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What Are Pound-Feet?

By February 9, 2022April 12th, 20222 Comments

Understanding the Carrying Capacity of Your Cargo Accessory is Important.

Understanding Pound FeetOverLoading any system, be it as large as a train or as small as a cargo carrier, can lead to a failure of that system. For systems with two or more support points, like train cars, trucks, and autos, weight as represented by pounds can be a suitable measure of Carrying Capacity. For cantilevered systems with a single support point, such as most bumper-mounted cargo accessories and the bumpers that support them, a more accurate measure of Carrying Capacity is the pound-foot. A pound-foot is equal to one pound supported one foot away from the support point. Two pounds supported one foot away from the support point is two pound-feet just as one pound supported two feet away from the support point is also two pound-feet.

Put simply: Weight times distance equals Load.

A great example of this is a diving board; the further out on it you walk, the more it depresses. See also https://en.wikipedia.org/wiki/Pound-foot_(torque).

Because bumper-mounted cargo accessories and the bumpers that support them utilize a single support point, pound-feet more accurately describe Carrying Capacity than do pounds. It is for this reason that we test and rate our products using pound-feet.  To determine your cargo’s Load, multiply the number of feet the center of your cargo is from the bumper times the weight.

Calculating Load in Pound-Feet

Generator Photo ExampleAs mentioned before, weight times distance equals Load. Using the example in the image to the right, we see two 50 pound generators on a tray. The center of the left-most generator is 0.625 feet (7.5 in/12) from the bumper, giving us 31.25 pound-feet of Load (0.625 ft. x 50 lbs.). The right-most is 1.67 feet (20 in/12) from the bumper, giving us 83.3 pound-feet (1.67 ft. x 50 lbs.). Adding the two values together gives us a total Load on the tray of 114.6 pound feet.

Tongue Weight vs. Carrying Capacity

Most hitches are rated for Tongue Weight in pounds due to their origins as towing devices; Tongue Weight is the downward force of a trailer’s tongue onto the hitch and is calculated in pounds. To convert Tongue Weight to Carrying Capacity, use 0.5 feet as the distance to the center of the Load (as this is the distance most hitch balls are from the hitch body) and the Tongue Weight as the Load. As an example, 500 pounds of Tongue Weight equals 250 pound-feet of Carrying Capacity.

Should you have questions understanding or calculating Load as pound-feet, please feel free to contact us.

2 Comments

  • Jeff Cupery says:

    On a 14 Ga (5/64) thick travel trailer steel bumper, I put your braces on that bolt to frame. The literature indicates it increases weight load to 400lbs. Is that 400 pound feet or 400 lbs?
    I want to put a bike rack on and carry 2 E bikes that weigh 55 lbs each and the rack weighs 38 lbs. Thinking steel bumper may be ify. Would your aluminum Heavy Hauler be a better choice?
    Thank you

    • Mount-n-Lock® says:

      Thanks for your inquiry. As you’ll see on our How Safe is My RV Bumper? page, the square tube bumper such as that found on many travel trailers (and some motor coaches) is pound-for-pound a very strong structural unit. The most obvious example of the strength of a square tube would be the tens of thousands of van semitrailers on the highway today. If you look closely at these vehicles, you will see that they have no frame underneath them (other than a short piece for attaching the wheel carriages to the underside of the van body). The 50,000 pounds of cargo within the trailer is carried exclusively by the thin-walled square tube that is the trailer body. The walls on some of these trailers are only 3/4 inch thick reinforced plywood which, for a wall height of over 8 feet, is proportionally 2 times thinner than the 18-gauge (0.0478”) wall of the 4-inch square RV bumper. If you have a 14 gauge wall, your bumper will be evn stronger.

      Our ratings are in pound-feet so the distance from the bumper will have to be considered when calculating carrying capacity. Two ebikes of 55 pounds each and centered 2 feet from the bumper will generate 220 pound-feet of load. Assuming the rack’s center of gravity is 1.5 feet out would add 57 pound-feet of load, for a total of 277 pound-feet.

      We hope this helps. Please let us know if you have any other questions!

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