Estimate Truck Rear GAWR Margin with 5th Wheel

[Bill76]

I'm looking into getting a truck and 14K to 15K 5th Wheel. Many thanks to others for input to my previous post ("Realistic Pin weight for a 5th Wheel trailer") which helped me start looking at Duallies in addition to 1 ton SRWs.

Now, I'm trying to determine if potential Truck & Trailer combinations will exceed the Gross Axel Weight Rating (GAWR) for the Rear Axel. This would be fairly straightforward if I already owned the truck and trailer. Use the info on the Truck & Trailer stickers, go weigh Truck then Truck with Trailer, then do the math. But, I'm trying to do this with spec's and as much real world data as I can so I can find a truck sufficient for potential trailers without overbuying the truck.

I've done a bunch of calculations and am shocked that all of the 1 ton SRWs & 1 ton DRWs I've looked at (Ford, RAM, & Chevy) show that the GAWR-Rear is exceeded for a particular 5th Wheel (15.5K GVWR). Besides a bunch of "theoretical" trucks from brochures, this also included a couple real world trucks (1 ton diesel duallies, 1 Ford, 1 RAM) using their door sticker data. This makes me question my method for calculating GAWR usages.

I've summarized my approach to estimating potential Rear GAWR Margin below. Any thoughts on the validity of my approach would be welcome.

1) Determine Curb Weights:
Truck's Curb Weight = GVWR - Payload

Truck's Rear Axel Curb Weight = GVWR * 42%
-- 42% derived from various data points


2) Determine GAWR:
GAWR-Rear Ford and RAM = 95% to 100% of Spec'd GAWR-Rear

GAWR-Rear Chevy - Not Spec'd (can't find in Brochure or Tow Guide), so:

GAWR-Rear Chevy SRWs = 61% of Spec'd GVWR (average of 2 "door sticker" examples)

GAWR-Rear Chevy DRWs = 70% of Spec'd GVWR (consensus of Ford & RAM examples)


3) Calculate the used payload (how much I would load In & On the truck):
Payload Used =
50% of stuff in Cab (Self, Wife, 2 Dogs, Misc other) +
100% of 5th Wheel Hitch (in Bed) Weight + 100% Trailer's Pin Weight
-- 50% assumes cab stuff spread across Front & Rear Axel


4) Determine if Rear Axel Weight s/b ok:
Compare GAWR-Rear to Truck's Rear Axel Curb Weight + Payload Used to see if GAWR-Rear is exceeded.

 

[donn]

More than likely the reason your getting overloaded numbers is the SRW. Tire load ratings will probably kill you no matter what else.
For that much fiver a dually would be a far better choice. For instance if with a SRW your tires load rating probably 3700 each times 2. For a dually tires are usually derated a bit. So instead of 3700 each you might see 3500 each. But thats now times 4.
If I were buying a new truck today, and I am not, but if so I would go buy a new Ram Laramie dually HO 6.7L Cummins, Aisin and 4.10 rear end, and quit worrying about towing or load ratings.

 

[Bill76]

Donn - Thanks for feedback. I'm not even factoring tire ratings into the equation, something else I need to consider.

However, even the Duallies I looked at all fail my Rear Axel weight calculation. One of the Duallies is actually a real world (i.e. I physically checked both load stickers on the door) 2019 RAM Laramie Longhorn 3500 DRW with the High Output Cummins Diesel. It was a 3.73, but even if I factor in a 4.10 it still fails.

That's why I'm looking for feedback on how I'm calculating things. Either I'm calculating it wrong or the whole Pickup Truck / 5th Wheel population is overloaded. I really doubt the whole rest of the world is wrong so I'm trying to figure out what I'm doing wrong.

 

[donn]

I think your going to fry your brain. LOL.
There is no way you can make it work like you think. In the real world, which is where we are folks tow up to 18,000 pound fivers every day with all brands of 3500 duallies. The difference is weight ratings across the board is that some brands are heavier than other brands. Rams will generally weigh less because they use a I6, vs GM and Ford use V8s. Cummins makes its torque starting around 1200 RPM. While V8s meed more RPMs to start building torque
I had a 2007 Ram 3500 dually Cummins/68RFE, 4.10 RE towing a 13,500, scaled fiver. Loved the way it pulled. I could run all day with the cruise control set except for the steapest of hills. BTW my fiver was pin heavy right at 3500 pounds. Truck hardly ever knew the trailer was back there.

 

[rbTN]

not sure where your numbers are from, but I owned a 2001 Dodge 3500 dually, payload was well over 5000, GAWR rear was close to 9k, this pdf shows even more for a 2019 so I don't see how a 15.5k 5er is going to put that much truck in a bind.

https://www.ramtrucks.com/content/dam/fca-brands/na/ramtrucks/en_us/towing/Trailer_Tow_Excel_2019_D2_OFFICIAL_INTERNAL_2-06-18.pdf

 

[rbrdriver]

I am towing a 16,500 pound fifth wheel 42' with a 2017 Ram 3500 dually (payload of 5400) just fine. It does have the H.O. cummins, aisin tranny and 4.10 rear and it handles it okay. It sounds like the fifth wheel you are considering might be a little smaller so any dually should be fine.

 

[Gary RV_Wizard]

I think your calculations are flawed, simply because there are too many unproved assumptions and variables involved. You will just fry your brain trying to get a meaningful answer that way.

As others have illustrated, there are plenty of dually rig configurations that have payloads in the 4000-5000+ range. That will generally be sufficient for a 15k-18k 5W.

Yes, you need to watch the rear axle GAWR when approaching max payload, but the truck engineers knew that when they calculated the payload. When you look at truck specs, it's typical that GVWR is less than the sum of the axle GAWRs. That's because they know the payload isn't always distributed perfectly.

And while I don't recommend it, you can probably exceed the rear GAWR a couple hundred lbs without too much worry. As long as the tires are rated enough for the extra load, that is. The axle ratings seem quite conservative.

 

[Bill76]

Thanks to all for your input -- I believe I found the problem with my GAWR estimates.

I agree that there are a lot of moving & variable parts in trying to calculate the GAW vs. GAWR margin. But, I figured I needed to do my due diligence to look at everything and I read several places to "not forget GAWR". I used the specs directly from the manufacturers sites (including the 2019 RAM spec link referenced above) and real world sticker and weight measurements where I could find them. I also looked at multiple 5th Wheel Trailers since the pin weight for one 15k trailer is often significantly different than another 15k trailer, even from the same manufacturer. And yes, I did almost fry my brain!

Even so, my GAWR numbers were WAY off (all 1 Ton SRWs showed 1000 to 2000 overload on rear axels, most 1 Ton DRWs showed 300 to 600 overload on rear axels) even though the Payload checks seemed reasonable. I knew something had to be wrong with my approach -- no way that all truck / fivers rear axels were overloaded or that all truck engineers messed this up.

So, I rested my fried brain for a couple days and let comments here roll in. Then looked things over again and found the problem. I used GVWR rather that truck curb weight to figure rear axel curb weight (see below, WRONG original and FIXED correct assumptions):

WRONG ----- Curb Weight Rear Calc'd = GVWR * Curb Weight Rear Factor

FIXED ----- Curb Weight Rear Calc'd = Truck Curb Weight * Curb Weight Rear Factor

Now, all GAWR numbers make sense. Things progressively get better moving up in truck capability. Reasonable to plenty of margin in 1 Ton SRW and 1 Ton DRW trucks. One complete real world example (have sticker values and measured weights of truck with and without trailer) looks consistent too which seems like a good sanity check. Understandably, the numbers I get are estimates but my intention is to use them as worst cases to help the selection process.

Again, thanks to all for the extra sets of eyes on this.

 

[Ernie n Tara]

Be careful using manufacturers published pin weights. These are, by necessity (the manufacturer can't know how it will be loaded), based on empty weight. Use the published trailer gross weight times 25 percent to be sure.

Ernie

 

[Bill76]

Thanks Ernie. Pin Weight was the first rabbit hole I went down, in thread "Realistic Pin weight for a 5th Wheel trailer". Now I'm looking at both 25% of Trailer GVWR and the % of Dry trailer weight the published pin weight is applied to the GVWR (usually runs 19% to 21%).

 

[rbrdriver]

Just remember also that whatever you put in your truck besides you such as tools, your fifth wheel hitch, extra fuel tank, generators, people, pets, etc. etc. counts against your payload. It adds up quicker than one can sometimes realize.

 

[Bill76]

Yep, I agree. I've got about 850 lbs figured in for this (self, wife, 2 dogs, hitch, and misc other).