The Backyard Approach To Tongue Weight

By William R. Gongaware

Contributor's Note: William R. Gongaware was interviewed for this story in the October 2000 issue of BoatUS Trailering. He passed away in August 2004.

William R. Gongaware taught engineering technology at J. Sargeant Reynolds Community College in Richmond,Virginia. Mr. Gongaware and his two assistants (grand children) explain how it's done using a little bit of engineering and a lot of common sense.

What's Your Tongue Weight?

If someone came up to you right now and asked "what's the tongue weight on your trailer," would you be able to tell them? And would it be correct? Do you even know what it means? Being able to answer these questions is important. Being able to get those answers is as close as your trailer and bathroom scale.

Tongue weight is just what it says: The weight of the trailer tongue on the hitch. It should be 7-12% of the weight of the trailer and boat fully loaded (including gas and, if applicable, outboard engine). If it is too light, the trailer will sway from side to side or surge forward and backward while being pulled. If the tongue weight is too heavy, the tow vehicle will be difficult to handle and, after a while, you will see excessive wear on the rear tires.

Here's what you need:

  • A bathroom scale with 300 pounds capacity
  • 2 five foot 2x4's, preferably southern pine or Douglas fir or better or a 4x6 of similar quality
  • 12 eight penny nails or equivalent screws no longer than three inches
  • 8 or more pieces of scrap hardwood (1x4x6 or longer) to be used as shims
  • 2 or 4 cement blocks or pieces of a 6x6 at least 10 inches long
  • One dark color marker (permanent ink, chisel tip preferred) and one pencil
  • Tools: torpedo level, hammer, measuring tape

Line drawing of how to determine trailer tongue weight

In this example, the trailer tongue is set on the 24-inch mark which means the weight registered on the scale will have to be multiplied by 2 to achieve the tongue weight. Make certain the 2x4s are level and at a height equal to that of the trailer hitch.

The Project Overview

In preparation, park the trailer and tow vehicle on a level surface. Chock the wheels and apply the parking brake. In order to make the system operate as safely as possible, relieve the tension between the ball and the coupler so the tongue can be raised smoothly with the trailer jack.

This project is based upon an Isaac Newton's law stating an object will remain in place, if the force pushing up equals the force pushing down. And because the tongue weight in many cases will be more than what a bathroom scale can handle, the design used here will weigh only a portion of the total weight. As a result, depending on where the trailer tongue is placed on the wood, the weight registered on the scale will be doubled (if the tongue is set at the "24" mark), tripled (if the tongue is placed on the "16" mark) or quadrupled (if the tongue is placed on the "12" mark) and will equal the weight of the trailer tongue.

The wood is supported at the 0 mark by shims and concrete block. At the 48" mark, the wood is supported by wood shims and the bathroom scale. The most important part of this entire project is don't be in a hurry.

Marking The Wood

All the nailing of the 2x4's is on the centerline of their length. Place the 2x4's one on top of the other and nail the boards together three inches from each end. Next, begin nailing the wood 6 inches apart alternately on opposite sides. Then using a pencil and measuring tape, start at one end and make a tic mark at 6, 18, 22, and 54-inch location. Starting at the first tic mark, use the color marker to construct a line on the sides and top of the wood at every tic mark location. Now, starting at the line marked "6," label it with an easy-to-read "0."

At the next line label it as "12, then "16" and "24" and finally "48" along the top of the sides. Now, looking at the side with the seam of the two boards showing, write "support," X4, X3,X2 and "scale" beginning at the "0" through "48-inch" lines respectively. This is the top of the lever upon which the trailer tongue will be placed.

Making It Work

Make certain the wood is level. And also make certain the height of the lever is equal to that of the tow vehicle's hitch (and if it is properly installed, the trailer itself should be level). Zero the scale, if this feature is installed. If it isn't, make note of the reading with just the weight of the lever and remember to subtract that from the reading with the trailer tongue.

Slowly jack the trailer tongue down so that the coupler is around a portion of the ball but the tongue on the lever (this is done for safety in the event the jack fails or the lever shifts). You are going to place the tongue on the "24" mark on the lever and watch the scale reading carefully. If it nears fifty pounds under the maximum capacity, move the tongue to the "12" mark.

Now, multiply the reading on the scale by 2 or 3 or 4 depending on where the tongue is sitting and remember to subtract the scale reading if you weren't able to set it at zero. There is your tongue weight.

Take the weight off the system before you disassemble.