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Old 01-18-2014, 02:59 PM   #1
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Western slope , Colorado
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Winter doldrums and thoughts to stir up those neurons….

We see a lot of threads on towing, tow vehicles, towed trailers, hitching, weight distribution, and the list goes on. It’s been cold here in Colorado, and the rest of the country, so the mid begins to wander and low and behold, a thought comes out! Have you ever wondered about GVWR (that’s gross vehicle weight rating) and why it is that for some reason the GVWR for a trailer is usually just less than the GAWR (gross axle weight rating) minus a little bit for the weight that is carried by the TV (tow vehicle). So, why is it that when you start looking at tow vehicles, the GVWR is usually much less than that of the combined GFAWR and GRAWR. GRRRRRRR!!!!

So here is my analysis on why, not really being an engineer, but thoughts that I wish I could have answered.

To look at a trailer, lets assume that you could load it up to it’s max weight. Without putting too much thought in doing it I would probably try to distribute the weight so most of it is over the axles or placed within so as to not get the tongue overly light or overly heavy, keeping it somewhere around that magical 10 to 15% that we all have been taught. So the load appears to continually be placed so the axles take the load up to the GVWR…. Seems ok to this point.

But, when we look at a TV(without the trailer attached), loading it is not that simple. The load carrying compartment is usually in the rear of the vehicle and there is little one could do to help distribute the load equally on the axles. You are destined to add weight (load) to the rear axle more and more until you get to the point of reaching the capacity of the rear axle, and in the process maybe lightening the load on the front axles. This is where I will use some numbers for clarification. Let’s say we have a ½ ton pickup truck with a FAWR of 3900lbs and a RAWR of 3900lbs with an overall GVWR of 7000 lbs. The axles have an overall carrying capacity of 800 pounds more than the vehicle could carry. The real weight of the vehicle is 3180 front axle and 2440 rear axle, which means that the total carrying capacity of the vehicle is 1380 lbs. So let’s pretend that I start adding all that weight to the bed of the truck, over the rear axle, without adding a driver and now I get up to 3820 lbs….pretty close to that manufactures number of 3900 lbs for the RAWR. So, is this how the manufacturer comes up with their GVWR, based solely on a limiting situation? (That was rhetorical and you don’t have to answer yet)

So my thought continues. Let’s assume that that we were able to do to the truck, what we could do to the trailer, load the axles more equally. Hey, we can do this with a WDH (weight distribution hitch). The principle with the WDH is that load (in this case force) can be redistributed from axle to axle by just applying a fulcrum and the tail of the TV. So let’s look at a hypothetical (ha ha) Airstream trailer hitched to the pickup truck. I am going to assume a 27’ FB classic that has a GVWR of 9,000 lbs. The pickup has a GCVWR (gross combined vehicle weight rating) of 15,000 lbs so in theory if you were to load them all up to the max you would exceed the CCVWR. The real numbers tell you a lot. So we have gone over the dry configuration on the truck, and in the fully loaded for vacation including water, food, etc., configuration, the trailer is 7,540 lbs. The axles seem to be way over on their carrying capacity and probably could have been 4,000lb versions. In any event, couldn’t see putting anything more in the trailer, but I could. So we take the weight of the trailer, add it to the truck weight, and we come up with 13,160 for the combined weight, well under the GCVWR, so I can safely tow???

Now that we have some individual weights let’s hitch up and get some real world totals. We measured the weight of the tongue at 790 lbs. Gives us a weight on the trailer axles of 6750. First now weight distribution and we get a FA weight of 2,860 lbs, RA weight of 3,600 lbs(gained 1,160 lbs???) and a trailer weight of 6,300 lbs. Hey, adds up to the 13,160 we got before, that’s a good sign. But wow, the rear axle is almost at its max of 3900 (And, I only put 790 lbs on the hitch????), and if I put in my toolbox, got in, with the wife, and let the dogs run around, there would be no joy in mudville. Probably exceeded the RA weight. To help out this problem we allow Archimedes principle to do it’s work and when we feel comfortable, adding about 4 or 5 links, we reweigh and come up with the new numbers…. FA 3,220 lbs, RA 3,200 lbs, and trailer weight of 6,740. Truck is level , but squatted some, trailer is level and still at the 13,600 lbs total (that’s good) but hey, now I am at 6,420 lbs total weight for the pickup, which leaves me now at only 520 lbs that I can carry in the truck. Not too much of a problem because most everything is back in the trailer.

Now that the preliminaries are over here is my thought….So, now that we have the trailer hitched up and have a fulcrum which allows for distribution of weight, does the pickup now in theory have a WDGVWR(weight-distributed gross vehicle weight rating, my coining of the term is right now) of 7,900 lbs….like the trailer, load(force not weight) can be distributed to the axles which have an overall higher weight rating higher than the initial manufactures GVWR. Let’s contemplate this theory that when a vehicle is equally loaded on its axles that it’s GVWR is close to the sum of its GAWR. Works for trailers, why not Weight Distributed tow vehicles??

I’m going to let this ramble along; my thoughts, hope my numbers jive, like to hear your thoughts, but may not be able to chime in frequently…warming up in front of the fire….

Disclaimer: Nothing in the above analysis implies that one should ever exceed the manufactures weight ratings.

"If you come to a fork in the road, take it."
Yogi Berra
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