Trailer Tongue Weight Voodoo

Almost three years ago I wrote a 3-part series titled, How Much Trailer Can You Tow? 

You can read them here:

Part 1

Part 2

Part 3 contains the information on Weight Distribution Hitches (WDH)

I have received many inquiries from trailer owners about Weight Distribution Hitches and there is a lot of confusion out there in trailer land. I have always advocated weighing your tow vehicle and trailer periodically to ensure everything is within specifications, and herein lays the confusion. People try to interpret the scale readings and assume a WDH will subtract weight from the tow vehicle’s rear axle and add it to the front axle; that is: the front axle of the tow vehicle will weigh more with the trailer hitched. We do not want this to happen. After completing my yearly maintenance on Monday, I loaded everything up, to include full water tanks and temporarily filled the refrigerator and headed over to the local CAT scale. I’ll use those measurements to hopefully explain how to weigh your set up, what weights you should be looking at, and how to interpret them.

The engineering and mathematical explanation of how WDH’s work is rather complicated, and this is not the forum to explain how it works in detail. But here are a few things to keep in mind:

  • Tongue weight is tongue weight, and a trailer hitch, whether it is load carrying or weight distributing, cannot change a trailer’s tongue weight.
  • A properly adjusted WDH distributes weight to both the front axle of the tow vehicle and the trailer axle.
  • How much weight a WDH distributes is where things get really complicated because we need to know the tow vehicle’s wheel base, distance from the hitch ball to the tow vehicle’s axles, to the trailer axles, angles, and all kinds of good stuff.

The good news is that we don’t need to do a whole bunch of calculations. We just need to know a few things. Let’s start by taking a look at what my 2012 Ford Expedition Manual says about setting up a WDH without air suspension:

  1. Park the vehicle (without the trailer) on a level surface.
  2. Measure the height of the top of the front wheel opening on the fender, this is H1.
  3. Attach the trailer to the vehicle without the weight distribution bars connected.
  4. Measure the height of the top of the front wheel opening on the fender a second time, this is H2.
  5. Install and adjust the tension in the weight distribution bars so that the height of the front fender is approximately halfway between H1 and H2.
  6. Check that the trailer is level. If not level, remove the trailer, adjust the hitch ball height accordingly, and repeat steps 3 -6.
  7. Warning: Do not adjust a weight-distributing hitch to any position where the rear bumper of the vehicle is higher than it was before attaching the trailer. Doing so will defeat the function of the weight-distributing hitch, which may cause unpredictable handling, and could result in serious personal injury.

Okay, so what on earth is Ford trying to accomplish with this procedure, since they don’t actually state it? The answer is:

We adjust the tension to get the front axle as close as possible back to its original weight before the trailer was connected to the vehicle. No more, no less.

It is that simple. This is what you need to know. You also need to understand that when you connect a trailer to a tow vehicle, the rear axle acts as a fulcrum, so the weight of the trailer tongue lifts the front of the tow vehicle making it front-lighter and the rear axle absorbs the difference.

no wdh
Trailer without WDH

For example, if you connect 900 lbs of tongue weight to a hitch it might remove 400 lbs from the front axle, making the total weight added to the rear axle 1,300 lbs. We want our WDH to return the 400 lbs to the front axle to ensure safe vehicle handling.

wdh
Trailer with WDH

In addition you need to know all the axle specifications for the tow vehicle and trailer, gross weight specifications, and the trailer’s tongue weight. Once you know look up the specs, you need to go weigh everything. Armed with this important information (specifications), you will make two passes on the scale. One without the trailer, and one with the trailer connected and the WDH bars tensioned.

A Cat scale has 3 platforms, so it can weigh 3 axles. On the first pass you will just weigh the axles of the tow vehicle, which will also give you the total weight.

cat scale

The second pass will give you the tow vehicle axle weights (which added together is the weight of the vehicle with the trailer attached) and the weight of the trailer axle. Add all three numbers and you get the combined gross weight of the tow vehicle and the trailer. We will use this information to calculate everything else we need to know.

So let’s review my data from Monday’s weigh-in.

weights no trailer weights combined

ANALYSIS

  • With the trailer connected and the WDH tensioned, the front axle weight decreased by 20lbs (3,000 vs. 2,980) versus the weight without the trailer. The WDH almost returned exactly all the weight to the front end. Good.
  • All axle weights are within specifications. Good.
  • Gross Vehicle Weight of the SUV with the trailer connected is 7,020 lbs vs. a max of 7,550. Good.
  • Combined gross vehicle weight of the SUV and the trailer is 14,620 vs. a max of 15,000 lbs. Good.

Now we need to calculate the trailer tongue weight. Take the sum of the axle weights of the SUV with the trailer connect.

In this example, it is:

2,980 + 4,040 = 7,020 lbs.

Now we subtract the total weight of the SUV when it was weighed without the trailer attached, and get:

7,020 – 6,130 = 890 lbs.

The max tongue weight allowed on the Expedition is 920 lbs. Good.

Tongue weight

Tongue weight should be 10% – 15% of the trailer’s gross weight. To calculate this, we add the trailer’s axle weight to the tongue weight we just calculated, or

7,600 + 890 = 8,490 lbs.

 which gives us 10.4% of trailer weight. Good.

The max weight specification is 9,000 lbs, and we are at 8,490, which includes about 850 lbs of fresh water. Good.

You should probably weight your set-up at least once a year, and every time you make significant changes in how things are loaded in the trailer.

I use a Sherline Trailer Tongue Weight Scale at home to weigh the trailer’s tongue weight. If it is in specifications I can adjust the WDH according to the 7 step procedure above and be confident that everything is set properly.

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