NAIAS Highlights: Production
T-bird convertible posed stiffness challenge
The evolution of the Ford Thunder-bird is now complete. From discontinued 1950s classic to 1999 concept, the car goes into production this summer as a 2002 model. The two-seat, rear-wheel-drive, V8-powered roadster comes standard as a convertible and offers an optional removable top with classic porthole windows.
A rigid, computer-engineered chassis and a fine-tuned four-wheel independent suspension system employing lightweight materials reduce unsprung weight and improve response. The rack-and-pinion steering gear is of variable-assist design, providing lower turning efforts at parking speeds and higher-level road feel at highway speeds. Four-wheel antilock disc brakes (ABS) are standard and employ electronic brake force distribution (EBD). Brake rotors are vented for optimal performance. Weight distribution is nearly an ideal 50/50 front to back, limiting potential for understeer and oversteer. An all-speed traction control system is available. With all of these features, the Thunderbird has what Ford considers "world-class" driving dynamics.
The Thunderbird is the first Ford Motor Co. North American car program born of the system called C3P - computer-aided design, computer-aided engineering, computer-aided manufacturing, and product information management. C3P, according to Ford, improves quality by providing more accurate, timely computer images of car and components before manufacturing. With C3P the number of prototype vehicles needed for testing is reduced.
A high degree of body stiffness is the basis for good ride and driving dynamics by providing a solid base for the suspension. Loss of stiffness means a less responsive suspension and more unwanted vibrations. Achieving the desired high levels of stiffness in convertibles presents a challenge, because they lack the stiffening element that a hard roof provides. The Thunderbird's stiffness had to be built from below.
Computer modeling through C3P identified potential areas that would benefit from added structural braces. One of the chassis engineering team's first moves was to incorporate a cross-car beam just behind the seats that integrates into the structure. Next they added a series of three steel bolted-on X-braces, hardware not found in a typical sedan with a fixed roof structure. One X-brace is below the engine compartment, where space is at a premium with the powertrain package, oil pan, oil filter, and steering apparatus. By integrating the X-brace into the crossmember for rack-and-pinion steering, the team preserved optimum ground clearance. The team also created a special midcar X-brace, expanding it as far forward and backward as possible while taking special care to route the exhaust systems over it.
The rear brace, a tube of 76-mm (3-in) diameter, is key to the car's stiffness. Bolt-on braces typically are added in the body shop, but Ford says the Thunderbird's very stiff rear brace is so critical to the body structure that it has to go on early in the assembly process to ensure precision fits along the body. Engineers and manufacturing specialists worked together to design the brace around such components as the hydraulic pump for the convertible top, a sling that houses the soft top when it's down, and a number of electrical modules. Instead of scattering them, they mounted the modules as a group right on the brace.
Other smaller, tubular braces were added across the rear-wheel kick-up area and the steering-column bracket for stiffness. The steering-column brace also enhances steering feel. Inside reinforcements were designed into the rocker section.
The fully independent suspension uses short-long arm designs (SLA) in front and back, which contribute to excellent steering geometry. The control arms and uprights are made of aircraft-grade forged or cast aluminum to reduce unsprung weight for improved responsiveness and help maintain full tire contact over rough, broken roads. To achieve the desired ride comfort and keep handling crisp, engineers chose lower spring rates than the average sport sedan, then put in larger-diameter shock absorbers. The Thunderbird's low-speed-tuning shocks help keep the car precise over small bumps in the road and maintain a relaxed ride.
For steering precision, stabilizer bars have molded-in flat sections on the bar and bushing that fit together like jigsaw puzzle pieces. The closer-fitting sections result in quicker response to steering adjustments.
The Thunderbird features power-assisted braking. Aluminum was chosen for the dual-piston front calipers to reduce weight. The all-speed traction control system involves a powertrain control module that senses wheel slip by means of the ABS system's wheel-speed sensors. When wheel slippage is detected, the system automatically responds through one or more of the following measures: applying brake to one or both drive wheels, retarding ignition timing, and reducing the fuel-injection flow rate. Brake intervention is used primarily at low speeds. At higher speeds, torque reduction is used.
The all-aluminum, 3.9-L, DOHC V8 produces 188 kW (252 hp) at 6100 rpm and 362 N•m (267 lb•ft) at 4300 rpm and is governed by a new-generation powertrain electronic controller. Designed for the Lincoln LS, the engine incorporates a lightweight driveshaft made from a high-strength steel tube with forged end flanges, using less material than equally functional conventional shafts. Other features include coil-on-plug ignition with platinum-tipped spark plugs and lightweight, low-friction pistons as well as dual exhaust. The engine meets federal low-emission vehicle standards.
Engineers drove a 1955 Thunderbird to listen to its distinct tone and then set sound targets for the 2002 model. They adjusted pipe, muffler, and resonator size and baffling to achieve the right note.
Mated to the engine is a five-speed, close-ratio electronically controlled transmission that features a one-piece, die-cast aluminum case for reduced weight and reduced powertrain bending characteristics. A quiet gear set is used in the transmission to minimize first- and second-gear whine, and a 3.58:1 final drive ratio gives good performance feel.
The Thunderbird noise, vibration, and harshness team developed a new cam drive system for the timing chain. The driveline was balanced to eliminate unwanted engine moan. Engineers also used laser holography to map the floor-pan vibrations that result from driving. As a result, mastic was applied only where needed to reduce excess noise, saving cost and weight. A computer-aided-engineering technique called statistical energy analysis was employed to understand convertible acoustics and build a balanced package. This allowed the team to develop a strategy for sound treatment even before hardware prototypes were available.
Top-down turbulence on the Thunderbird is minimized with the steep angle of the windshield.
With airflow visualization, engineers were able to ensure the wind crossing over the windshield doesn't create unwanted cabin turbulence that would interfere with conversation. The Thunderbird antenna is embedded in the windshield to reduce wind noise.
Additional isolation from wind noise results from the doors' drop-glass feature. When the Thunderbird door is opened, the side glass lowers just slightly. When the door is closed, the glass goes back up to fit snugly into the seal system. At high speeds in many other convertibles, the glass can pull away from the seals and allow noise to seep in. But the sealing outside the Thunderbird's glass reduces this risk and adds a barrier against water leakage.
The wraparound glass on the windshield of the '50s Thunderbird was handmade. But today's added safety requirements made developing the windshield for the 2002 Thunderbird a challenge. The depth of angle, overall size, and amount of curvature in the glass were established using a new proprietary technology capable of pressing and sag forming glass to get very complex shapes.
The Thunderbird is the first Ford convertible to come equipped with combination head and chest side airbags. High-and mild-strength steel were used strategically to maximize energy absorption properties in crashes, and the driveshaft was designed in two parts that "telescope" for the same purpose.
Specifications for the Thunderbird include a 2724-mm (107.2-in) wheelbase; front track width of 1537 mm (60.5 in); rear width of 1529 mm (60.2 in); and overall mass of 1699 kg (3775 lb), which increases 39 kg (88 lb) with the removable top.
Ford also showed the Thunderbird Sports Roadster concept, which features a sloped rear deck with molded fiberglass tonneau cover and integrated headrests.
- Patrick Ponticel
