Wasatch Lance
Park City, Utah
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This is a subject that I have never seen addressed in this forum, and I think many people operate under some wrong assumptions.
Question: If you attach a trailer with a 500 lb tongue weight to the rear of a pickup truck, how much does it increase the weight on the rear axle?
Answer: If you think the answer is 500 lbs, read on!
A typical pickup truck uses a receiver hitch that is mounted to the truck several feet behind the rear axle. If you then want to tow a trailer, you install a ball mount and attach the trailer.
The thing is, the distance between the center of the hitch ball and the centerline of the rear axle acts as a lever. The effect is to increase the weight on the rear axle by (in some cases substantially) more than the weight on the hitch ball.
I know some are saying "No way, if the trailer tongue weighs 500 lbs it can't weigh more than that when attached to the vehicle."
Well, while it increases the weight on the rear axle by an amount over the actual tongue weight, it decreases the weight on the front axle by the same amount. Think of a lever and a fulcrum. The rear tires are the fulcrum and the distance from the rear axle to the hitch ball is one end of a lever. The other end of the lever is the front axle.
To calculate the effect of trailer tongue weight on axle loading you need three numbers. The trailer's tongue weight, the tow vehicles wheelbase, and the distance between the centerline of the rear axle and the hitch ball. If you use a hitch extension, you need to factor that length in as well.
I'll use my truck as an example. The wheelbase is 172.4 inches. I use a 42 inch hitch extension when towing a trailer with the truck camper on. With the hitch extension, the distance between the centerline of the rear axle and the center of the hitch ball is 99.5 inches. Now, let's do some math.
The ratio of lever to wheelbase is 99.5/172.4 which equals .577146. Let's call it .58. Now, multiply .58 times the trailer tongue weight. (I'll use 500 lbs) .58 x 500 = 290.
So the actual increase on the rear axle is not 500 lbs, It is actually 500 + 290 = 790 lbs. Similarly, the front axle's weight is decreased by 290 lbs.
For those of us who are "pushing" our axle loading, or are debating the purchase of a new truck, it's worth it to understand the physics involved and how it actually changes the axle loading.
I hope this information helps...
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BradW
Flat Rock, Alabama
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http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/15964794.cfm
Caution: Possibel defective spelling above; spell checker is DOA.
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emaav
Hillsboro, Oregon
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Wasatch, very nicely explained. Thank you.
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SoCalDesertRider
SanDiego, CA, USA
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Very good post. 
As Brad pointed out, we already have a post about the same topic in the Truck Camper University section of the forum. Still, it is good that it gets mentioned once in a while in the main forum to increase awareness for those who don't read the TCU.
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Raften
Northern Calfornia
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So could you weight the rear axel weight with and without the trailer attached and see the increase over 500? Or does it only show in a dynamic situation?
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JerryBowers
Mendocino County, CA
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Raften wrote:So could you weight the rear axel weight with and without the trailer attached and see the increase over 500? Or does it only show in a dynamic situation? Weighing the truck and separately weighing the rear (or front and rear) axle(s) with and without the trailer attached will give you the actual (higher) weight on the rear axle as described above.
Happy RVing,
Jerry
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Horseplay
NC
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That's why you use weight distribution bars.
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Golden_HVAC
Fulltime, CA, USA
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Horseplay wrote:That's why you use weight distribution bars.
Using weight distribution bars will allow transferring weight from the rear axle to both the front of the tow vehicle and the trailer axles. It works like a lever, with the chains pulling up on the bars, causing the hitch to pivot forward, transferring weight forward, and at the same time the chains pulling up on the bars causes a few pounds of weight to get transferred to the trailer axles.
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