Crash tests: Not all SUVs built alike

NEW YORK (CNNMoney.com) -- While all offer reasonable protection from front impacts, there are big differences in side impact protection among six truck-based SUVs, according to the Insurance Institute for Highway Safety.

The worst-performing vehicle in the test was the Chevrolet Trailblazer, which earned an "Acceptable" rating for front impact protection, the second-highest of four possible ratings, and a "Marginal" rating for side impact protection, the second-worst rating possible. The TrailBlazer earned a "Poor" rating, the worst possible, for whiplash protection in rear impacts.

The Insurance Institute points out that the Chevrolet Trailblazer (top) and Jeep Grand Cherokee lack body-protecting side airbags. Both got Marginal ratings for side impact protection.

Results for the TrailBlazer by General Motors (Charts, Fortune 500) also apply to the closely related GMC Envoy, Isuzu Ascender and Saab 9-7X SUVs.

In its announcement, the Insurance Institute pointed out that the Trailblazer and Chrysler's Jeep Cherokee lacked body-protecting side airbags although both offer head-protecting side airbags.

The Grand Cherokee earned the top "Good" rating for front impact protection but, like the Trailblazer, got a "Marginal" rating for side impact protection. The Grand Cherokee earned a "Good" rating for rear impact protection.

The Grand Cherokee earned top 5-star scores in all of the federal government's crash tests, however, a Chrysler spokesman noted.

The Insurance Institute is a private organization funded by auto insurers.

"No single test can determine a vehicle's overall safety performance, and Chrysler continues to pursue every opportunity to improve the crash worthiness of its vehicles," Chrysler said in a prepared statement.

General Motors also pointed out that the TrailBlazer had earned 5-star ratings in the federal government's side impact test. (The TrailBlazer earned 3 and 4 stars for protecting the driver and passenger, respectively, in the government's front impact test, however.)

"The performance of some of these models in the side test was surprising," said Institute senior vice president David Zuby. "SUVs should have an advantage in side crashes because the driver and passengers ride higher up than in cars. People often think they're safer in one of these vehicles, but many cars hold up better than some of these midsize SUVs in this test."

The best performing SUV was the Nissan Pathfinder when equipped with optional side airbags. It earned the top rating of "Good" for both front and side impact protection and a "Marginal" rating for whiplash protection.

Without side airbags, side impact protection was rated "Marginal" in the Pathfinder.

The Nissan XTerra, when tested with and without side airbags, performed as well as the Pathfinder except that it was rated "Poor" for whiplash protection.

The Toyota (Charts) 4Runner, likewise, earned "Good" ratings for front and side impact protection but got a "Poor" rating for whiplash protection.

The Ford (Charts, Fortune 500) Explorer earned a "Good" score for front impact protection, but an "Acceptable" rating for side impact protection and a "Poor" rating for whiplash protection. The Explorer's score also applies the closely related Mercury Mountaineer.

Truck-based SUVs like these are different from the car-based crossover SUVs which are now more popular among buyers. Examples of car-based SUVs would be the Toyota Highlander, Ford Escape or GMC Acadia.

Truck-based SUVs have a weight-supporting chassis or frame that is separate from the body. That's the best design for off-roading or heavy towing. In car-based crossover SUVs, weight supporting structures are incorporated into the vehicle's body, which allows the vehicle to be made lighter for better fuel economy and handling.

The Insurance Institute's front impact test involves an offset impact at 40 miles per hour. The vehicle strikes the barrier with just one side of its front end, so impact forces are concentrated in a relatively small area.

For side impact safety, vehicles are tested using a moving barrier that strikes the vehicle at a speed of 31 mph. The barrier is raised off the ground to mimic an impact from an SUV or pick-up truck.

Because of the height of the barrier, vehicles without head-protecting side airbags do not perform well in the Institute's side impact test. Even some vehicles with those types of airbags still do not do well.

To measure rear impact whiplash protection, the Institute measures the height and shape of the headrest to see if it would, theoretically, protect an average sized man. If the headrest is judged to be "Good" or "Acceptable" according to that test, it is then further tested in a 20 mph rear impact using a crash test dummy.

Vehicles with seats that are not judged "Good" or "Acceptable" based on measurement alone are given a rating of "Poor."