Ok, I'm going to be hauling a water tank on a flatbed gooseneck behind my F-350 DRW. At one point in doing research for this project I came across an equation that utilized the center of balance for the load and the distance from the kingpin to the trailer axles to determine exactly where the load needed to be placed on the trailer in order to have the proper load on the kingpin, but I didn't write it down and of course I can't find it again. Does anyone know what that equation would be?

I'm trying to determine two things: where on the trailer my water tank needs to be centered, and how much water I can put into the tank without exceeding the GCVWR. I've got all the values of the truck and trailer weights and have determined that when empty, the trailer transfers 23.36% of it's weight onto the truck. I just need to know exactly where to put that tank since I'm going to be running right at the legal limit and need to be careful not to overload my truck's rear axle (trailer is rated for 24,000, so I'm not worried about overloading it).

 

Do you have a scale nearby?

 

Yeah, I've already weighed everything...

Truck weight with no trailer is 9200 pounds. Truck weight with empty trailer is 10,800, which means 1600 pounds got transferred from the trailer to the truck. 1600 pounds is 23.36% of the total trailer weight, which is 6850.

The water tank holds 2000 gallons and weighs 800 pounds empty.

Water is 8 pounds per gallon.

Trailer GVWR is 24,000 pounds, truck GVWR is 11,200, with a rear GAWR of 9750. Gooseneck hitch is rated for 30,000 with a 6500 pound kingpin weight.

I know how to get the total volume of water I can legally carry in my tank (I know it won't be all 2000 gallons), I just don't know the formula to get that tank properly placed on the trailer to get my center of gravity such that I won't have too much kingpin weight, but I will have enough to give me good traction on our hilly gravel roads.

 

Ok, here's what I've worked to so far...

I need to have no more than 3000 pounds on the kingpin when fully loaded.

In order to keep the entire rig at 26k or less, I need to haul no more than 1200 gallons of water (10k pounds). That puts me 50 pounds over 26k right now, but I know I can take 50 pounds worth of tools and crap out of the back seat of the truck! :-)

I also found an equation that showed that center of gravity was directly proportional to the distance from my kingpin to the axles, and the percentage of the trailer weight on the kingpin. So I know from weighing everything this morning that 23% of the weight of the trailer falls on the kingpin when empty, which implies that 23% of the distance from the kingpin to the center of the axles is where center of gravity would be, which is 58" forward of the center of the axles (overall length is 322 inches, with 247 inches from the center of the kingpin to the center of the axles.)

Now, how do I figure out where to put the center of the load (tank is 12 feet long so CG on it is at 6 feet) on the trailer so that I do not exceed 3000 pounds on the kingpin? (I already have 1600 pounds on it from empty trailer weight, so I can't add any more than 1400 pounds of load).

My mind is totally confused... Am I way overthinking this?

 

yep

for the loading, it's just statics. Assuming a moment free tandem and hitch, just make the free body diagram and solve for your forces... however this is theoretical. The best way is to use a scale.


Why are you limiting yourself to 26k? You're already driving a class A combination... Load it up to the GCVWR.


Also, you have to take unsprung weight out of the Center of Mass equation, especially if you're using empty weights. The axles and wheels are a non-negligible portion of the weight.

 

What are you hauling this water for? You may be looking at needing a CDL even if you keep the actual weight of everything below 26,000 because the GVWR of truck and trailer are over 26k combined. You will also need a medical card, DOT numbers, flares, etc.

 

and likely a tanker endorsement for that volume. ( >1000usg)

 

Yup most likely to be strictly legal you'll need Class A CDL with a tanker endorsement, but may be ok if private use, depends on the laws of your state. You need to decide how legal and safe you want to be. But what you want to do the way you currently have it planned is very likely not legal and COMPLETELY not safe.

Safety is the much MUCH more important thing here and the important part of that is that you're WAAAAAY safer filling that 2000 gallon tank and being overweight, then you are only putting 1200 gallons in it. A half full tank will act like a big pendulum and be very VERY unstable. Picture a 10,000(8.34lbs/galx1200) pound weight hanging from a cable swinging from the top of the tank. This get's even worse if the tank is long, now hang that cable from a rolling gantry the length of the tank. That is how stable a half full water tank will be. So either fill it all the way or get a smaller tank.

If filled all the way 2000 gallons of water 16,680lbs + 800lb tank = 17480lb load. On a 6850lb trailer is 24330lbs, just over your 24,000 GVWR of the trailer.

Pin weight, the factory numbers are the factory numbers, and for legal reasons that's all you have. But for what the truck can take look at the tires and add up the weight rating of the rear tires. And yes, you'll want as much pin weight as the truck can handle.

From there it is a simple percentage equation with the distance from the center of the trailer axles to the pin, and the center of the tank. Which it looks like you have worked out. BUT you FIRST need to work it for either a smaller tank, or a full tank, depending on your choices. But DO NOT try to haul a half full tank. You'd be MUCH better off with multiple smaller tanks then one large one if you decide you can't risk hauling the weight of the 2000 gallon tank full.

The bigger question here is why haul this tank, is the goal to move water, the tank, or both. And is this a one time, short distance thing, or something you are going to do regularly over long distances, on the highway, in traffic and such?

 

I'm already up on all the legal and licensing stuff. My understanding from reading Colorado law is that a cdl isn't needed if the unloaded combination is under 16,000, and the loaded combination does not exceed 26,000 which I can make happen by unloading 50 pounds of tools from the bed of the truck. But I'll go back and re read it. If I am already class A, then I'll push it to the max weight which will be about 1800 gallons.

 

Also, the tank is baffled. I've already hauled several times with it just under half full and had no sway problems. I used to drive fire tankers and am very familiar with the dangers of partially filled non baffled tanks! This will be a regular use system for getting water from the local filling station to our 10,000 gallon storage facility at our lodge. Wells don't keep up with demand. It's a ten mile drive on two lane state highway and one lane dirt county road with a few decent western Colorado grades. Tank is already purchased so no option to go with other tanks.

 

BruteFord, where my brain is having trouble is figuring out the math on where the tank needs to be in relation to the axles and kingpin...

 

So you're just trying to figure out how to maximize your haul with the truck, trailer, and tank combination you have?

In which case I can't offer an more, you have the numbers and the math. It's all about where and how much you want to push the limits.

 

Ok I'll add for that you need to work out what are the solid numbers. The most solid of which is the pin weight. If 3,000lbs of the tank load is your number then that's it and go from there. Next I still say fill the tank, the trailer can take it, most of all on such a short trip and it means fewer trips and a weight that can be planned for as the full weight will always be the same.

So if full a 17,480lb load of which we want 3000lbs on the pin. So that's 17.16% (3000/17480=.1716) of the weight. 247" from center of pin to center of axles, so center of the tank 42.39"(247x.1716) forward of the center point of the axles.

But if you want to limit to a 10,000lb load then it changes. 3,000 is 33.33% so in that case to get 3,000lbs on the pin you want the center of the tank 82" forward of the center of the axles.

Because of the load differences(full vs. 1,200gal), load capabilities of the trailer, and the fact that your very likely illegal anyway without a CDL then I say plan go all the way to 2000 and mount the tank closer to the first example at 42.5-44" forward and if you want to haul less full you can you'll just have less pin weight, but you can and likely is best to haul full.

 

Wait I just re-read your previous post, you are limited to 1400lbs of the load on the pin? That changes the numbers a lot!!!

If full, 17,480lbs 1400 of which on the pin is 8% of the load. 8% of 247" is 19.8". So center of the tank 19.8" forward of the center of the axles.

If limiting to a 10,000 load that's easy math, 14% of the load on the pin so 14% of 247" is 34.5"

 

Gotcha, thanks... That's where my math was falling down. I've already started looking into a cdl and will probably go that route as soon as I can. For now I'll mount the tank where it needs to go for full, and haul at 26kish... Thanks for the help!