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How to Determine Actual Weight Capacity of Tandem Axle Trailer


I just bought a 18 ft trailer with two 3500lbs axels. The a frame coupler is a 2 in and is rated for 5000lbs with a 700 lb tounge weight. The dry weight of the trailer is 1400 lbs. If I subtract 1400 lbs from the 5000lb capacity of the tounge, does that mean that I can only load 3600 lbs of weight on my trailer?


Expert Reply:

Your trailer should have an identification/safety sticker that indicates its gross vehicle weight rating (GVWR) which is the most the loaded trailer can weigh. If it is a brand new trailer this will unambiguously tell you its capacity; if it was pre-owned it is possible replacement parts were installed.

It does sound as though your trailer was over-spec on the axles (a total capacity of 7000-lbs) relative to the coupler on the trailer which you said is rated for 5000-lbs trailer weight. Whenever the different parts of a trailer - frame, coupler, axles, suspension, hubs, wheels or tires - are rated with different weight capacities always the lowest-rated component determines overall capacity, as in the "weakest link in the chain" so to speak.

With that said, there is no guarantee that changing to a higher-rated 2-inch A-frame coupler such as 8K-rated # BD43805W0317 or # CA-5210-B will actually increase the trailer weight capacity to that of the two axles (7,000-lbs). This WOULD be the case if the coupler turned out to be the lowest-rated component part on the trailer. Unless you know this to be the case I would not assume it to be so...the trailer's frame could turn out to be the weakest link. This is what separates true legitimate mechanical engineers (and trailer designers) from folks like you and me!

So, with the current 5K coupler on the trailer your math is correct.

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