What's wrong with this simple method of weighing?

[excelrvguy]

When I was President of an RV manufacturing company, I weighed many trailers. I've always subscribed to a simple method of weighing our 5th wheels. I was often met with resistance and engaged in several debates about my method being wrong, but no one could ever explain why.

I never felt that weighing the truck & trailer, then weighing the truck empty, then weighing the trailer.... blah blah blah was an inefficient way to weigh, not to mention being a real hassle.

When I weigh a 5th wheel, I pull the unit across the scales until the rear truck tires are off the scales, but not the jacks of the 5th wheel. Then I let the jacks down on the unit, just BARELY taking the weight off the hitch. (Not enough to actually lift the hitch.. Like maybe 1/8" to where I could barely see daylight between the hitch plate and the pin box)

Then I get a weight stamp. This would be the total weight of the unit. I would raise the jacks back up and have them stamp the weight with only the axles on the scales (I haven't moved a thing).

Example = Total weight 14,500
weight on axles 11,600
leaves only the pin weight. 2,900

I'm not talking about weighing each axle or each wheel. To me, this method saves time and effort and hassle.

What say you?

[Vanished]

The trailer has a total weight.. In the OP's post example of 14,500..
He has two weights of 2900 (landing gear) and 11,600 (axles) as examples..
The question we don't know is where is the center of mass of the trailer.. The total weight of the trailer is the distance * force of the axles (11,600) plus the distance times force of the landing gear (2900).. the question we are trying to answer is what would the pin weight be? the answer is it also has to be 2900 lbs total - but it's further from the center of gravity - so if it's 16' from the center of mass compared to 12' for the jacks (both random examples) in theory it should be 2175 lbs if the examples above are used..

The OP's method can correctly calculate total shipping weight and axle weight but does indeed give an incorrect pin weight..

[RustyJC]

In theory, nothing's wrong with your method. In case 1, you have the total weight of the trailer on the platform - it doesn't matter what it's resting on; jacks, wheels or anything else. In case 2, you have the axle weight of the trailer on the platform - the difference is pin weight.

In practice, for this to be absolutely accurate, in case 1 you must have no pin weight whatsoever left on the truck, but you must also not lift too high such that the pin is pulling upward on the jaws of the hitch and transferring some of the weight of the hitch/truck to the front jacks of the trailer.

Rusty

[fj12ryder]

Okay, let me see.

One way is you weigh both truck and trailer as a unit, then you drop the front legs and raise the trailer enough to take all the weight off the truck. That gives you the complete trailer weight and then some math will give you the pin weight.

The other way is basically the same except you pull off the scales drop the legs the same way but you unhook the trailer. Now you weigh the truck and simple math will give you the pin weight.

Neither one seems unduly complicated to me. With one you don't drop the trailer and the other one you do. Big deal. Either way works, nothing much different between the two.

[azrving]

Ok, so I already know my pin weight from doing it the reglur way. Who is going to the scale with their rig to do it both ways? OP, IT'S YOU 🙂

[rhagfo]

Mandalay Parr wrote:
It's not the pin weight but rather the Jack weight.
Pin weight would be less.

Not so, total weight minus axle weight with landing gear raised puts weight on pin and axles so the result is true pin weight.

Too many over thing simple math and physics.

[Mandalay_Parr]

rjxj wrote:

Mandalay Parr wrote:
It's not the pin weight but rather the Jack weight.
Pin weight would be less.

I see what you are thinking. 🙂 You are saying in effect there is a lever formed from the axles to the pin and the lifting weight at the landing gear would be more than lifting at the pin.

Yes, exactly.
Jerry Parr

[azrving]

excelrvguy wrote:

Mandalay Parr wrote:
It's not the pin weight but rather the Jack weight.
Pin weight would be less.

That's the debate that always comes up... Jack weight would be different than pin weight because of the lever effect. My point is that with the jacks down, I'm weighing the TOTAL weight of the trailer. Then the weight on axles. doing the math tells me that what's not on the axles, has to be on the pin.

The trailer doesn't weigh more because it's sitting on it's jacks vs if you had a tripod under the pin. Total weight would be total weight... correct?

Why haven't you or the others you have dealt with ever tried both methods, same rig same time and compared numbers?

[drfife]

excelrvguy wrote:

Mandalay Parr wrote:
It's not the pin weight but rather the Jack weight.
Pin weight would be less.

That's the debate that always comes up... Jack weight would be different than pin weight because of the lever effect. My point is that with the jacks down, I'm weighing the TOTAL weight of the trailer. Then the weight on axles. doing the math tells me that what's not on the axles, has to be on the pin.

The trailer doesn't weigh more because it's sitting on it's jacks vs if you had a tripod under the pin. Total weight would be total weight... correct?

You are correct.

[excelrvguy]

Mandalay Parr wrote:
It's not the pin weight but rather the Jack weight.
Pin weight would be less.

That's the debate that always comes up... Jack weight would be different than pin weight because of the lever effect. My point is that with the jacks down, I'm weighing the TOTAL weight of the trailer. Then the weight on axles. doing the math tells me that what's not on the axles, has to be on the pin.

The trailer doesn't weigh more because it's sitting on it's jacks vs if you had a tripod under the pin. Total weight would be total weight... correct?

[romore]

The difference if any would be minimal, IMHO not enough to be a factor for our purposes. This method saves a lot of time and effort, I like it.

[Artum_Snowbird]

All I could possibly suggest would be to weigh the truck, front and back and total, then hook up and do your routine. It should read the same as you call it in my opinion.

[azrving]

Mandalay Parr wrote:
It's not the pin weight but rather the Jack weight.
Pin weight would be less.

I see what you are thinking. 🙂 You are saying in effect there is a lever formed from the axles to the pin and the lifting weight at the landing gear would be more than lifting at the pin.

[Mandalay_Parr]

It's not the pin weight but rather the Jack weight.
Pin weight would be less.

[azdryheat]

Makes sense.

[azrving]

So at first you are getting a total weight of the FW then you are getting the FW axle weight. Total minus axles = pin. When you first put the landing gear down, the scale is picking up the FW axles and landing gear? Correct?

Seems right to me. I already know the weight of my truck and it's not going to change much compared to the FW so yes quick and easy.