Learn Me Calculating Trailer Tongue Load

I've got a new for me 70's ? Ford 8 foot pickup bed trailer that I am using to haul water to fill a buried and insulated 1500 gallon plastic water tank I am using for a cistern at my cabin.

The trailer tank is 300 gallons (8 lbs per gallon of water = 2400 lbs) and I read on the internet that the trailer probably weighs 1000 to 1200 lbs. The trailer hitch is 93" from the trailer axle and with an empty trailer and there is no weight on the tongue (it is virtually effortless to lift the hitch end).

The trailer tows great unloaded at 75 mph and loaded at 65 mph. (After I delaminated and blew the tread off a 20 year old passenger car tire and nursed it full of water 25 miles on asphalt highway and 3 miles on gravel on the bald belts crawling the last few blocks on a deflated carcass and bought some new proper 10 ply tires.)

How much tongue weight do I want on my hitch? Somewhere I've read 300 lbs and others say 10 to 15 % of gross trailer weight.

Weigh-Safe.com: Tongue Weight Calculator

So, if the gross trailer weight is 3400 to 3600 lbs, the tongue weight should be 340 lbs to 540 lbs.

10% = 0.10 x 3400 = 340 lbs

15% = 0.15 x 3400 = 510 lbs

10% = 0.10 x 3600 = 360 lbs

15% = 0.15 x 3600 = 540 lbs

If I move my tank centerline 12" in front the axle centerline is it just a simple ratio to calculate the tongue weight?

12"/93" = x/3400 lb

.1290 x 3400 lb = 438.7 lb tongue weight?

Are you moving the axle or just the water in the trailer? If you're moving the water in the trailer, you won't be affecting the trailers CG, so the denominator should be 2400 lbs. 

If the empty trailer is zero tongue weight, then.....

(Weight of tank and water) x 12"= 93" x tongue weight

It's a simple lever calculation.

Water is a live weight. Don't get too close to the limits. 

SV reX said:

Water is a live weight. Don't get too close to the limits. 

Just curious, would this still be true if the tank is full?

3,500 lbs. on a 1/2 ton axle with no trailer brakes?  That's sketchy as heck even with the CG correct.

In reply to APEowner :

Thanks for the replies guys. It's been a long time since I have tried to solve an Algebra story problem.

It's a Ford 9" rearend if that makes a difference. The brakes and it appears the pinion are removed. My pickup has an electric brake controller so I guess it wouldn't be too hard to add E brakes.

I do fill it up brim full so there isn't all that much sloshing.

spandak said:

SV reX said:

Water is a live weight. Don't get too close to the limits. 

Just curious, would this still be true if the tank is full?

Have you hauled liquid before?  It's different!  As SVRX said, live weight.  You hit the brakes and the weight transfers forward...significantly.  So much so that the feeling will be that your brakes grab and then as the weight shifts...your brakes aren't grabbing any more, and your pushing the brake pedal even harder then, hoping for more stopping power.  

That's what happens forward.  Then, there is side to side!  Get some sway going and the sloshing in the tank can increase that sway to the point of disaster!  

So, as SVRX says, don't push the load limits.  This isn't just hauling firewood.  You're hauling a load with a mind of its own (or at least a load that has changing dynamics.)

As for full, the tank seems to be domed so that it will never be actually, "full".

The air pocket is hard to calculate and my drawing isn't too accurate. It's hard to remember but I believe the tank is 325 gallons. When I fill it with the 2" rural water supply, it takes only about a minute to pump the water and I have to set the amount at 275 gallons and it shuts down an inch or so from the top of the fill spout.

Who knows how accurate the pump is but if it is close, the air pocket might be 50 gallons. It isn't too bad hauling, I only drive 55-60 mph and do long braking distances and easy slow turns.

I usually only have to drive 12 miles round trip per load, but lately, the local dispenser has been down for repair so I have been having to drive 100 miles round trip. 

I once hauled a bunch of wet ash in a Toyota Pickup trailer and I didn't want to load it at the front of the trailer because I was lazy and didn't want too much trouble shoveling it out of the back so I loaded it all near the tailgate. The tailgate was down and the trailer got the death wobbles almost to the point of flipping over. By the time I got slowed down and under control, all of the ash had ejected itself out of the trailer.  I don't want that to happen with 2400 pounds of water.

In reply to VolvoHeretic :

The death wobble happens when a load is too tail heavy.   It can be accentuated by a shifting load that triggers it. 
 

Live loads are a shifting load. Always. Liquids, livestock, etc. 

50 gallons of headspace means 417 pounds of water that can shift.  208 lbs of water can hit one side at the same time the opposite side becomes 208 lbs lighter.

Set your load a little bit tongue heavy, but don't get near your vehicle towing limits. 
 

You haven't said what you are towing with. If it's a modern F250 you are probably fine. If it's a B2000 you're crazy. 
 

I once had a trailer like that. It had a 1 ton axle, so we figured it could haul. We added 4' tall sides and loaded it with roofing shingles. 
 

It passed me when we were headed down a hill with a load. I watched it jam it's tongue into an embankment and flip twice end for end.  It was scary. I was very thankful there was no one on that sidewalk it crossed. 
 

I admire you doing your homework. Just leave yourself some headroom for the capabilities of the truck. 

In reply to SV reX :

Thanks. I'm using a 2009 extended cab GMC Sierra 1500.

Now I need advice on how to install brakes on the trailer. It seems as though all of the electrical trailer brakes kits are for spindle type axles, nothing for a flange type of axle. I have a controller that came with the truck. Since there are no brakes whatsoever, I am open to any suggestions.

Also, should I go to a 2-5/16" ball? I need to get a large upside down drop hitch for the pickup so that the front of the trailer is higher that the back and the drain suctions most of the water out.

Stock drum brakes with a Hydraulic Brake Actuator?

ETrailer.com: Demco Hydraulic Brake Actuator - Drum - Zinc - 2" Ball - 7,000 lbs

This is their cheapest one and seems nice enough except for the plastic ball release lever that looks like it will snap off with little effort.

I think you might be better off swapping in an axle that's rated for the weight you're hauling that has electric brakes already installed.  Something like this Link

Ok, if it's true that I want 15% of the dead load (1000 lbs) and live load (2400 lbs) sitting on the hitch, and the dead load is centered over the axle, I need to move the live load until it equals 15% of the total load (3400 lbs) which is close to moving the tank 20" in front of the trailer axle. I think. 

I will allow that just moving a 250gal tote full of waste oil in the bed of a pickup was a really queasy uneasy feeling.  Like, I started to get flashbacks from reading John's post.  And that was with the load in the bed, not on a trailer making the zigs turn into zags.

My gut feeling is to get as much tongue weight as you feel like you can get away with and forget percentages.

I agree with Pete. It's not about the percentages, it's about feeling comfortable and confident in the load you are towing. 
 

Try it. Start with the math, then do a test drive with a 25% load. If it doesn't feel good, shift the load. If it feels good, try a heavier load. When you're done testing and it feels good, you are golden. 
 

One other thing... don't forget that the pickup bed is higher than a typical utility trailer (which means the center of gravity of the load is higher), and the rear suspension of that 70's vintage truck may not not have originally been intended for a 2500 lb load in the bed.

Trial and error testing is your best method.

So, I have calculated the tank placement in the trailer to test for 10%, 12.5%, and 15% tongue loads which I read is the range of total trailer live and dead load that is recommended sitting on the trailer hitch. I am also just guessing a dead empty tank trailer load of 1000 lbs. I have hauled 4 loads about 320 miles a little lighter than the 10% tongue load and there where no issues with speeds up to 70 mph, (80x4) with half of it loaded and the other half unloaded.

I won't have a chance to weigh the trailer and the actual empty tongue weight or test the other tank placements until next year when I have to fill up the cistern again. Maybe I should have a minimum of at least 50 or 100 lbs dead load on the hitch at all times?

Thanks everyone for the suggestions and discussion. I hope my math is correct. I also don't know exactly where the pickup box is located in relation to the wheels which isn't important right now.