RV Shopping: Weights and Measures

Lisa Fritscher May 25, 2012 No Comments

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Weights are boring but critical for safety and comfort

So you have decided to take the plunge, joining millions of others in the RV lifestyle. While it is important to find just the right floor plan to meet your family’s needs, don’t fall in love too fast. The layout and design features of an RV affect your family’s happiness, but the weights and measures affect your family’s safety. Although rules also apply to truck campers and motor homes, towable RVs such as travel trailers and fifth wheels require special considerations. Here I will focus on travel trailers, as those are the RVs with which I have personal experience. Always check with the manufacturers of both your tow vehicle and the trailer for specific advice on your particular combination.

Learning the Acronyms

Although it is not nearly as much fun as looking at photos and planning your trip, everyone in the market for a towable RV needs to learn a variety of acronyms. Understanding how these factors work together is a critical early step in RV shopping. For the tow vehicle, these numbers are on a sticker in the driver’s side door frame. For the RV, these numbers are on a sticker somewhere on the RV. In older models, the sticker was generally inside a kitchen cabinet, but in newer models it is typically on the outside near an entry door or storage compartment. Although general numbers are usually available online, the number may vary between individual units due to factory-installed options. Read the sticker on every single vehicle and RV you are considering.

You need to know the capabilities of both the tow vehicle and the trailer

Gross Vehicle Weight Rating (GVWR): Both the RV and the tow vehicle have these. The GVWR refers to the maximum fully-loaded weight that the RV or vehicle can safely hold. This includes people, animals, furniture, accessories, water, propane, and even clothing. If it is inside the walls or attached to the frame, it counts toward the GVWR.

Maximum Trailer Capacity (MTC): This is the maximum trailer weight that your tow vehicle can handle. In theory, the GVWR of the trailer should not exceed the MTC of the tow vehicle. However, as long as the unloaded weight of the trailer is significantly below the MTC, you can keep the trailer within a safe range by carefully managing its payload.

Unloaded Weight (UW), Dry Weight (DW), Curb Weight (CW), or Base Weight (BW): This rating goes by a number of different names, but like the GVWR, both the RV and the tow vehicle have these. This is the weight of the RV or the tow vehicle without anything in it. But this can be confusing, because different manufacturers calculate this differently. Many, but not all, vehicle manufacturers include the weight of a 150-pound driver in this weight. Some also include a full tank of gas. Some RV manufacturers list the weight of the RV before any options are added (such as an awning or gas grill), while others list the weight of the fully-equipped RV when it leaves the factory. Since this term may mean many different things, the best way to get an accurate weight is to weigh the vehicle or RV yourself. We will discuss how to do this a little later.

Payload Capacity (PC) or Cargo Capacity (CC): This is how much weight you can safely put into the RV or tow vehicle, including options, accessories, people, pets, and personal items. The CC is equal to the GVWR minus the unloaded weight of that vehicle or RV. If the payload capacity is clearly marked on the vehicle or RV, then you can skip the step of getting your own unloaded weight. The UW will equal the GVWR minus the CC.

Gross Combined Weight Rating (GCWR): This number applies only to the tow vehicle. It is the maximum total weight of the tow vehicle plus the RV, including all of the people and items you add.

Rear Axle Weight Rating (RAWR): This number is only found on the tow vehicle. The RAWR is the maximum amount of weight that the rear axle of the vehicle can safely support. When towing, this number includes the tongue weight of the RV. It also includes the normal, unloaded weight on the rear axle as well as any payload that you carry inside the rear of the vehicle.

Tongue Weight (TW): This number applies only to the RV. This is the amount of weight that the RV places on the rear axle of the tow vehicle. For stability and safe towing, the TW of a travel trailer must be between 10 and 15 percent of the total loaded trailer weight. Different specs apply to fifth wheels. The dry tongue weight quoted by manufacturers is often misleading, as the TW goes up as the trailer is loaded for camping.

Hitch Receiver Weight (HRW): This number applies only to the tow vehicle. It is the amount of tongue weight that the tow vehicle can safely handle. Note that this number may be lower than the available payload capacity on your RAWR.

How the Weights Work Together

When taken together, the above ratings paint a clear picture of whether your chosen RV and tow vehicle are a safe and effective combination. Here are some real-world numbers to help demonstrate how this all works.

Dad and I drive a 2007 Ford Expedition Eddie Bauer with a Heavy Duty Tow Package. According to a combination of our vehicle sticker and the Ford Towing Guide, these are the numbers for our Expedition:

 

GVWR (Gross Vehicle Weight Rating): 7,400
MTC (Maximum Trailer Capacity): 9,200
CW (Curb Weight): 6,140
CC (Cargo Capacity): 1,260
GCWR (Gross Combined Weight Rating): 15,000
RAWR (Rear Axle Weight Rating): 4,250
HRW (Hitch Receiver Weight): 920

 

Armed with those figures for our Expedition, let’s look at the numbers for our travel trailer, a 2012 Forest River Flagstaff 27RLSS:

 

GVWR (Gross Vehicle Weight Rating): 7,436
UW (Unloaded Weight): 6,143
CC (Cargo Capacity): 1,293
TW (Tongue Weight): 474

 

We know right off the bat that the TW of the Flagstaff is a bit misleading. When towing a travel trailer, for stability reasons it is critical that the TW be 10 to 15 percent of the total trailer weight. Therefore, with the trailer empty, the TW should be at least 614 pounds. Fully loaded, it should be at least 743 pounds. We can accomplish this by loading our heavier items toward the front of the travel trailer, but it is something we need to look at when comparing the capabilities of the Expedition to the needs of the trailer.

Your tow vehicle and trailer must work together

Now we can do some basic calculations without ever taking either the vehicle or the RV to a scale. The GVWR of the trailer is 7436, which is a good bit less than the MTC of the Expedition, at 9200. So the trailer weight itself is fine. The GVWR of the Expedition plus the GVWR of the trailer is 14836, below the 15,000 pound GCWR of the Expedition. So even if we load both vehicles to their maximum, we are still within specs. The minimum safe TW for the GVWR of the trailer is 743, below the 920 pound HRW of the Expedition. Again, the numbers work out. The maximum safe TW for the GVWR of the trailer is 1115. That’s higher than the HRW of the Expedition. But that’s okay. The dry weight of the tongue is only 474, so it will be easy to load in such a way that we do not exceed the HRW. If the dry weight of the tongue was higher, it might be tough to keep the TW down once the trailer was loaded.

GVWR (trailer) < MTC (Expedition)
GVWR (Expedition) + GVWR (trailer) < GCWR (Expedition)
GVWR (trailer) x .1 = minimum safe TW (trailer)
GVWR (trailer) x .15 = maximum safe TW (trailer)
Minimum TW (trailer) < HRW (Expedition)

Next we have to go to the scales to weigh the Expedition. We know that the RAWR of the Expedition is 4250—but how much weight is it already carrying? Can we put 743 or more pounds of tongue weight on it without causing damage? Aligning the Expedition on a CAT scale so that each axle is on a separate scale gives us the answer. We’re currently carrying 3240 pounds on that rear axle.

RAWR (Expedition) – Actual Weight on rear axle = remaining CC on rear axle
4250 – 3240 = 1010

We have over 1000 pounds to play with—our max HRW is only 920, so no way is the tongue weight on that trailer going that high! We also learn that the current loaded weight of our Expedition is right at 6400 pounds.

GVWR (Expedition) – Actual Weight (Expedition) = remaining CC of Expedition
7400 – 6400 = 1000

Again, our HRW is 920, lower than the available CC of 1000 pounds. So the additional tongue weight will not exceed the Expedition’s GVWR. The travel trailer appears to be well within our vehicle’s capacities.

Trailer Weights

Our trailer can handle the payload we need without overstressing our tow vehicle

Now we need to look at the trailer itself. Dad and I are full-timers, meaning that we live in the RV year-round. According to experts that we have heard at various seminars, two people who full-time generally carry around 1500 pounds of “stuff.” That’s everything from dishes to electronics, and clothing to furniture. If you do a lot of dry camping, in which you do not have utility hookups, you will also carry water in the fresh water tank. A full load of water adds approximately 250 pounds, depending on the size of your tank! Since we rarely camp without hookups, we only carry around 1/3 of a tank, or about 80 pounds of water.

We were able to eliminate quite a few RVs from our list based on weights alone. Some of the travel trailers that we looked at had dry tongue weights of well over 900 pounds! Since we can only handle a maximum of 920 pounds fully loaded, those RVs did not meet our needs. Likewise, many of the travel trailers had cargo capacities of only a few hundred pounds. There is no way that we could have carried everything needed for full-timing in that little capacity.

The Flagstaff that we finally selected is a bit light on cargo capacity, at just 1293 pounds. Our previous travel trailer could handle 2200 pounds. But this was an area in which we were willing to compromise, since the trailer is otherwise perfect for our needs. We just have to be extremely careful when loading it. But since our Expedition weights are not nearly at capacity, we carry some items that we use less frequently inside the vehicle instead of the RV.

Using the CAT Scales

Weighing the combination, loaded for camping, is the only way to know for sure

Except for the current weights on the Expedition, we were able to find all the numbers we needed without visiting a scale. Once you purchase your travel trailer, however, it is imperative that you make the trip. CAT scales are available at truck stops, where semi-truck drivers use them to ensure that their trucks are within legal weight limits. Open to everyone, these scales are the best place outside of specialized RV weighing facilities (offered at some rallies) to determine the actual weight of your tow combination.

Load up the tow vehicle and RV as you would for camping. If you plan to dry camp, fill your fresh water tank. Fill the propane tanks and your vehicle’s gas tank. Add the clothing, shoes, toys, games, books and other supplies you plan to carry. Don’t forget the firewood, grill, bicycles and anything else that you would take on your ideal camping trip. Hitch up and drive (carefully) to the nearest CAT scale.

CAT scales actually consist of multiple smaller scales that are divided by painted yellow lines. Position your combination so that each axle is on a separate scale. The front axle of your vehicle should be on the furthest forward scale, the rear axle on the next scale and all the trailer axles on the third. For purposes of calculations, all the trailer axles will be considered together as one unit. Push the call button and tell the attendant that you need to weigh your combination. When you get the go-ahead, pull off the scales and park in a safe location, then go inside to get the results. As of 2012, an initial weigh costs $9.50, while reweighs within 24 hours cost $1 each.

Tow the trailer to wherever you are storing it and unhitch. Do not unload anything in your tow vehicle. Immediately return to the CAT scale in your tow vehicle. If it is a long distance, replace whatever gas you used. Now position the vehicle on the CAT scale as you did before, ensuring that each axle is on a separate scale. Push the button and give the attendant the previous weigh number from your receipt. Park safely and go inside to get your new receipt.

With the two receipts in hand, you have all the information you need to determine your actual weights and make adjustments as needed. The bottom number on your first receipt is the Gross Weight of the tow vehicle and the RV. Subtract the Gross Weight on the second receipt (just the tow vehicle) from the Gross Weight of the combination. Next, subtract the Trailer Axle Weight on the first receipt to get the Actual Tongue Weight of the trailer.

Gross Weight (with trailer) – Gross Weight (without trailer) – Trailer Axle Weight = Actual TW

Add the Actual TW of the trailer to the Trailer Axle Weight on the first receipt (combination) to get the total weight of the trailer. Multiply the total weight of the trailer by 0.1 to determine the minimum safe TW, and by 0.15 to determine the maximum safe TW. Compare those numbers to your calculated actual tongue weight.

(Actual TW + Trailer Axle Weight) x 0.1 = minimum safe TW

(Actual TW + Trailer Axle Weight) x 0.15 = maximum safe TW

Confused yet? Here are some examples from our combination, taken at a CAT scale a month ago. We had the dealer transfer our weight distribution sway control hitch system from our old trailer to the new one.

 

Tow Vehicle and Trailer Combination

Steer Axle 2,800
Drive Axle 4,160
Trailer Axle 6,760
Gross Weight 13,720

 

Tow Vehicle Only

Steer Axle 2,900
Drive Axle 3,360
Gross Weight 6,260

 

The Steer Axle is the front axle, and the Drive Axle is the rear axle. The Trailer Axle is the weight carried on the axle of the trailer, but does not include the tongue.

The gross weight of the combination is 13720 (first receipt), while the gross weight of the tow vehicle without the trailer attached is 6260 (second receipt). The trailer axle weight (first receipt) is 6760. So using the formula, we can calculate the actual tongue weight.

Gross Weight (with trailer) – Gross Weight (without trailer) – Trailer Axle Weight = Actual TW
13720 – 6260 – 6760 = 700

The trailer axle weighs 6760 and the tongue weighs 700. That means our total trailer weight is 7460 pounds.

Trailer Axle Weight + Actual Tongue Weight = Total Trailer Weight

6760 + 700 = 7460

Remember that the Tongue Weight should be 10 to 15 percent of the total trailer weight.

(Actual TW + Trailer Axle Weight) x 0.1 = minimum safe TW

(Actual TW + Trailer Axle Weight) x 0.15 = maximum safe TW

That means our TW should be between 746 and 1119. The maximum TW that our Expedition can handle is 920, or roughly twelve percent. But our concern is that our TW is too low. Ten percent of 7460 is 746, so our tongue weight is 46 pounds lighter than it should be. On our next trip, we’ll need to shift some weight forward onto the tongue. Thankfully, our RAWR is 4250, so we can handle another 90 pounds on the rear axle.

RAWR (Expedition) – Actual Weight on rear axle (first receipt) = remaining CC on rear axle
4250 – 4160 = 90

We can easily shift 50 pounds or so forward without damaging the axle. If the tongue weight was too high, we could reverse the process and shift weight backwards onto the trailer axle.

But there’s another problem. The GVWR of the trailer is 7436 and we’re carrying 7460. That means we are carrying 24 pounds too much inside the trailer. We need to move some items into the vehicle, since we’re not exceeding its capacity.

So we could simply move 24 pounds from the trailer into the Expedition and 50 pounds from the back of the trailer to the front of the trailer. We have 90 pounds available on our rear axle, and 50 plus 24 is 74, so we’re still (barely) within specs. But what if we want to buy more groceries? What if we pick up a few souvenirs? Won’t we go overweight again?

There is a second possibility. The tongue weight needs to be at least ten percent of the loaded trailer weight. Therefore, if the loaded trailer weight goes down, so does the necessary tongue weight. What if we move a bunch of weight out of the trailer?

Looking at the numbers again, we can do another equation. By adding together the front axle and rear axle numbers on the receipt that includes the trailer, we can determine how much the tow vehicle weighs including the weight of the trailer tongue.

Steer Axle (first receipt) + Drive Axle (first receipt) = Gross Weight Tow Vehicle with Tongue
2800 + 4160 = 6960

The GVWR of the Expedition is 7400. That means we can still add 440 pounds of cargo to the Expedition, as long as we don’t add it all to the rear axle. Since Dad and I travel alone, we can fold down our second row seats and put cargo there.

If we move 200 pounds of weight from the trailer axle to the Expedition, then the total trailer weight goes down to 7260. Now we only need to shift 26 pounds forward onto the trailer tongue to get to ten percent. If we move 300 pounds, then the trailer weight goes down to 7160. We just have to be careful to balance the load in the Expedition so that more of the weight goes onto the front axle than the rear axle. But how do we balance everything? The weight distributing hitch is a valuable tool that no RVer with a heavy trailer should ever be without.

Related Articles:

RV Shopping: Weight Distribution and Sway Control

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Lisa is a full-time travel writer. She lives in an RV with her disabled father and writes about their experiences. Although she has no children of her own, Lisa loves being an Aunt to her own relatives and the children of all her friends. You can follow her adventures on her blog, Travel Confessions.

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