By Jim Henry

Producing safer convertibles is no easy task. The challenge is to combine eye-appealing roofless looks with passenger protection. And it is certainly not a new problem. For as long as convertibles have existed, the safety of the occupants, particularly in a rollover, has been a concern.

While convertibles comprise only a small part of the automobile market, their enthusiastic buyers are pushing manufacturers to come up with new designs all the time. To meet this demand, manufacturers are taking advantage of new and stronger materials, innovative designs and a number of auto safety features found in hardtop cars to deliver the next generation of safer convertibles.

In hardtop cars, the roof plays an important part in dissipating the force from a collision while also keeping the vehicle's body from flexing or twisting.

Naturally, the lack of a roof in a convertible gives rise to some safety concerns. To address this problem, automakers are using a number of active and passive security features.

Passive solutions include:

• A super-strong "A-pillar" that surrounds the windshield and functions as a roll bar. This protects the heads of front-seat or two-seat occupants. The materials used in today's convertibles draw on advances in engineering to produce improved steel alloys for lighter and stronger pillars.
• A reinforced body — especially the floor. For cars that are offered as a convertible or a hardtop, the convertible version usually weighs considerably more because of that reinforcement. A stiffer body is not only safer, it makes the car handle better, and it reduces squeaks and rattles.
• Hardtop safety features including front and side air bags, crumple zones that absorb the energy of a collision and reinforced doors to protect occupants in side impacts. And many luxury models have seat belt "pre-tensioners" that tug on the belts to hold occupants in place in a collision.
• A fixed roll bar towards the rear of the car. This protects the heads of front- and rear-seat occupants in a rollover.

 Active safety features include:

• A pop-up roll bar toward the rear of the car. Again, this provides protection for front- and back-seat occupants.
• Collision avoidance features including antilock brakes, traction control, stability control and adaptive headlights that redirect the beams as the steering wheel turns. Some of these are optional, but many are becoming increasingly common.

Crash Test Ratings

Convertibles are subject to many of the same government tests as hardtops, including front and side crash testing and a test to measure the likelihood of a rollover.

Based on those tests, the National Highway Traffic Safety Administration (NHTSA) last fall awarded the 2008 Ford Mustang Convertible five-star ratings in all five categories including front-impact driver safety, front-impact passenger safety, side-impact driver safety, side-impact rear passenger safety and low likelihood of rollover. It's the first convertible to earn five stars across the board, according to Ford.

The five-star NHTSA endorsement doesn't necessarily mean the Ford Mustang Convertible is the safest convertible on the U.S. market; it's just the highest-rated model NHTSA has tested. NHTSA doesn't crash-test test all cars. Crash-testing is mostly self-administered by the auto companies, after which they certify to NHTSA that their vehicles met the standards.

In fact, convertibles are specifically exempted from the NHTSA roof-crush test that's applied to other passenger cars. A tougher roof-crush test has been under discussion since 2005, but even the stricter version would exempt convertibles. And there's no requirement for a pop-up roll bar, either. The Ford Mustang Convertible, for instance, doesn't have one.

Surprisingly, such rollover-related safety measures are not mandated by U.S. government regulations. However, automakers have come up with many of the key safety measures for convertibles on their own. Mercedes-Benz, for instance, performs a "drop test," where it drops cars upside-down at an angle, to measure how strong the A-pillars are.

Incorporating Safety Technology

The two-seat Mercedes-Benz SL roadster had one of the first pop-up roll bars, starting in 1989. The original system used a sensor that was essentially a weighted pendulum. If the sensor detected that the car was far enough askew from horizontal to possibly roll over, it triggered a roll bar that popped up in about three-tenths of a second. The system worked with the fabric top up or down.

The safety features in a convertible have to deploy rapidly to be effective. Air bags and some seat belt pre-tensioners use so-called pyrotechnics that ignite a substance similar to gunpowder and use the force from the expanding gases to deploy the safety feature as quickly as possible.

In contrast, the pop-up roll bar is spring-loaded with a heavy spring. The spring is compressed when the roll bar is in the down position. A ratcheting device keeps it deployed once it's in the up position. Today's SL and several competitors use essentially the same concept, only with more sophisticated electronic sensors.

Pop-up roll bars are not exclusive to the most expensive luxury cars, either. The 2009 Mercedes-Benz SL550 starts at $96,775 suggested retail, but less-expensive cars also feature pop-up protection, including the Volkswagen Eos, starting at $29,555. It has spring-loaded beams that pop up behind the rear head restraints.

In addition, a growing number of convertibles have folding hard-tops, including the current SL, the latest BMW 3-Series Convertible and many others. These provide some additional rollover protection when up, but still not as much as a fixed roof.

These varied efforts by the manufacturers are helping them deliver a new generation of convertibles that look good while addressing the long-term safety concerns associated with this type of car.

Bio: Jim Henry is a veteran freelance automotive writer. He was a one-man bureau in New York City for Detroit-based Automotive News for many years. He is also a past president and a board member of the International Motor Press Association.