By Roger Marble
I’ve been monitoring a forum thread about a guy who wanted to hang a 250-pound toolbox off a mount from the rear bumper of his trailer. He just didn’t understand the implications of changing the hitch load if he made such a change or the shift in tire loads between axles.
This 30-second video from Facebook shows the potential of not doing the calculations and for ignoring science.
Distance from axle to toolbox could be a serious issue when you consider how levers work.
I have also seen some Class C motorhomes with platforms attached to the trailer hitch receiver. Some have mobility scooters on them, some have toolboxes and some even have motorcycles. These items might be 100 pounds, but I am afraid that the total might approach 500 pounds in some applications. While the hitch and platform might be rated for 500 pounds or more, the back end of the RV may have lower limits.
Here we see a 5,000-pound hitch on an RV with only a 250-pound load limit.
Load capacity is limited by the weakest link or component
It is important to remember that just as with axles, springs, and tires, the load capacity is limited by the weakest link or component. For axles, the weakest component is many times the tires. You may have an axle rated Gross Axle Weight Rating (GAWR) of 3,000 pounds and the tires may only be rated for 1,510 pounds. But swapping the axle with a 4,000-pound unit will not raise the GAWR unless you also change the tires and probably also the springs, brakes and spring mounts.
This same situation can occur for the hitch. Class C motorhomes are many times built on the “cutaway” frames of standard vans. But the RV company extends the wheelbase by cutting the rear of the frame then welding in a couple pieces of steel to make it longer. This does not result in a longer-but-equal-strength structure. In fact, it may mean the total frame strength is now lower.
Back to the question of what happens even if you don’t have load limitations on the frame or receiver. Hanging weight off the rear bumper does TWO things. First, it obviously increases the load on the rear tires. Second, it decreases the front tire loads and transfers that load onto the rear axle and tires. You need to be sure that this addition is not overloading any of your tires, as both of these conditions are potentially dangerous.
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Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
##RVT1016
FYI, I have a neighbor who has a fixture to carry his Harley off the back of his RV. Wonder if he did the math?
Let’s take this important discussion a step further and look at a real world math example. My truck has a 160 “ wheelbase, and the hitch receiver opening is about 50” behind the left-to-right center of the back axle. Let’s say I install a carrier into the hitch and load it down with 250 lbs. of stuff (including the carrier) and that the load’s center of mass is 30” behind the receiver opening. This would place the load 80” behind the rear axle. The added load on the rear axle would be 250 x (160 + 80) / 160 = 375 lbs., and the load on the front axle would be reduced by 375 – 250 = 125 lbs. HTH, / JG.
I expect to see electric wheelchairs and motorcycles out in the road when the “bumper hitch assembly” fails. No matter how big the rig or automobile/truck, folks always seem to need more space for more stuff.