   #9
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Typical CAT scales weigh in 20 lb increments. 29 lbs 15 oz is 20 lbs, 30 lbs is 40 lbs.

I agree with Brian B-P. If the tongue has negative weight, then some point behind the axel (fulcrum or pivot point) is going to be applying some portion of that negative weight to the ground. You can't just hang a scale in mid air and measure the negative force as weight and then expect to do use the number to figure trailer weight. Weight of TT is the combined weight of all points of contact with the ground. It don't matter if tongue jack is up or down, stabilizer jacks up or down, trailer is balanced on the tires, trailer is balanced on one tire, balanced on it's rear bumper, or on it's roof. Weight is a measurement obtained by adding the observed weight at each point that is supporting it, when an object is at rest (not moving). It doesn't matter how light or heavy the tongue is.

Curt
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