Hitch ratings, why?

[jayhaitch]

My 2001 Ford Expedition factory hitch has a weight carrying limit of 5000 lbs/500 lb tongue weight, but 8850 lb/885 lb limit weight distributing. I don't understand the physics of this. Mass is mass.

I understand how the spring action of a weight distributing setup opposes the gravitation force at the hitch/ball mount point. So I assume this means the downwards vector of an 885 lb tongue weight is counterbalanced through the weight distributing spring by an opposite vector of 385 lbs to net 500 lbs. Correct me if I'm wrong someone, please.

But if I'm right, what does this have to do with total trailer weight? The hitch, hitch bolts, truck frame, etc, is still being pulled by 8850 lbs of weight instead of 5000 lbs whether the weight distributing system is installed or not. Inertia still exists. How does this limit variation work?

Thanks for clarifying.

 

[Bird]

I don't understand the physics of this.

And I'm not at all sure I could explain it, but . . .

When you pull that trailer without the weight distributing hitch . . . let's assume you have that 500 pounds of tongue weight. Now you're not only putting extra weight on the rear axle/suspension, you're actually removing a little weight from the front wheels because the rear axle is the pivot point. And when you step on the brakes, weight shifts. As to what weight shifts, how much it shifts, and which direction it shifts . . . , well, maybe that depends on a number of factors such as whether or not the trailer has brakes, how effective those brakes are, how hard you step on the brake pedal, etc.

But when you pull that same trailer with the weight distributing hitch, that same 500# of tongue weight is no longer pushing down on the rear suspension, and lightening the load on the front suspension. Instead, it's "distributing" that 500# to both front and rear truck suspension and back to the trailer's suspension.

So, yep, mass is mass and weight is weight, but your tow ratings are not simply based on what your vehicle can "pull". Instead they're based on what the vehicle can safely, pull, stop, and control in most road and weather conditions, and weight distribution, whether you're talking about a trailer, or just talking about the load in or on a vehicle, greatly affects the overall handling of that vehicle.

 

[RayH]

Many hitches are rated to match the vehicle. If you sit a pickup hitch right next to the Expedition hitch they are probably constructed basically the same but the pickup hitch may be rated to tow more. The weight rating of the hitch doesnt represent the failure point of the hitch itself.

 

[jayhaitch]

My Expedition has a maximum tow rating of 6900 lbs, so the max hitch weight is a moot point. The trailer I've ordered is GVWR 7000 lbs, but surge brakes, therefore no weight distributing system allowed. I guess this means I'm limited to 5000 lb gross trailer weight. At least until I get the Dodge Cummins I'm hankering for

My GC2310 is only 2600 lbs, and the trailer weight is around 1000-1100 lbs (aluminum), so I've got lots of extra capacity. I was ready to get a single axle trailer, but I believe in leaving a margin for safety, and I could only find 3500lb GVWR single axle trailers. I went to a tandem axle setup. If I could have found one I would have seriously considered a 5200 lb single axle trailer. But two 3500 lb axles should tow more smoothly and be easier to load anyway.

The ball mounts and hitch balls I've seen are either 5000, 6000, or 10,000 lb rated. The 10,000 lb one is a heavy sucker! I think I'll get the 6000 lb rated setup. Then the lowest rated point is the hitch, but everything else has a nice safety factor built in.

 

[jayhaitch]

Forgot to say thanks, Bird. That was a great explanation. I never thought of the rear axle of the truck as a pivot point. Makes sense.

Reminds me of my 1978 Ford Fiesta. 3 guys, the 6'4" guy in back, rear cargo area loaded to the roof (literally), coming back from a camping weekend. At 105 Kph I had power steering! At 100 kph or less, back to manual steering. Curious, that

 

[George2615]

The 7K trailer you selected will be fine. Just remember your truck is rated for 6900. So if your trailer weighs 1100 subtracted from the 7000 trailer rating = 5900. your truck is good for up to 6900 so deduct another 100 from the 5900 for a legal max weight on your trailer of 5800#. 5800 load plus the trailer 1100 = 5900. Your tractor is 2600 so you have plenty of leeway.

 

[jayhaitch]

I seem to remember reading somewhere that 1 cubic yard of gravel weighed around 2000 lbs. If so, I can also haul 2 yards of gravel at a time and still be within the hitch weight carrying limit.

I also assume there is a pretty good safety factor built in, so if I haul 5500 or 6000 lbs for short distances infrequently, with a 6000 lb ball mount and ball, my hitch won't rip out of the frame, right? (I know, I know, just follow the rules

 

[Bird]

Jay, until about 4 years ago, I had a 1981 Ford F250 (3/4 ton) pickup, standard cab, 8' bed. And I went to town once and bought one yard of pea gravel. I'm not sure just how accurate the guy was with his front end loader, but I went back home, about 25 miles, with the back end of the truck a bit low and the steering was probably as light as your Ford Fiesta.

 

[roxynoodle]

Bird's explanation of the WD hitch was very good. As another example, think of the people who can lay down on a bed of nails. They don't get punctured because their weight is evenly distributed. However, if the same person were to step on one nail, we know it would puncture his/her foot. Distributing the trailer's weight allows it to tow more than having it all hooked to one point. How's that for a comparison?

 

[jayhaitch]

Excellent analogy Roxy! Point clearly (and sharply!) elucidated in my visual memory

Only 2 more weeks till I get my trailer. Chrismas in September