RV Dry Weight & Estimation – The Big Error

[DW_Gray]

Okay, I’ve read enough RV related forum comments and viewed enough RV dealers’ websites to cause me to want to rant. Rather, I’ve come up with a better approach. I’m about to revise my article, Before You Buy That RV, Truck or Other Tow Vehicle, by adding a second equation that is overlooked by many RVers. The reason for this second equation is that some trucks have a high GCWR but the pin weight of the trailer will exceed the truck's rear GAWR. Before I do, I’d like some constructive feedback. Below are the draft paragraphs I intend to add to the article. If you that have time, please read and try some of my new calculation formulas. I may not be the smartest dude on this forum, but my goal is to provide information that will assist in educated decision making.
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My personal opinion is that all the websites that attempt to assist RVers with estimating towing weight is entirely the wrong approach. Estimating is just that, estimating. I recommend you not waste your time with weight estimation. You have to use numbers that actually mean something. In connection with towing weight safety the five most important numbers you should use are Maximum Trailer Weight (sometimes call Maximum Loaded Trailer Weight), Gross Vehicle Weight Rating (GVWR), Gross Combined Weight Rating (GCWR), Gross Vehicle Weight (GVW) and a new term I present to the RV industry, Gross Hitch Weight Rating (GHWR). I'll explain more about the new term below.

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MORE ON TRUCK REAR AXLE GROSS WEIGHT

Most RV trailer websites list the hitch weight of the trailer they're selling. That listed hitch weight is based on the trailer's dry weight. You and I know that no RVer will be towing a dry RV to a campground. Actually, you can never be sure of the true dry weight unless you weighed it at the nearest truck scale. Using the dry weight and hitch weight provides little value to knowing if you're looking at a safe towing weight combination. This new second equation is calculating the trailer's Gross Hitch Weight Rating (GHWR). This is the most weight that any trailer should be pressing down vertically on the tow vehicle hitch or on the rear axle. Below is the calculation formula to obtain the Gross Hitch Weight Rating (GHWR) for 5th wheel or gooseneck trailers.

5th Wheel/Gooseneck Towing: Trailer GVWR X 25 percent (.25) = GHWR

Example: 5th Wheel/Gooseneck Trailer GVWR: 18,000 X .25 = 4,500 GHWR

Once you've calculated your trailer's GHWR you need to ensure your truck can tow this amount of weight by not exceeding the truck's rear Gross Axle Weight Rating (GAWR).

5th Wheel/Gooseneck Towing

Some vehicle brochures or owner's manual or towing guides will show the curb weight of the rear axle. Unfortunately these listed weights are not realistic. Therefore I recommend you weigh your truck's rear axle at the nearest scale to obtain an accurate rear axle weight. To ensure your rear axle can safely tow the trailer's GHWR, perform the following calculation formula.

(GHWR + Rear Axle Weight) - Rear GAWR = (A negative is good and you're under GAWR) (A positive is bad and you're over GAWR)

Okay, here's an about-face. Above I said "Honestly, there isn’t any need for a specific calculator to assist you with knowing how much your vehicle can tow." I've changed my mind. I'm currently working a on new calculator that will simplify the above information and have you input some real data, not estimates, that help you know before you buy.

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[JohnDar]

Your formula would be of value to someone trying to figure what truck to match to the desired trailer, before either are purchased. Here's some results, using your equation, for 1) "book" values, and 2) real (RVSEF) values for my rig and TV.

1) 16000 x .20 = 3200. (3200 + 3560) = 6760. 6760 - 8200 = -1440 lbs. The 3560 Rear Curb Wt. is from a CAT scale and the 8200 lbs. Rear GAWR is from the vehicle door.

2) (2950 + 2650) = 6600. 6600 - 8200 = -1600 lbs. The 2950 is the RVSEF calculated pin wt. and 2650 is the measured rear axle load minus the pin wt. The report actually gives the rear axle weight with trailer as 6600.

The 160 lb. difference in the "safety factors" is due to the rig only weighing 14525 lbs. loaded as we would travel (empty tanks). The dry weight is stickered as 12115 lbs. That was before adding the w/d, slide toppers, central vac and suppressor at the dealership.

Will these changes be added to your "app" as a free upgrade, or will one have to repurchase the "app"?

 

[danemayer]

5th Wheel/Gooseneck Towing: Trailer GVWR X 20 percent (.20) = MVWR

Examples:
5th Wheel/Gooseneck Trailer: GVWR 18,000 X .20 = 3,600 MVWR

Conventional Trailer: GVWR 9,800 X .15 = 1,470 MVWR

Once you've calculated your trailer's MVWR you need to ensure your tow vehicle can tow this amount of weight on your conventional hitch (tongue weight (TW) rating) or will not exceed your 5th wheel/gooseneck's rear Gross Axle Weight Rating (GAWR). TW is the maximum vertical weight allowed on the conventional hitch assembly.

Dave,

Our Rushmore has a GVWR of 16,200. That would put the MVWR at 3240. We were weighed at Goshen last year and came in with an actual vertical load of 3500 on the hitch. That maxes out the hitch's rating and the rear axle rating. Left-to-right, we were 25# over on one side and 25# under on the other.

So 3500/16200 would put us at 21.6% and it would seem that we are not overloaded. Btw, the trailer actual weight was 15,000#, which includes Washer/Dryer, Generator, lots of gear in the basement, enough stuff for a 10 week trip, and probably 35 gallons of fresh water.

Are you rounding down to leave us a 260# margin on vertical load, or is there some other rationale for 20%

 

[TedS]

Since all weight is vertical, I think your MVWR should be MHWR. Maximum Hitch Weight Rating. More descriptive of what it is.

 

[DW_Gray]

Dave,

Our Rushmore has a GVWR of 16,200. That would put the MVWR at 3240. We were weighed at Goshen last year and came in with an actual vertical load of 3500 on the hitch. That maxes out the hitch's rating and the rear axle rating. Left-to-right, we were 25# over on one side and 25# under on the other.

So 3500/16200 would put us at 21.6% and it would seem that we are not overloaded. Btw, the trailer actual weight was 15,000#, which includes Washer/Dryer, Generator, lots of gear in the basement, enough stuff for a 10 week trip, and probably 35 gallons of fresh water.

Are you rounding down to leave us a 260# margin on vertical load, or is there some other rationale for 20%

Oh boy, working on the project since 6 A.M. has proven I failed to look at my own information. You are right, the percentage in the formula should be 25 percent (.25). I'll fix that if I can get the editor to work for me. It's been a long day. So really you are good to go.

 

[DW_Gray]

Since all weight is vertical, I think your MVWR should be MHWR. Maximum Hitch Weight Rating. More descriptive of what it is.

That was my first thought. But that term is already designated directly to the vehicle hitch. Therefore it's imperative to have a separate term solely for the RV that is new and unique. I don't want there to be any confusion.

Make sense?

 

[DW_Gray]

Your formula would be of value to someone trying to figure what truck to match to the desired trailer, before either are purchased. Here's some results, using your equation, for 1) "book" values, and 2) real (RVSEF) values for my rig and TV.

1) 16000 x .20 = 3200. (3200 + 3560) = 6760. 6760 - 8200 = -1440 lbs. The 3560 Rear Curb Wt. is from a CAT scale and the 8200 lbs. Rear GAWR is from the vehicle door.

2) (2950 + 2650) = 6600. 6600 - 8200 = -1600 lbs. The 2950 is the RVSEF calculated pin wt. and 2650 is the measured rear axle load minus the pin wt. The report actually gives the rear axle weight with trailer as 6600.

The 160 lb. difference in the "safety factors" is due to the rig only weighing 14525 lbs. loaded as we would travel (empty tanks). The dry weight is stickered as 12115 lbs. That was before adding the w/d, slide toppers, central vac and suppressor at the dealership.

Will these changes be added to your "app" as a free upgrade, or will one have to repurchase the "app"?

Formula correction: 1) 16000 x .25 = 4000. (4000 + 3560) = 7560. 7560 - 8200 = -640 lbs.

2) (2950 + 2650) = 6600. 6600 - 8200 = -1600 lbs. The 2950 is the RVSEF calculated pin wt. and 2650 is the measured rear axle load minus the pin wt. The report actually gives the rear axle weight with trailer as 6600.

To compare with RVSEF weights, use the formula below. I did it with my RVSEF weights and there was 75 pounds difference. I think the RVSEF scale margin is +/- 50#

RV GVWR X .25 + Weighed rear axle curb weight - rear GAWR.

 

[TedS]

I understand trying to distinguish tow vehicle from rv. But, 'V' is usually known to represent 'vehicle' as in GVWR (gross vehicle weight rating). And GVWR is used both for the tow vehicle and the RV. The two are usually preceeded by vehicle or RV to distinguish one from the other. I think the same would be done with MHWR.

MVWR could be mis-interpreted as Maximum Vehicle Weight Rating.

 

[danemayer]

MPWR? P=payload

 

[DW_Gray]

I understand trying to distinguish tow vehicle from rv. But, 'V' is usually known to represent 'vehicle' as in GVWR (gross vehicle weight rating). And GVWR is used both for the tow vehicle and the RV. The two are usually preceeded by vehicle or RV to distinguish one from the other. I think the same would be done with MHWR.

MVWR could be mis-interpreted as Maximum Vehicle Weight Rating.

Ted, I appreciate your feedback very much. After additional thought and term research, I've sided with you on this one. I made the changes in the OP above to concur with the term Maximum Hitch Weight Rating.

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Well, the next day I decided an additional improvement. The new term is now Gross Hitch Weight Rating (GHWR).

 

[TravelTiger]

Let me see if I understand.

1. I take the trailer I am looking at, say a BigHorn 3585 RL, and take the Gross Vehicle Weight Rating (GVWR) from the sticker on the side of the trailer (or advertised on mfg. website)

2. Then I have a known, my 1-ton dually truck, and I weigh it without a trailer, I need the rear axle weight (with a full tank of fuel, loaded with people and stuff we'd carry, and a 5ver hitch in the bed).

3. Then I need what the truck door panel says for Rear Gross Axle Weight Rating (GAWR).



With these 3 numbers, I can figure out if I can tow it safely?


Ok, here goes the math.

RV GVWR X .25 + Weighed rear axle curb weight - rear GAWR.

15500 (Bighorn GVWR) x .25 + 3350 (RVSEF weighed rear axle) - 8550 (truck rear GAWR) = -1325, so I am good, yes?



--Erika

The question might still arrise, what if I don't have a hitch yet? How do I know what I'd carry in the truck on a trip? I guess you need to make sure there is plenty of margin for another 500lb of "unknown"?

 

[porthole]

As someone once said, "Trust, but verify."

President Ronald Reagan

 

[Westwind]

Now I understand why everyone's head hurts after trying to come to the bottom of the first statement.

 

[DW_Gray]

Will these changes be added to your "app" as a free upgrade, or will one have to repurchase the "app"?

Good question. For now, I'll make it available on the FWS website. I'll monitor how often it is used and feedback and then consider it as stand-alone app or an inclusion as a FWS upgrade at no additional cost. I'll just have wait and see. Hopefully I'll create a product will be highly useable for those who want to know before they buy.

 

[DW_Gray]

My OP is updated with some changes. Thanks for all the feedback. After spending most of the day yesterday working the formula's and proving them, I decided to drop the formula for conventional towing. There are too many variables for conventional towing to create a simple-to-use calculator. I'll consider that sometime in the future. Besides, I'm not sure there are as much overloading issues with conventional towing as there are with fifth wheel towing. I'll have do some more research on that.

No headaches allowed.

 

[mmomega]

Just some extra info to keep in mind. I know the axles Ford uses are rated for higher than what they show on the door or their website.

The sterling 10.5 used in the F250/F350 SRW are rated from Ford @ 6200 and 7280 lbs respectively. When the axle is actually rated for 9750 lbs from Visteon.
the Dana 80 used in the F350/F450 DRW are rated from Ford @ 9000 lbs. it is actually rated at 11000 lbs from Dana.

if you check the source book you find that the axle rating numbers listed by Ford are matched to the ratings of the rear leaf springs, not the actual GAWR.

i have not checked GM or RAMs numbers to see how they match up.

 

[DW_Gray]

Thanks JD. I'll check into that. I may have to make a note of that on the calculator.

 

[lwmcguir]

Just some extra info to keep in mind. I know the axles Ford uses are rated for higher than what they show on the door or their website.

The sterling 10.5 used in the F250/F350 SRW are rated from Ford @ 6200 and 7280 lbs respectively. When the axle is actually rated for 9750 lbs from Visteon.
the Dana 80 used in the F350/F450 DRW are rated from Ford @ 9000 lbs. it is actually rated at 11000 lbs from Dana.

if you check the source book you find that the axle rating numbers listed by Ford are matched to the ratings of the rear leaf springs, not the actual GAWR.

i have not checked GM or RAMs numbers to see how they match up.

This is correct and due to the ride. If you spring it for the axle rating it will ride like the proverbial lumber wagon. We had never had a Ford axle fail to date.

 

[DW_Gray]

Okay, I want make sure I understand this.

For Fords, in this example, the rear GAWR certification on the door may state 6200# or 7280#. But the real rating for the actual axle (Visteon) is 9750#.

It's my understanding that the certification label is the only acceptable weight rating as far as any state is concerned, legally. Therefore, legally the rating will be 6200# or 7280# regardless if any other component has a higher rating. Unless I'm missing something or Ford does something different, the general rule always is the lowest weight rating in an assembly of components will be the rating.

The certification rating is the only acceptable form of legal weight measurement. That being so, there is no concern for other components with higher ratings. It will not be used in the axle load calculation, only what is shown on the certification label.

 

[mmomega]

My post was more of a useless knowledge type post since the law will still go by the rating Ford places on the vehicle.

This info is probably more useful for those that use these trucks out on the farm or for private use where the truck won't see much road use.