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# Understanding a Trailer's Various Capacities: A Trailer is only as Strong as its Weakest Link

A trailer consists of many individual components, each of which must be sufficient in capacity in relation to the Gross Vehicle Weight Rating (GVWR). NATM trailer manufacturing members have been inspected to verify compliance in this regard, but what is this process? This small part of the compliance verification program can be quickly and easily completed by anyone from the manufacturer to the dealer, to the end-user. But first, one must examine the physics behind these capacities.

A trailer’s GVWR is typically the sum of the weight of the trailer and the cargo capacity or, as a formula:

Gross Vehicle Weight Rating (GVWR) = Weight of Trailer + Cargo Capacity

In other words, what the total weight of the trailer will be when fully loaded. This is extremely important because each component must be rated or capable of withstanding that weight or a failure of that component is likely. This equation is essentially a weakest link examination:

The lowest rated component (the weakest link) of the trailer must be equal to or greater than the GVWR.

Take the following trailer for example:

GVWR: 10,000 lbs.

Coupler: 7,500 lbs.

Tires: 12,000 lbs.

Rims: 13,500 lbs.

Axles: 10,000 lbs.

Safety Chains: 15,000 lbs.

If the trailer, when fully loaded, weighs 10,000 lbs., this weight would be greater than the coupler capacity, creating significant risk of a coupler failure. The coupler is the piece of the trailer that physically connects with the tow vehicle, and a coupler failure would result in at least minor damages and, at most, a completely detached trailer in motion on the roadway.

With this simple process NATM consultants complete the following capacity verification:

1. Determine the GVWR of the trailer The GVWR is required to be stated on the trailer’s VIN label. This label should be located on the front half of the trailer on the left side (roadside). Keep the GVWR in mind as you will be comparing this number with all other capacities.

2. Verify GAWR is equal to or greater than GVWR The Gross Axle Rating (GAWR) is also required to be listed on the VIN label. Once found, verify that the GAWR is equal to or greater than the GVWR. If multiple axles are present, be sure to add the capacity for each axle together and compare this number to the GVWR.

3. Verify that the coupler capacity is greater than or equal to the GVWR The coupler should be stamped on the top with its capacity. Verify that this capacity is equal to or greater than the GVWR. Sometimes a finish may make the capacity hard to read or an accessory, such as a jack, could be covering the capacity. The manufacturer should be able to verify and/or provide documentation of the capacity of the component.

4. Verify that the safety chain capacity is greater than or equal to the GVWR The safety chains are what attaches the trailer to the tow vehicle and engage in the event of an uncoupling. These chains are sometimes marked with their capacity, but the manufacturer should be able to verify and/or provide documentation of the capacity. Further, the entire attachment method should be rated at or above the GVWR including the hooks, attachment method of the chains to the frame, etc.

5. Verify that the tire capacity is greater than or equal to the GVWR Each tire should have its tire capacity on the wall of the tire. Find this number and multiply it by the number of tires on the trailer for the figure that will be verified as equal to or greater than the GVWR. For example, a single axle trailer will have two tires, so the capacity of a single tire should be multiplied by two. A dual axle trailer will have four tires, so the capacity of a single tire should be multiplied by four. The sum total of the tire capacities is what is required to be equal to or greater than the GVWR.

6. Verify that the rim capacity is greater than or equal to the GVWR The same process used for verifying tire capacity should be used in verifying that the rim capacity is equal to or greater than the GVWR. For instance, a single axle trailer will have two rims, so a single rim capacity multiplied by two is the number to be compared with the GVWR. Oftentimes the rim capacity will be stamped on the inside of the rim making verification difficult. If a spare is available, this may be an easier way to find the capacity and avoid accessing the underside of the trailer. If neither option is available, the manufacturer should be able to verify and/or provide documentation of the capacity.

The above considerations are extremely important when verifying the towing capacity of a trailer. To eliminate the need to independently verify these capacities, consider purchasing trailers which have been manufactured by a company certified compliant by NATM. A trailer manufactured by a certified compliant company will usually display the NATM decal as an easy way to identify its compliance with these and all other program requirements.

For further questions regarding NATM’s compliance verification program, contact NATM Technical Director Terry Jones at Terry.Jones@NATM.com or (785) 272-4433.