Drop frame van RV

Are we talking about a trailer setup or a truck frame setup? Some reason I thought we were talking about customizing a truck body, but regardless, same principal applies except we're talking about the empty/laden tongue weight change versus the steer axle weight change

I used to refer to the web site below frequently in a previous life.

Trailer Weight and Balance Equations and Calculator | Engineers Edge | www.engineersedge.com

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Just buy 30ft + RV and pull a 1 or 2 car enclosed trailer, why reinvent the wheel?

 

I am talking about a weight and balance issue with a trailer, I ask this because camper 5th wheels and small truck gooseneck trailers carry about 20-25% pin weight and, are arguably dynamically stable. I do understand that the height of the center of gravity is a factor, RV's and stackers are not particularly low centered vehicles.

What I want to do is get some insight on how vehicle dynamics seems to differ in light duty and heavy duty trailer design. Even the Euro combination vehicles have different axle placement, I really don't like triple axles though. The idea of a spread with dump on the back has some appeal.

The crux of the question is, what is the adverse effects of loading a trailer with one third the weight on the king pin and two thirds on the trailer axles?

Maybe I am crazy but, this is not about getting an RV and towing a car trailer. This is more of taking the idea of a toy hauler 5th wheel to another dimension.

Steve

 

A few thoughts:
You will want as much weight on the drive axle as possible. I pull some specelized trailers that have almost no pin weight because the axles are almost in the center of the trailer. On dry roads it is squirrely, on poor roads it’s down right ignorant.

Have you pulled a spread axle before? They pull down the road nice and straight but would be a nightmare in a camp ground. Absolutely put a dump valve on at least the rear axle if not both axles.

Are you sure the rear suspension of the truck is only rated at 17k? Seems low for a single axle?

 

There's probably not an issue with that configuration per se. But my concern would be does king pin weight get lighter as you load the trailer? Because as a toy hauler, the bulk of the weight probably gets applied behind the trailer axles. Then we get into actual expected gross laden weight. Unless we're talking about a very bizarre short trailer, as long as you have sufficient towing vehicle (1 ton tandem or bigger) weight distribution should not be an issue

Also, I don't like the idea of 3 axles either on a travel trailer, however, you are limited by readily available tire weight ratings of the appropriate size. If you're pushing the weight limits, you may need to consider 3 axles if dual configuration is not an option.

Lots of guys running the older and heavier 2-axle luxury 5th wheels of yesteryear are having problems with blowouts as the typical real-world weights of these rigs, and summer heat causes problems with 16 inch load range G tires. You see very few of the bigger "toy haulers" of today with only 2 axles. It's just too much to ask of these 4 tires under full load configuration.

 

The drop frame van idea uses water tanks to keep the weight constant on the pin, with the drop a good deal of the weigh in the trailer will be down low at or below king pin height.

I have a 42' camper that grosses out at 24K, it has 3- 7K axles, going to 17.5 solved the tire problem. The trailer is nice but, I want to take my cars with me to events. There are no toy haulers that could carry my cars and, they are pretty small and spartan inside.

My truck was tandem and I singled it to haul the camper, it is short at 200" wb, the idea to do a drop frame van came after I turned the truck in an RV hauler!

In looking at trailers for sale, I came across several older Kentucky and Great Danes on Racing Junk that were being pulled by single axle tractors or toterhomes on class 7 chassis with single axles. A toterhome with a 15' body couldn't be that light on the back axle?

My rough calculation for close tandem on a 48' trailer is 48" king pin setting and 36' rear axle placement, that is about the rear overhang of a 53' trailer with the axles all the way forward. The front axle of the car would be near center of the tandem, that back axle of the car 5' from the back of the trailer at 1600 pounds. The car and bikes would at most put 2500 or so beyond the tandem at 9-10 past the center of the tandems, 100 gallons of water 20' ahead of the tandems would offset that. I have done a lot of work on this idea and I have the faculities to do the work. I ask these questions because, you guys have far more diverse experience with unusual load configurations in the real world.

Steve

 

Steve, a couple questions:
you say you're limited to 17K on your drives because of the suspension but I'm not sure that's really the case. I assume it's half a tandem that you've modified, but tandems are 34K by law, is the single-axle version really limited to 17K? I'd assume it would actually be a 20K axle because it's a single axle. You know more about your axle than I do but I'm curious and the answer would be helpful.

What's a scale ticket look like for your rig as it is? how does the rear axle scale with the trailer you currently have?

I run a somewhat similar setup, single axle tractor and single or tandem axle trailers with it. It's a day cab so I don't have experience with how a truck like yours scales with a sleeper and a single-axle, but... depending upon how much camper you install into that trailer, I can't imagine with a tandem out back you'd be hitting any sort of limits on weight. I can fit a car a and a lot of stuff into our 28' pup trailer (kentucky single drop with 19.5s) and not be even close on weight (but again, our tractor is light). How much does all the RV stuff weigh? Also, I'm not saying handling doesn't matter, but if it's not for commerce, I'd assume you can take your time getting places, and reducing speed helps handling a lot so maybe you don't need to worry so much as long as it scales.

I think one of the reasons the axles are far forward on a lot of RV trailers is for tight turns.

Just a few thoughts, questions. I've got some experience with related details, but you've got a specific set of details there.

 

Mugurpe,

Flex-Air suspension IS limited to 17K single, from a weight standpoint could I run 18K and be OK? That still wouldn't make the cut, my truck scales at 10200 steer 1/2 fuel, 4800 rear axle.

So my 12K pin weight is really about max, the trailer shell with no RV mods is about 13K. I have accurate weights on all the appliances, slide outs, windows, interior furnishings, electrical, plumbing, HVAC and, vehicles. I have estimated the weight of the framing and reinforcement to add the slide out to the trailer. The total added weight comes to 15-16K, so the RV conversion trailer should weigh about 29-30K with 1/2 fresh water and gen fuel. The car I plan to take most often weighs 3800 and a couple bikes 900-1000, call it 36K loaded for a trip. I can put 12K of that on my drive axle, everything else has to be on the trailer axles. That comes out to that 1/3-2/3rds number I mentioned. Without cars or bikes the pin would still be at 12K and the trailer axles would be lighter.

The RV trailers have their axle placement forward for a few reasons, turning is one of them, ability of pick up trucks to tow them, many new 5th wheels have as little as 17% pin weight and, frame strength. That is one reason I asked about the uneven loading on a van, the RV and light truck 5th wheel and gooseneck trailer seem to favor 20% pin weight and they do go down the road often at amazing speeds!

I really don't want to build a trailer that has poor handling, the whole concept of picking everyone brain is proof of that. That said, just because it is not customary or generally done, is it unsafe? Regarding that, would a spread give me that with the understanding that having dumps on both axles would allow the trailer to turn without damaging the tires/axle scrubbing in turns. Or conversely have a close tandem at 36' or so? The close tandem would have a lot of weight behind the axles.

I have not pulled a spread axle trailer but, this seems like it may be the answer to the issue?

Steve

 

For the scrubbing it depends on how many miles you're planning on getting out of a set of tires. As an RV, you may never do enough slow speed driving to scrub the tires enough for it to matter. I assume since fun is the plan, it's more about getting where you're going and enjoying yourself there than packing in 100,000 miles a year. They also make tires for high-scrub applications. If the spread axle solves problems I'd say do it and use the appropriate tire. I also don't get the idea you're strapped for cash so a set of 8 trailer tires every couple years isn't the end of the world.

Since you're doing the fabrication yourself, you could also drop $1000 and get a single portable scale. While you're mocking stuff up, you could test the weight distribution with the scale before you weld everything up finally. You'd be able to tell if just moving the tandem and minor changes to the arrangement of the goods in the trailer would be enough, or if going to a spread would do it. I've never pulled a spread axle so someone else will have to make suggestions about that. I do pull drop frames and they have a low center of gravity and ride nicely, especially if you have belly boxes and load them (maybe good for the water tanks & car spares and such).

 

Kentucky puts spreads on their race hauler trailers:
Kentucky Trailer
Kentucky Trailer