Volvo C70: Safety from the top down

You only have to spend a few minutes with Volvo's safety engineers to appreciate why the automaker waited more than half a century before building only the third production convertible in the company's history--the C70.

"The strength and energy absorption capability of a car's roof are critical to maintaining the integrity of the passenger compartment. From the beginning we said if Volvo is to make an open car, it must be safe," says Stephan Ryrberg, who heads up Volvo's Safety Centre in Gothenberg, Sweden. A mechanical engineer, he has been actively involved in developing automobile safety systems at Volvo for more than a decade.

To compensate for the lack of a roof, Volvo engineers redesigned the entire body structure, reinforcing the doors, door sills, and the lower B-pillars. The car's unique rollover protection system features a pair of roll bars concealed behind the rear seats that automatically deploy within 0.2 sec, sturdy A-pillars that double as roll bars, and crash-sensor-activated pyrotechnical pretensioners on the seat belts.

Body makeover. The absence of a roof over one's head is of obvious concern in a rollover situation, which we'll get to later. But what many people may not realize is that a car roof plays an essential role in helping to distribute the energy in accidents of all kinds. "Collision energy is transmitted mainly along load paths consisting of the upper and lower side members and sideframe, as well as the subframe and the roof," Ryrberg explains.

In a hard-top car, the forces in a frontal collision travel along the upper side members to the A-pillars, which distribute them to the roof and also towards the rear--through the side doors to the B-pillars. In a rear-end collision, the load path is mainly through the C-pillars to the roof and roof rails.

Obviously, the elimination of any of these paths means that the energy must go somewhere else. It all makes sense when you consider that an automobile's body structure is basically a truss. This strong, rigid framework of beams is designed to withstand both low and high energy collisions. But take away even one component, and the entire structure is compromised.

To compensate for the missing elements in the C70, Volvo engineers reinforced the doors, door sills, and the lower B-pillars. By using boron steel, engineers were able to get stronger sections without adding to the weight of the pillars or increasing their size. To make up for the loss of the load-dissipating ability of the B-post, engineers also reinforced the horseshoe-shaped rear body structure.

So effective are these design changes, claims Volvo, that the C70 is comparable to the hard-top Volvo S70 in front, side, and rear collisions. But what about rollover situations?

Restraining orders. With no roof overhead, figuring out how to protect occupants in a rollover situation was no small task. Ryrberg says that Volvo engineers had two primary design goals: keep all occupants in the car--and keep them safe.

Key to the car's rollover protection system is a pair of spring-loaded, steel rollover bars (actually, they're hoop-shaped), concealed behind the rear-seat head restraints. These automatically deploy at lightening speed (0.2 ms), when sensors detect high acceleration forces and an unacceptable vehicle pitch.

Detecting the latter is not as easy as it sounds, because a car can overturn in virtually any direction and rollovers are often preceded by free-flight. Engineers finally settled on a combination of three sensing elements. Two "spirit level" sensors--essentially glass tubes filled with liquid and containing a small air bubble--are mounted at different angles and different locations. The vehicle's attitude can be determined by measuring the amount of light passing through as the bubble moves from one side to the other. A third sensor element, consisting of a simple, spring-suspended mass, detects vehicle position in free-flight situations involving slow rotations.

Engineers also reinforced the A-pillars so they could double as rollover bars at the front of the car. They did this by increasing their overall cross-section and adding tubular inserts that are extremely strong in axial compression. So robust are the pillars now, says Volvo engineers, that they exceed the U.S. Federal Motor Vehicle crush resistance standard.

In order to further restrain the occupants in a rollover situation, engineers also equipped each three-point seatbelt with a pretensioner, located next to the belt reel. Activated by pyrotechnics (a powder charge, in this case) when rapid deceleration is detected, the pretensioner automatically takes up the slack in the belt. Peak loads to the body are reduced by tying the occupant, in effect, to the deceleration of the vehicle.

"In many cases, it is the strength of the structure that protects the occupants in a rollover," Ryrberg concludes. "With the C70, it is the strength of our entire design."

C70 shines on rainy days

By Karen Auguston Field

Riding around in a convertible in 35C weather is about as fun as it looks. By swaddling myself in enough bulky clothes, turning the heater (and heated seats) on full blast, and keeping the windows rolled up, I was able to tolerate what in my mind were Arctic-like conditions. It was unfortunate that the C70's very un-Volvo-like looks are so attention-grabbing. For the entire drive, I was uncomfortably aware of how foolish I looked, miserably huddled behind the wheel of such a great-looking car.

Wintry temperatures weren't what Volvo had in mind when it invited the press to test drive the C70 in Phoenix . Nonetheless, I did get a chance to drive it in the kind of conditions in which a car's performance really matters to me: rain, fog, and yes, even snow, in the hills outside of town. I'm a big fan of front-wheel drive, and with the added help of the low-speed traction control system, the car handled beautifully, even on wet, slippery roads.

Extreme variations in the weather gave us several reasons to raise and lower the convertible top--a couple of times we did it just for the heck of it, because it's fun to watch. An electromechanical actuator (including five separate motors) makes for a quick open/close cycle. During one hasty stop at the side of the road we even timed it: 10 seconds later, we had a roof over our heads and were raring to go.

Should the electronics fail, the roof can be raised manually with a hand-crank. I'm skeptical of Volvo's claim that the procedure takes no more than five minutes, but maybe they are talking about people who are more mechanically inclined than me.

Even at high speeds, the C70 is surprisingly quiet for a convertible, particularly with the detachable windscreen in place. All the better to enjoy the stereo system (400W-amp, 10 speakers, in-dash CD changer), which is awesome for a standard package.

Although Volvo has completely sold me on the looks, feel, and safety features of the C70, in the end I have to admit I'd rather have the sedan version--it has a permanent steel structure overhead to protect my cranium. Call me boring. Or maybe I'm just worried about figuring out how to use that hand-crank if the top ever gets stuck in a down position.