Towing is complicated, isn’t it? Every time I think I know everything, I go back to SAE J2807 or the FMVSS, and I learn something new!
One of my Readers, “Charlie,” recently asked me a series of insightful questions. I thought his questions were very insightful, so I’ve edited our conversation and posted it here. I hope it will be helpful for everyone else, too. You can never learn too much about safe towing and weight capacities!
Charlie Asks: “Does a Weight-Distribution Hitch Return Weight to the Front Axles? And Is Cargo Capacity Calculated with the RV Moving or Stationary?”
Regarding a pull-behind camper using a weight-carrying hitch, when the trailer tongue weight is placed on the truck hitch, it seems that this will cause some amount of the tow vehicle’s front axle weight (albeit perhaps small) to also transfer to the tow vehicle’s rear axle. Is this correct? (The concept of pushing down on a toy truck’s rear bumper until the front end gets light enough to come off the ground.)
I have not found any websites that mention this aspect of weight loading on a tow vehicle’s rear axle (that is in addition to the trailer weight being applied). Is it because it is such a small and insignificant aspect? In my particular case, based on the tow vehicle’s actual axle weights and axle limits, it seems like the trailer weight causing some shift of front axle weight to the rear axle of the tow vehicle could be helpful. Is there a “rule-of-thumb” or way of estimating the axle weight shift produced when applying the tongue weight to the hitch?
In one of your articles, you mention “occupants are considered payload” related to RVs. So does this mean that occupant (user) weight is a factor contributing to “overloading” and exceeding a trailer payload rating?
Is an RV trailer’s cargo rating based on stationary use with the occupants inside, or is there any distinction for cargo loading when towing and unoccupied?
I’m just trying to make an educated and safe decision on selecting an appropriate camper to pair with my truck. There is so much to consider!
Ross Replies: Yes, A Weight-Distribution Hitch Has a FALR %. Cargo Capacity Is Calculated with the Passengers …
Yes, a weight-carrying hitch will act as a lever and fulcrum, transferring some of the tow vehicle’s front axle weight to the tow vehicle’s rear axle. And it’s not a small amount. It’s a significant number. In fact, this is the primary reason for weight-distribution hitches! As I wrote in my article about misconceptions about weight-distribution hitches, a WDH will usually “return” a certain portion of the weight BACK to the front axle. This portion is usually between 50 and 100%, and it’s called Front Axle Load Restoration. This is why it’s important to calibrate/install your WDH correctly.
Additional weight on the tow vehicle’s rear axle is good up to a point. It can improve cornering traction, reduce fishtailing, and improve straight-line braking performance, but you obviously don’t want to max out (or overload) your tow vehicle’s rear axle capacity.
I don’t know of a useful rule of thumb to calculate the rear axle load transfer. It’s geometry, basically. It’s based on the vehicle wheelbase and hitch overhangs, among other things. You can watch the YouTube video I link to in the above article to see how the math works out.
To my knowledge, there is no explicit mandate in the FMVSS whether passengers (as payload) should be calculated when an RV is stationary or mobile? That’s a great question! As far as manufacturers go, I think several assumptions are made. With a motorhome, it’s assumed occupants will be on board when traveling; with a travel trailer, it’s assumed there won’t be occupants when traveling. But technically, it doesn’t charge the cargo capacity. The cargo capacity is based on the GVWR, and no RV manufacturer is going to give you license to exceed the GVWR whether moving or stationary.
Personally, I wouldn’t worry (within reason) about occupant weight when an RV is stationary, leveled, blocked, and stabilized. Obviously, RVs don’t fall apart when you invite all your neighbors inside for a party! Are you above the GVWR? Yes, you are. Is your RV going to fold like a house of cards? I hope not!
But when an RV is moving, and the tires are heating up and the frame is flexing and the springs are bouncing, I would watch my payload like a hawk and give myself as much margin as I could.
Thanks for the great questions, Charlie!