I have a boat on a single-axle trailer. If I keep the system (boat plus trailer) level and take 2 weight readings, one at the tongue and one a few feet closer to the axle, can I approximate the total weight of the system?
Say L1 = 13.5 feet, W1 = 47 lbs.
L2 = 9.4 feet, W2 = 70 lbs.
I realize this is likely to be very inaccurate. Really just a mental exercise. Anybody?
Yes, it's a statics problem.
As long as you measure all the points where the loads go to the ground, and you maintain it's level, the sum of the forces will be the weight. The locations don't really matter for that kind of measurement.
This does not work because any weight below the fulcrum (wheels, tires, axle, etc.) Is not a part of the "system" in the mathematical exercise.
In reply to alfadriver :
Well, I don't have a scale of sufficient capacity to put under the axle. Any way to calculate total weight based on the relationship of weight to axle distance? Or do I need to know the location of the center of mass in order to calculate that?
alfadriver said:Yes, it's a statics problem.
As long as you measure all the points where the loads go to the ground, and you maintain it's level, the sum of the forces will be the weight. The locations don't really matter for that kind of measurement.
Now I see you are not measuring the weight at the axles.... But at different points along the tongue, and can you use that to estimate the weight.
alfadriver said:alfadriver said:Yes, it's a statics problem.
As long as you measure all the points where the loads go to the ground, and you maintain it's level, the sum of the forces will be the weight. The locations don't really matter for that kind of measurement.
Now I see you are not measuring the weight at the axles.... But at different points along the tongue, and can you use that to estimate the weight.
Yes. Nordic is clearly correct in stating that this would exclude the unsprung weight.
In reply to 1988RedT2 :
When i draw it out, it kind of does.
I wish I could draw it easily and show the statics, but I'm in a meeting.
In reply to alfadriver :
Perhaps later, then. By all means, you should make the meeting a priority. 
Yes it's statics and yes you can solve.
If you weigh 1 or 2 places on the tongue and the only other support is at CL of axle, then yes you can but you will also need to know/guess the distance from the CL of the axle(s) to the location of the CG of the boat and trailer.
based on the numbers provided: the CG is about 1 foot in front of the axle and the boat/trailer (minus tires/wheels/axles) comes out at about 586 pounds.
1988RedT2 said:In reply to alfadriver :
Perhaps later, then. By all means, you should make the meeting a priority.
The meeting sucks, which is why I can keep track of this thread. LOL
I am not an engineer. Well, my title has often said "software engineer", but I do not have a BSME...
I started to write that I didn't think it would work, and then talked myself into it while reasoning it through. Then started actually playing with levers and numbers, and... I don't think it works without knowing the CoG position of the load.
The principle is that the center of gravity of the load is somewhere between the tongue and wheels, and thus when we change the lever arm a known amount, we can work out the actual load from the scale of the change in measured weight relative to the scale of the revised lever arm.
Taking the distance from tongue to wheels as 100% of the lever, and starting with a hypothetical 1000 lb load 25% of the way from the wheels to the tongue, we'd have a tongue weight of 250 lb. (from our 1:4 lever) If we then took a measurement at 75% of the way from the wheels to the tongue, we'd get 333 lb. (1:3 lever)
BUT if the 250 lb load at the tongue were caused by a 2,000 lb load 12.5% forward of the wheels (1:8 lever), then our measurement at 75% would be... 333 lb. (1:6 lever); So we get the same values at the tongue and and a fixed second location for two wildly different loads.
The problem is that we don't know our leverages (unlike the example above, where we were able to pick them), because we don't know the CoG of the load. If you can tilt it enough directions to learn the CoG fore/aft location, then I think you might be able to work it out.
EDIT: So I think I agree with Sparkydog, excepting that knowing the CoG seems nontrivial to me.
EDIT-AGAIN: And I assume you'd have a hard time taking the measurements you need to work out CoG without scales capable of doing the measurement you want in the first place. That is, turning it so far that you could measure at a bunch of different angles, but always such that you were measuring the "light" end... I suspect it would be awkward and not terribly accurate. If I were stuck with a bathroom scale and a need to know total mass, I'd be figuring out how to use levers to take the measurement more directly.
Here's the free body diagram. M is the mass of the trailer, F1 is the force at the front point, F1' is the force at the moved front point, F2 is the force when F1 is touching the ground, F2' is hte force when F1' is touching the ground.
and the statics:
Sum of the forces = 0
F1 + F2 - M = 0
F1' + F2' - M = 0
Sum of the moments = 0
F1d1 - Md2 = 0
F1'd1' - Md2 = 0
Putting numbers in:
47 + F2 = M
70 + F2' = M
634.5 = Md2
or 658 as measured the 2nd time...
Which really doesn't solve the statics problem.
In reply to Ransom :
But if you could measure the upward force at the rear of the boat/trailer, couldn’t you then calculate it? Keep in mind this is all way above my head...
Weigh station.
I do somehow feel that the location of the CG must be known for this to work. This boat has an old 90 HP V4 Johnson outboard on it which has to go over 300 pounds, and the builder of the trailer made it such that the tongue is quite light. Boat is an old fiberglass bass boat from the 70's, about 15 feet long. Trailer is homebuilt by a competent welder, and it is not built light. I was mentally ball-parking the rig at around 3000 pounds, maybe as little as 2000, but wanted to narrow it down.
Hardly an issue given that I currently pull it with the Dooley. Might be an issue down the road if I choose to make any adjustments to the fleet.
Where do you guys weigh heavy stuff? I already asked at the county landfill, and they said theirs was broken. Go figure.
Truck stops or scrap metal yards.
Given the current estimate of weight, here's a cheap way to weigh it:
3 bathroom scales
2 3-4' 2x6 boards
2 6" 2x4s
Lay one end of 2x6 on the scale, the other on the 2x4, put the trailer tire half way between, and double the reading on the bathroom scale. Third scale under the tongue jack. Won't be super precise, but will get you reasonably close. If you have only one scale, you could do each measurement separately with blocks under the other tire and tongue to keep the angles the same.
Edit: I was basing that on the 650lb estimate, not the 2000-3000 lb estimate. By putting the tire 1/4 of the way between the 2x4 and the scale, the capacity goes to 4x the max of the scale, so 1000 - 1200 depending on the scale. Accuracy suffers a lot, though.
Could we just have him run a 1/4 mile with the tow vehicle on its own and then again with the trailer attached? Curb weight of the tow vehicle and the times should give us enough information to use one of those awesome online calculators to get it semi-correct.
Pete Gossett said:In reply to Ransom :
But if you could measure the upward force at the rear of the boat/trailer, couldn’t you then calculate it? Keep in mind this is all way above my head...
I don't think so; it would still just be a different lever arm; if the rear was pulling up 500 lb, 6' behind the axle, that would be the same information as it pressing down 500 lb 6' ahead of the axle (these numbers totally unrelated to my example above, only referring to the question of measuring behind the axle).
pres589 said:Could we just have him run a 1/4 mile with the tow vehicle on its own and then again with the trailer attached? Curb weight of the tow vehicle and the times should give us enough information to use one of those awesome online calculators to get it semi-correct.
I like this idea! Not sure the boat would pass tech, though. And my truck is slow, in spite of the 4.10 gears. Empty weight of 5200 lbs. and a tired carbed 350 doesn't help.
In reply to 1988RedT2 :
Going from statics to statistics, to get good data from the drag race method, you need at least 3 runs in each configuration, so that you can get JUST a standard deviation, which will at least give you a confidence interval that you can determine that there is a time difference that is statistically significant. THEN you can use that data to calculate the weight change.
Probably need more runs than that to deal with the environmental factors.
I can picture the end result of testing being something more in the insurance claim area than the satisfactory answer zone.....
Wow I don't think I've written an equation for at least a couple of decades!
Ladies and gentlemen, I give you... the Sparkydog Boat Trailer Equation!!!
In reply to Sparkydog :
I got to thinking about this, but I wouldn't have known the equations without a lot of wayback machine research. But yeah, with careful measurement of the lever arm lengths, it should be possible to calculate the overall weight based on the difference between weights at 2 points along the tongue.
You'll need to log in to post.
