Engineering News

Chrysler gears up for new convertible roll-out

Auburn Hills, MI--Early next year, Chrysler executives will officially lift the veil from the long-awaited Sebring convertible. And when they do, the sleek and stylish Sebring is sure to wow the crowd.

Oohs and aahs aside, however, the real value of Chrysler's new convertible will not be visible to the assembled crowd. That's because the strong point of the Sebring is that it's a convertible underneath, as well as on top. Unlike just about every American convertible ever made, it is designed to be a convertible. It has no coupe counterpart; wasn't built off another platform; wasn't created in parallel with another vehicle.

Sebring's design methodology serves as a reminder that Chrysler isn't afraid to break the rules set forth by Detroit's clubby engineering fraternity. Up to now, building a convertible has been simple: Slice the roof off a coupe, add a cloth top and some reinforcing steel, and you had a product. More recently, some automakers designed their convertibles in parallel with a coupe.

Chrysler engineers, however, weren't content to build off an existing line, or a parallel one. "This will be one of the very few convertibles on the market that doesn't have a coupe sibling," notes Chrysler spokes-man Jeff Leestma.

The costly decision to build the vehicle from scratch is raising a few eyebrows around the industry. "We can't imagine why Chrysler would do it that way," says one industry executive who asked not to be named. "It's not the most cost-effective way to go."

Some observers, though, say that the new program looks uncannily similar to another Chrysler effort of the early 1990s. "This is the same company that came out with the Viper in 1991," notes Don Schroeder, senior technical editor for Car and Driver magazine. "From a corporate standpoint, the Viper appeared to make no sense whatsoever. Yet, in retrospect, it's probably the best thing Chrysler did in the early 1990s."

Substance over style. Indeed, many Chrysler insiders quietly believe that the Sebring convertible could have an impact similar to that of some of the company's other recent successes: Viper, Neon, Cirrus, Stratus, and the LHs. And the reason is substance, rather than style. Beneath the skins of the Sebring JX and JXi convertible lies an uncharacteristically stiff chassis. For drivers, that translates to better noise, vibration, and harshness characteristics. To a lesser extent, it also means improved vehicle dynamics: acceleration, braking, and handling.

Though it shares the name of the Chrysler Sebring sport coupe, the two have little in common. In truth, the convertible more closely resembles the JA platform--Cirrus and Stratus--than any other Chrysler vehicle. Its powertrains are the same used in those cars: a 2.4l, 16-valve DOHC in-line four-cylinder engine in the JX; and a 2.5l, 24-valve, V-6 in the upscale JXi. It also borrows on the now famous cab-forward design of the LH sedans and the Neon. As a result, the company's engineers created the rarest of convertibles--one with a roomy back seat. "We wanted to produce a car that four adults could enjoy," Leestma says. "Rear-seat passengers won't have to sacrifice comfort."

The car's real innovation, however, lies in its chassis. Though Chrysler hasn't yet revealed the vehicle's weight, the company claims that its engineers have designed a convertible that's stiff without being tank-like. That's a departure from conventional convertibles, which typically compensate for the loss of the roof with a significant addition in weight--usually at the rear end. The 1994 Chrysler Le-Baron convertible, for example, weighed 3,122 lbs, compared to 2,972 for the LeBaron four-door sedan.

Extra weight aside, however, stiffening plays a vital role. That's because convertibles have no B- and C-pillars, which, combined with the roof, resist torsion and bending loads. Behind the modified rear seat, convertibles also have no package shelf area, which resists lateral loads. As a result, an unstiffened convertible chassis would lose so much bending strength that it would actually sag several millimeters.

To stiffen a convertible chassis, engineers typically call for welding of extra structural members to areas at the bottom and rear of the car. Those areas include the transmission tunnel, rocker panels beneath the doors, and in back of the rear seat. Inclusion of structural steel in those areas helps regain some of the torsional and bending strength that's lost.

In the design of the new Sebring convertible, Chrysler engineers took rigidity a step further. As in past convertible programs, they stiffened the rocker panels and tunnel areas. But because the Sebring was designed from the ground up as a convertible, they were also able to modify the front end. That's a departure from past programs, where existing front-end packaging constraints prevented modification. Designing from scratch also enabled them to add structure at the rear quarter panel--another area that would be off-limits in a modified coupe.

By adding structural members to the front, Chrysler engineers balanced the mass of the car, effectively countering the additional steel in the rear. Ultimately, they hope that greater balance will translate to improved performance characteristics, particularly in the vehicle's handling.

Even more important was the use of advanced analytical techniques in the design of the new car. Sebring engineers employed finite element analysis (FEA) as a means of optimizing the balance between weight and rigidity. Through a union of CATIA CAD data and FEA software, they stiffened the vehicle through trial-and-error placement of structural members. Designing the chassis on-screen, they also minimized the weight.

Such procedures may have helped Chrysler engineers design from scratch without piling up exorbitant costs. "With techniques such as finite element analysis, there's been a great deal of improvement in structural efficiency," notes David E. Cole, director of the Office for the Study of Automotive Transportation at the University of Michigan. "Analytical techniques allow the engineer to analyze the design without having to build a vehicle to see if it works. Building a vehicle (in hardware) takes a long time and is very expensive."

LeBaron successor. The design process employed on the Sebring convertible contrasts sharply with the methodology used on the LeBaron more than a decade ago. Experts attribute that difference to dramatic changes in the engineering culture and to Chrysler's financial position. "Chrysler was fighting for its life at the time the LeBaron convertible program was launched," explains Cole. "They weren't able to plan very far ahead."

Still, the LeBaron convertible succeeded beyond all expectations. Though it was really a modified K-car, the LeBaron grew to be the most popular convertible in the U.S., reaching sales of more than 300,000 between 1987 and 1995.

Though no other American automaker has created a separate program for its convertibles, others have built specialty vehicles in parallel programs. Ford, for example, designed its Mustang convertible in parallel with the coupe. Chevrolet did the same with its Camaro and Corvette convertibles. American and Japanese carmakers also design their right-hand-steer vehicles in parallel with left-hand-steer models.

Whether the programs are parallel or separate, designing from scratch offers numerous advantages, says Cole. "You can do it at lower cost, you can be more efficient, and you can have better execution in the overall design," he says. "There is no question that it's the best way to design a vehicle."

--Charles J. Murray, Senior Regional Editor

  Mercedes moves beyond ABS

Arjeplog, Sweden--According to engineers at Mercedes-Benz, incorrect steering is responsible for 50 percent of all automobile accidents; they blame incorrect braking for another 10 percent. That's why the German automaker introduced its Electronic Stability Program (ESP). Available in Europe in the S-600 coupe, ESP provides active, rather than passive, safety.

Developed with Robert Bosch GmbH, the ESP system works during acceleration, coasting, and braking on wet or icy roads. It eliminates the need to countersteer during a skid, and allows braking pressure to be applied to individual wheels. "It's like an invisible hand that pulls you back on the right course," says Hermann Gaus, director of passenger-car development for E- and S-class cars.

The system uses two computers, each with a 48-kbyte capacity, and is programmed for normal driving conditions. Five sets of sensors--for wheel speed, brake pressure, steering angle, lateral acceleration, and yaw--monitor changes in conditions and send the information continuously to the car's computer. When conditions exceed set parameters, the system kicks in, determining what is needed to stabilize the car. ESP can compensate for over-steering or under-steering, adjust braking pressure on the front and rear axles, and signal the engine and transmission electronic systems to make adjustments such as reducing engine torque. The calculation process takes 40 milliseconds.

Of the five sensors, the one for yaw is most critical because it must accurately measure rotation of the car on its vertical axis. Engineers adapted the ESP yaw from systems used in the aerospace industry. Eight oscillators, housed in a small steel cylinder, continuously change position in response to the car's movement. This, in turn, generates the signal that allows the system to calculate rotation speed and determine if a correction is needed.

--Ariane Sains, European Editor, Sweden

Test drive gets mixed reviews

Cold, dry air blows across the 75-cm-thick ice of Lake Hornavanas as I slide behind the wheel of the Mercedes S 600 Coupe and fiddle with the heater. I'm here with a group of journalists to test drive the coupe and sports car equipped with the Electronic Stability Program (ESP). Designed to work on wet and icy roads, Mercedes bills ESP as a generation beyond anti-lock brakes: a steering-braking correction system that can control the car in a skid better than a trained driver.

I am skeptical. As a teenager learning to drive in a suburb of Boston, the routine for getting out of a skid was drummed into me: Turn into it and don't jam on the brake. I don't think the car can do better.

To test the limits of new systems, Mercedes-Benz and several other car companies operate test sites from December to March in the Lappland town of Arjeplog, where the ice is guaranteed to be thick and the lake becomes an open road.

Unlike some of the other drivers, who think this is the next best thing to the Indy 500, I start out cautiously. Getting the feel of the car, I begin to turn the wheel, trying to precipita