During most of the decades that people have towed Airstreams, the load capacity of the tow vehicle has not been a concern. In the 1960s and 1970s particularly, owners towed Airstreams millions of miles with sedans from that era. Those cars had huge trunks and they were often filled to the brim since the trailers had virtually no exterior storage.
The result was vehicles that were likely over their rated load capacity by quite a bit, but it’s hard to point to widespread problems that resulted from this. Few seemed to worry about it, perhaps thinking that 150 to 200 pounds of extra load per tire would never be enough to break anything.
Would you ever consider carrying more than the rated weight capacity on a vehicle? You might quickly think, “Never!” but it’s likely you have quite regularly. Solo SUVs, vans or cars loaded for family vacation are often well over their weight rating, as are millions of commercial pick-ups and vans. Most people never realize this because they don’t weigh their cargo (and passengers) before every trip, but fortunately these excesses don’t usually cause problems. My point is only that load capacity is not something to totally ignore or exceed by a wild margin, but I wouldn’t exaggerate its importance.
There are two acronyms related to a vehicles load capacity:
GVWR means “Gross Vehicle Weight Rating.” In theory, this is the maximum weight a vehicle can be when fully loaded.
GAWR stands for “Gross Axle Weight Rating.” In theory this is the maximum load an axle can carry. Sometimes you will see RGAWR or FGAWR for “Rear Gross Axle Weight Rating” or “Front Gross Axle Weight Rating.” Axle ratings may be limited by tire capacity, the axle capability, or because the automaker is concerned about vehicle dynamics when a certain weight is exceeded.
If you have been reading this column for a while you will be aware that I have some issues with the Society of Automotive Engineers (SAE) standard for hitches as well as some of the methods and motivations used to develop tow ratings. Because they have very little time to pay attention to it, it seems few auto engineers really have any in-depth understanding of trailer towing. What little testing they do perform is with improperly connected weight distribution on receivers that are too weak to perform properly, largely on a test track and without much real-world experience.
Often a lighter vehicle with independent suspension (like this Porsche Cayenne) can tow better overall than a heavier truck, even when it’s close to the manufacturer’s maximum weight rating.
However, I will say I think most of the engineering in modern vehicles is nothing short of amazing in areas where engineers are allowed to devote a lot of attention and the marketing or legal department does not get to trump their decisions. Most vehicles are over-engineered, especially in regard to the chassis and running gear.
The inconsistencies of manufacturers weight ratings begin to show when you look at them closely. For example, on many vehicles the front and rear axle ratings added together total more than the GVWR. Currently you can purchase a ¾ ton 4x4 diesel crew cab with a dry weight of 7,429 pounds. The front axle rating is 5,500 pounds and the rear is 6,010. This means that if you could distribute the weight perfectly the truck should be able to carry 11,520 pounds – but the GVWR is only 9,600.
So according to the GVWR, that big heavy-duty truck can carry just 2,170 pounds. Why is it de-rated so much? We can theorize that when the truck is driven solo with more than 2,170 pounds of load it becomes more unstable than the manufacturer is comfortable with. But this theory doesn’t hold up when we look at the shorter wheelbase regular cab version. It should tend to be less stable, yet it has more payload (2,500 pounds). Conversely the same truck with two-wheel drive has even less payload, despite this version being lower to the ground and having independent front suspension instead of a live axle (which means it handles considerably better). It seems that many of these ratings are generated by methods that aren’t solely engineering-based. I believe there’s some “It’s how we have always done it” involved.
Often, we will see two vehicles with identical suspension components, tires, transmission, etc., but with different axle weight (GAWR) ratings. For example, this might be seen in short and long versions of the same SUV. This is helpful, because when we are setting up a very high hitch weight trailer, we will often use the short version of an SUV that we know had a higher rated capacity in its longer version. The shorter version allows us to transfer the hitch weight more easily, while knowing that we are not overloading any suspension components. Again, I don’t want to imply that you can completely ignore weight ratings. I would not want to see an axle 1,000 pounds over its rating, and you never want tires over their rating.
Sometimes we will make changes to the Airstream itself to reduce tongue weight. When Airstream was making units with slide-outs, which had very high tongue weights, we ordered them without the front battery compartments and would move the batteries to a location under the bed. Since a double-axle Airstream does not really need a spare (you can tow on three wheels if necessary) we also eliminated the spare tire and carrier. These are pretty simple changes to make if needed.
Let’s look at some real-world weights and ratings. One of my favourite tow vehicles is the Chrysler 300. The towing weights listed here are my actual 300 connected to an Airstream 34-foot Classic loaded for vacation with four people in the car. In the truck were two sets of golf clubs and a few miscellaneous items. For comparison, I also weighed the car solo (without the trailer), when driving to a meeting with four people in it and suitcases in the trunk.
When towing with this car we are 310 pounds over the GVWR and 70 pounds over the rear axle rating, and well below the tire ratings. But note that the rear axle is 100 pounds more overloaded on the solo trip (170 pounds versus 70), and also note that the car is considerably less balanced than it is when towing. This underscores how easy and commonplace it is to overload vehicles with just ordinary passengers and cargo, and how passenger/cargo loading tends to put more weight on the rear axle than a trailer with proper weight distribution.
Let’s say that the 70 pounds excess weight on the rear axle really concerns me, so I go instead to a truck-based SUV. The Ford Expedition is the one I would recommend. It has a very robust 4-wheel independent suspension and with the right tires, in my opinion, it is the best handling truck-based tow vehicle.
Here is the same table with a customer’s Ford Expedition and an Airstream 30-foot International loaded for travel with a family of four and about 300 pounds of stuff in the rear cargo area.
Certainly the Expedition had considerably more interior space than the Chrysler 300 and is a better off-road vehicle, but if weight ratings or safety are your primary reason for buying an Expedition instead of a Chrysler 300, then not much has been accomplished. The Expedition is still 249 pounds over the GVWR and 30 pounds over the rear axle rating.
Also, with the Expedition you will be driving a vehicle all year long that weighs 1,400 pounds more, has a much higher centre of gravity, and yet still has the same size brakes. Although very stable for a large SUV, the Expedition still does not come close to the Chrysler 300 for stopping distance or accident-avoidance agility, and it has a much higher chance of rolling over.
On either the Ford Expedition of the Chrysler 300 it would be possible to bring the axle weights under the rating with a little adjustment of loading and torsion bars. However, let’s say that I am still concerned about being near the weight rating, and so I decide to get a ¾ ton truck.
The next chart is for a Dodge 2500 4x4 crew cab with the same trailer as above, and with similar loading and the addition of a fiberglass box cover and a bed liner. Due to the length of the truck, it is harder to transfer weight to the front wheels. On the other hand, Cummins diesel engine ensures there will still be plenty of weight on the front wheels.