You've probably seen this before: An ill-equipped tow vehicle, listing upward or to the side, trailer swaying dangerously back and forth; and to complete the picture, a blissfully ignorant driver behind the wheel.

On big holiday weekends like the one coming up, when the highways are filled with packed vehicles and families eager to get away, it's awfully easy to forget about packing the right things and properly loading your own vehicle, let alone making sure that boat, set of jet-skiis, or pop-up camper you're towing along is safe.

With the trend of downsizing, as families trade their truck-based SUVs like the Ford Explorer or Chevrolet Tahoe for more carlike and often smaller models such as the new 2010 Chevrolet Equinox or Honda CR-V—or even back into smaller sedans—the number of vehicles towing smaller trailers out on the highways this year might actually increase. Renting a trailer once or twice a year, after all—for getting that raft to the river, or your kid's belongings to college—might be well worth the year-round fuel savings of a smaller, passenger-oriented vehicle.

It shows in the figures. While the RV industry is in the second year of a pronounced downturn, RV rentals are up 12 percent in 2009, according to the Recreation Vehicle Rental Association, with camping trailers a large portion of that increase. U-Haul also has reported an increase for in-town rentals (referring to trailers that aren't being used for moving) this year.

Another sign that people will be out in full force with travel trailers: Reservations at RV-oriented parks and campgrounds are up three to five percent versus the same time last year, according to the National Association of RV Parks and Campgrounds.

"The lifestyle is definitely still alive and well," said Courtney Robey, public relations manager for the Recreational Vehicle Industry Association (RVIA). People are typically staying closer to home, she says, but smaller, towable RVs are proving popular. "Families can save up to 61 percent by using an RV for vacation," said Robey, "and that's including the RV’s ownership costs."

That likely leaves a lot of motorists wondering if they have enough vehicle for their trailer, enough trailer for their needs, and all the right hardware to make it safe.

Time to crunch some numbers

The answer is probably yes, but there are a lot of things to make sure of first. Seeing how the numbers add up is crucial, and the best place to start with that is your glovebox. Pull out the owner's manual and look for weight ratings, or a section on towing. If you've misplaced that, look at the inside of the driver's doorsill, or you'll find most late-model vehicle manuals on automakers' Web sites. First note the maximum gross trailer weight, then note your vehicle's maximum gross vehicle weight rating (or GVWR). Next, to get an idea of how much weight your vehicle can carry safely, in terms of occupants and cargo, subtract the curb weight from the gross weight. Newer vehicles will more clearly state in the doorsill the maximum combined weight of cargo and passengers.

And even if your trailer weight is well within the tow limit of your vehicle, don't forget to add the tongue weight (the downward force on the hitch), which is usually roughly ten percent of the total weight of the trailer if you've it attached correctly. As a double-check, you might also find a gross combined weight rating (GCWR) figure, which is the maximum allowed total weight of both the vehicle and trailer; keep in mind that this figure might be for braked trailers.

There's no general rule of thumb for the towing capacity for a type of vehicle. Some mid-size sedans—like the Ford Fusion, for instance—aren't approved for any towing, while some car-based crossover vehicles are approved for 3,500 pounds or more. If you're in doubt about the weight or the trailer, or the weight of your loaded vehicle, you'll find drive-on scales at trailer rental outlets or vehicle inspection stations. For unbraked trailers, the general rule is that the trailer shouldn't weigh any more than the empty weight of your tow vehicle. With braked trailers, you can exceed that if you follow those other cautions, but you should seek expert help in how to install and drive with them.

The important point to take away, according to Valaurie Hopper, marketing communications manager for Cequent, a major supplier of towing hardware, is to get the numbers right, and "never exceed that capacity, no matter what."

There are advantages to getting your tow hitch through the new-car dealership; usually it's part of a package that includes substantial upgrades to the cooling system and suspension. But if you didn't, Hopper recommends purchasing your hitch, hitch ball, mounts, coupler, and clamp all through the same shop—or at least assessed by an expert who can go over each piece—so that you can be sure they're the proper size and that it all meets your designated weight rating. "Whichever has the lowest rating between the hitch, ball mount, and other parts," says Hopper, "that’s your actual weight rating."

Only as strong as the weakest link

Your towing setup is only as strong as its weakest link, said John Nielsen, director of AAA's Auto Repair and Buying Network of AAA. "You can’t use a Class 2 hitch on a Class 3 receiver," said Nielsen. Today, Class 1 hardware is good up to 2,000 pounds, Class 2 up to 3,500 pounds, and Class 3 up to 5,000 pounds; Class 3 hardware uses a larger two-inch hitch ball.

The owner’s manual usually also includes some guidance on proper loading—a final important consideration. Just like you shouldn't place all of your heaviest cargo items at the back of your vehicle, you shouldn't load the heaviest items at either end of the trailer. Ideally, the heaviest things in the trailer you're towing should be just in front of the axle and about 60 percent of the trailer's weight should be ahead of the axle. 

If too much weight is at the back of the trailer it can produce a negative load at the tongue, effectively lifting up at the hitch and creating dangerous "sway" conditions. At too-fast highway speeds, sway can very quickly lead to a sideways whip of the trailer that can pull the tow vehicle out of control. Some new vehicles come with trailer sway control, an enhanced version of electronic stability control that can help avert sway before it gets out of control.

How fast is too fast? Some experts advise not exceeding 25 percent lower than the speed limit, and 55 mph is considered the upper limit for many small-wheel trailers. However Bret Dick, a chassis engineer at General Motors, says that with a new or well-maintained late-model vehicle—even a smaller crossover vehicle like the 2010 Chevrolet Equinox—and a properly loaded trailer, you’ll be able to do the speed limit. "If you have a newer vehicle and it’s been maintained, you’re generally in good shape," he said.

"But if in doubt, take it slow," said Dick. "Slower is better here."

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