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Who taught YOU to drive?

Piling on too much weight is bad for cars, too

By Peter DeMarco
June 6, 2010

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How we squeezed 13 grown men into a minivan, I still don’t know. It was foolhardy to even try, but brainy ideas aren’t exactly the hallmark of bachelor parties, are they?

“They say it seats five comfortably,’’ the best man, Jake, quipped as we climbed in. Arms and legs flailed in every direction. Guys sat on the floor, rode backward, and balanced precariously on each other’s laps. With every bump in the road, groans of pain rang out.

The toll on the poor Kia Sedona was just as bad. With our combined weight of about 2,400 pounds, the undercarriage nearly kissed the ground. The van struggled to reach 50 miles per hour, and on turns it swayed like a rowboat.

We made it to our destination in part because it was just a couple of miles away, and in part because Jake, the designated driver, was exceedingly cautious. But it wasn’t easy, he later told me.

“The added weight gave the effect of being extremely topheavy, so that the slightest turns caused the Kia to lean and lurch far more than you’d expect,’’ he said. “Think of how you walk when you’re trying to balance a heavy object on your head. It was as if I wasn’t driving drunk, but the car was.’’

It was a stupid stunt, and we certainly broke seatbelt laws. How much trouble, though, were we really asking for? How much weight can you pile into the average car before bad things start to happen?

This week, we look at vehicle payload capacities. How do you know how much weight your car can safely carry? How does excess weight affect handling and mileage? Can you actually damage your car by overloading it?

I’ll say it again: We shouldn’t have done this, even if it was fun. Understandably, the people at Kia were not pleased when I explained the situation.

“The Kia Sedona is designed to safely hold seven passengers, not 13 passengers,’’ a company spokesman e-mailed me. “More than seven passengers should not be transported in the Sedona under any circumstances because there are only seven seat positions with corresponding seat belts and airbags. . . Extra weight on a vehicle may affect handling, stability, braking, towing capacity, and rollover resistance.’’

For a more detailed explanation I sought the advice of two industry experts: Subaru’s Dave Sullivan, and Mark Johnson at General Motors.

Johnson leads a team of engineers who try to design lighter-weight GM vehicles, while Sullivan heads the production of Subaru’s Legacy, Outback, and Tribeca lines.

To figure out how much weight your car can carry, just read the driver’s door jamb. (For older vehicles, you might need to dig out the owner’s manual.) The first essential number to search for is payload capacity, which covers all occupants, cargo inside and on the roof, fluids (including gas) in the vehicle, and the “tongue weight’’ of any trailer you pull (usually about 10 percent of the combined weight of the trailer and its cargo).

The second essential number is your tire load rating, which tells you the air pressure you should put in your tires to support the vehicle’s maximum payload.

There are more numbers listed on the door jamb, such as the gross vehicle weight rating, but for practical purposes they are less important.

Maximum payloads are determined by myriad elements, including engine horsepower and torque, vehicle size, and braking ability, and whether the vehicle has two- or four-wheel drive, a truck’s “platform’’ base (stronger) or a “unibody’’ car base, and truck-style leaf springs (again, stronger) or an integrated suspension.

“Every vehicle is different, and every model is different,’’ said Johnson.

So, what happens when you exceed your payload capacity?

For starters, your vehicle won’t have a balanced ride and you’ll lose some of your steering abilities.

“You’ll see people driving down the road with a heavy trailer and a lot of gear in the back of their SUV, and the front end of the car looks like it’s pointing 20 degrees up at the sky,’’ said Sullivan. “Because you’ve overloaded the rear and you’ve underloaded the front, your front tires are not going to have the proper traction, adhesion, or ability to steer.’’

For every 50 to 100 pounds of weight you add to your vehicle beyond your maximum payload, you’ll also experience a decrease in fuel economy.

For smaller cars, you could lose as much as a half-mile per gallon for every excess 100 pounds. (Mileage estimates are calculated for partial payloads, not the maximum payload, so in truth your fuel economy will suffer even before your vehicle is fully loaded, Johnson said.)

Add a good deal of excess weight, and your headaches multiply. Our Kia Sedona’s maximum payload was 1,532 pounds, meaning we exceeded it by more than 60 percent. With that much weight, the engineers said, we could have blown the transmission on a steep hill, cracked the metal on the arms that hold the tires in place, or smacked the undercarriage on a bad pothole, causing structural damage to the entire vehicle.

We could have tipped over on a sharp curve, as our minivan’s overwrought suspension system was rendered nearly useless. Or the structural welds on the vehicle could have failed. Or we could have blown our tires.

“As we found out with the Firestone case with Ford Explorers back in the day, if you’re underinflating or improperly inflating your tires and running them with excessive load in the vehicle . . . they will shred up,’’ Sullivan said.

The horror list went on, including stuff I’d never considered, such as the applied pressure of our bodies landing on our seats after being sent airborne on a bounce. “It’s mass times velocity, so you multiply the effects,’’ Sullivan said.

The one consolation Johnson offered was that our driver, Jake, did the right thing by driving slowly and cautiously. “Just like in driver’s ed, you’re supposed to adjust your driving habits to the conditions, whether it’s the weather or vehicle loading,’’ he said.

Good advice, for sure. But at my next bachelor party, I’m advocating we take cabs.

Peter DeMarco can be reached at demarco@globe.com. He also updates a Facebook page, “WhotaughtYOUtodrive?’’