Oh this one can elicit strong opinions. First ... you cannot change the GVWR (it's a rating), no matter what you do to the rig. The manufacturer is the only one who can do that (to my knowledge). Increasing the ability to carry more is a different question. You mentioned axles, wheels, bearings and tires as areas where capability can be increased, but frame strength and rigidity at critical structural areas (please review what Howard and Linda are going thru right now) are all necessary inclusions to decide carrying capacity. Generally, the manufacturer's RATINGS should not be exceeded. To do so is to add risk to the equation and that becomes a personal decision as to the amount of risk versus the amount of reward. Not to encourage you one way or another, I will say that any 5th wheel as heavy as what you are describing should be towed by a truck suitable for the load (one ton DRW at a minimum) and a high quality cushioning hitch (air bags or foam cells depending on which brand you prefer). A cushioning hitch goes a LONG way toward reducing frame and pin box stress. Cushioning hitches reduce the amplitude of applied forces to the pin box kingpin (which is the tip of a lever) which reduces potential induced metal fatigue (the root cause of cracks and fractures which lead to failures). There are several Engineers on this forum who could probably explain it better than I did, and probably give much better advice as well.

-- Edited by RonC on Tuesday 6th of June 2017 12:11:11 PM

vannchan wrote:

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The fifth wheel I am looking at has a listed UVW of 14,200, a hitch weight of 2,900, and a GVWR of 16,500.  It has 2 7,000lb axles with G-rated tires.  The trailer has a large amount of storage, both a pass through up front, plus possibly 10’ worth of pass through storage in the rear below the living area.  If I wanted to increase my carrying capacity, could I upgrade the axles to 8,000lb ones, as well as upgrading the tires to match and allow us 2,000lb more to carry? (I do not think I would add that much, but I would rather have more capacity than I need rather than being right at the maximum.)  Or is there more to it than that?

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 There is much more to it than that I'm afraid. However, increasing the axle capacity and tire capacity is totally acceptable and with proper evaluation a good / excellent idea. Remember, axles actually are not "7K."  They are two 3,500lb rated SIDES.  So one can be over on a side and not over in total on the axle.  Very common problem.  Therefor going to 8K axles one can increase side capacity to 4,000lbs.  A good idea in most cases as well as the higher capacity, safer "H" tires.

That said, no one except the OEM (the company that built the trailer) can legally increase the total weight carrying capacity of the trailer - no matter how much steel an after market company / dealer installs.  The same is true for the pin box.  One can put in a higher capacity pin box or axles but that does not increase the weight capacity of the total frame of the trailer.  When one increases capacity in one place it transfers the weakest point of the frame to someplace else - which then can break.  The frame is engineered as a system and the results are a broken frame.  Seen many.

But, I reiterate, going from 7K to 8K is not a bad idea UNLESS 8K axles are simply too much for the trailer at hand.  The very best way to know about this is to visit someplace like MORryde who will not try to sell you something that is not appropriate for the trailer in question.  Normally upgrades from 7K to 8K axles are very common.  However, not always recommended depending on the rig. 

Based on your 16,500 GVWR I would venture to say upgrading to 8K would be a good idea but I would also want wheel weights to make a better evaluation.  MORryde would do that.

All of the above is just my opinion emphasizing I would get a specific opinion from MORryde if it were me.  That's what I did when I upgraded our previous rig.

(I also agree with Glenn about contacting the OEM for their take. He's correct about the tires but I doubt it on your rig based on experience only.)

Bill

As FastEagle noted, the VEHICLE manufacturer is the only one who can change the rating (the documents provided are for a motor vehicle, not a trailer), but the same principles apply.

A little more to ponder on carrying capacity.

There is something called FOS (factor of safety) that is used in the engineering world. Something that has a FOS of 2, would be able to withstand double it's stated rating before failure. Pressure vessels typically have a FOS of 10-20. Steel used in buildings typically has a FOS of 5-6. In my working life, among other things, I had a manufacturing division reporting to me. They made custom engineered products. I reached out to one of my former engineers and asked what he thought about all this. He said it would be a complete guess and, like me, had no idea what the FOS is for a 5th wheel frame might be, but the lowest FOS he had ever heard of is 1.3 ... so 30% higher than rated (except in the Aerospace Industry where the FOS can be as low as 1.2 due to the premium placed on weight) and as FOS goes up generally weight goes up also. No guarantee that an RV frame is built to that standard (or any other one for that matter), but it does give insight. I'm reasonably sure that a 5th wheel rated at 16,000 GVW could certainly be loaded to the number with no issues at all. There is some FOS built in, but how much? That really is the big question. I know what I would be comfortable with, but others have their comfort level and some don't think you should ever "borrow" from your safety margin. It's really very sensitive and personal. So no recommendation from me ... just information.