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Beyond The 25-Hertz Body
The 25 Hz (Hertz or cycles per second) body was a measure of dynamic body structural stiffness. This measurement identified the lowest natural (resonant) frequency of the full body structure. If the body was vibrated at its lowest natural frequency, it would resonate, or vibrate in harmony, with it.
In general, the higher this minimum frequency, the stiffer the body structure, and the more solid and vibration-free the ride. Increased stiffness also improved handling while reducing buzzes, rattles, and other noises.
This measurement was used as a starting point for targeting body structure characteristics. One problem was the passenger feels and hears inputs from vibration and noise sources that range from 1 to 20,000 Hz. Fortunately, the most dominant inputs were in the first 500 Hz. However, there were still over 100 natural (resonant) frequencies in that range.
While 25 Hz was accepted as a good target, its acceptance was primarily based on simplicity, and until recent years, limited computer capacity to answer the question, "What does the driver perceive?"
We now know that there is little correlation between the lowest natural frequency of the full body structure and good ride (vibration-free perceptions). While increasing the frequency by stiffening the structure may result in a better ride, with lower noise and vibration, another vehicle of a different basic design may have a lower frequency and be better in these attributes. In fact, the first mode frequencies of many of today's vehicles are lower than 25 Hz, yet they are subjectively as good or better than the "25 Hz" vehicles.
The other significant issue with the 25 Hz goal is that in order to meet it, other desirable features such as weight, cost, and interior room may have to be compromised.
Chrysler engineers expanded their objectives to address these issues. They still had targets for the lowest natural frequency, but these were given less emphasis than what the driver feels and hears. Simply put, a larger force at the driver's seat or steering wheel, or a higher sound pressure, causes a poorer subjective impression.
These amplitudes can be predicted using Forced Response Dynamic Analysis. Chrysler now uses this analysis as part of its design process. The analysis simulates road inputs and vehicle responses. For ride and vibration, Chrysler measures amplitude at the seat track or steering column — where subjective-to-objective correlation has been developed for optimization.
Similarly, during structure design, Chrysler set impedance goals for all of the suspension and power train attachments to the body structure. (Impedance is a measure of the structure's ability to oppose energy flow-potentially audible vibration-from the source to the passenger compartment.) Together with acoustic transfer functions and other tactile response goals, there were about 100 structural performance objectives for each vehicle.
Beyond housing passengers and supporting the vehicle's functional systems, the body shell as a structure supported the suspension, attenuated noise, controled shakes, and managed energy a crash. Only recently had it become possible to address all of these issues through computer simulation during vehicle design. Structural analysis required CATIA's mathematical data base and supercomputers capable of handling the enormous amounts of data used in structural Finite Element Analysis (FEA). DMA (Digital Model Assembly) facilitated computer simulation by accurately locating all attachment points.
The number of detail features fine-tuning the Intrepid/Concorde's performance was a direct result of the iterative process of analysis and optimization made possible by computer simulation.
Structural stiffness added a solid feeling to the ride and helped make handling precise by allowing the suspension to work without deflecting the body. In the Concorde and Intrepid Unibody shells, torsional stiffness was 37 percent greater and bending stiffness was 46 percent greater than the prior body, which was best-in-class when introduced.
Noise attenuation through localized stiffening had been the major focus for structural analysis and development on the new Concorde and Intrepid. The iterative refinement and analysis process tuned the stiffness of structural paths from external noise sources, such as the engine and suspension system, to the passenger compartment, preventing the noise from being audible. Structural refinement was especially important because a large portion of structure-borne noise occured at frequencies below 400 Hertz (cycles per second), which were hard to filter. It also reduced the need for added pads to control higher-frequency noise.
A new technique, P/A analysis, helped engineers predict the intensity of structure-borne noise that vehicle occupants would hear. P/A was the term for the partial pressure (noise level) received at the occupant's ears per unit of structural acceleration occurring at the point of attachment for a noise source. The acceleration was the result of a force applied to the structure by the power train, suspension, etc., due to hitting a bump, engine vibration, etc. The structure transmitted noise by deflecting in the vicinity of the noise source. Stiffening the structure in these locations reduced deflection and noise transmission. The P/A analysis helped engineers understand the interactions between structural elements and noise inputs and the contribution that each input made to the overall noise level. P/A analysis used a combination of computational and experimental measurements to determine the effect of structural changes on audible noise. Finite Element Analysis (FEA) modeling, which determined the acceleration at each attachment point, was coupled with mathematically represented experimental audio data measured in the NVH laboratory. P/A analysis computed noise level transmitted to occupants as follows:
The results, expressed as total noise level for each set of test conditions, were reported in a chart to facilitate analysis. Frequently, a structural change in one area affected the response in another area, necessitating multiple iterations to obtain optimum results.
Shake is a low-frequency vibration characteristic that is visible in the seats, instrument panel and steering wheel, or felt in the seats and steering wheel. Using FEA, body structure engineers predict the response characteristics of these masses across the spectrum of vibration frequencies that tend to excite a shake response. Structural refinement minimized the level of vibration input at the mounting points for these masses.
Impact energy management requirements are mutually exclusive with those of suspension support, noise attenuation, and shake control. Their integration through the SST (Synthesis of Simulation Technologies) process facilitates the best possible combination of capabilities.
The following features contribute to structural stiffness:
The 1998 Concorde and Intrepid had "two millimeter" bodies, meaning that all measured characteristics were maintained within two millimeters of the designed position. An accurate body shell contributed to the customer perception of good workmanship by providing flush body panels with tight and uniform gaps and by making trim and other mating parts fit well. Furthermore, it meant that doors fit properly, reducing potential for wind noise and water leaks.
To provide car line differentiation, Concorde and Intrepid had different body-in-white (BIW) assemblies. Each BIW had unique front fenders, hoods, roofs, quarter panels, and trunk lids. The Concorde had the first high-volume aluminum body panel (hood) ever used on a Chrysler vehicle.
The door panels had one-piece construction for dimensional control and high quality fit and finish (including door sealing). One-piece aperture panels also reduced the number of major stampings in the body side from 15 to 10, reducing cost while increasing body stiffness.
Throughout the body, part consolidation had reduced the number of stampings. A typical example was the quarter inner panel construction where one stamping replaces three on the prior body. In another area, the center pillar overlapped the sill, eliminating three additional stampings previously used. Dimensional accuracy was enhanced by reducing the number of pieces because additive tolerances associated with multiple parts and their respective welding operations did not exist.
One objective for the 1998 Concorde and Intrepid was to eliminate weight wherever possible. The new Concorde and Intrepid body shells were stronger, stiffer, and provided more features than their predecessors, including double-shear suspension mounts and integrated side impact protection, but weighed little more than its predecessor. This resulted from the use of lightweight materials and from optimizing the structure consistent with the added requirements. Extensive use of high-strength steels for the front and rear longitudinal rails, center pillars, and other areas affected by impact requirement reduced body shell weight by an estimated 40 pounds (18 kg) compared to mild steel.
The aluminum hood on Concorde reduced weight by 19 pounds (8.5 kg) and made the Concorde body shell equivalent in weight to that of the smaller Intrepid. Intrepid's steel hood was also lightened by optimizing the inner panel structure.
Hoods had single-pivot rear hinges, which were adjustable for accurate hood fit and gas prop counterbalance supports. Latches were placed at the leading edges of the hoods to permit the hoods to fit closely around the headlights without the possibility of contact if the hood was slammed shut. The secondary release mechanism was readily accessible and had a yellow handle for visibility.
Trunk Lid And Mechanism
New four-bar trunk lid hinges with gas prop counterbalancing increased usable trunk volume and improved trunk lid-to-body fit. Computer-designed hinge and prop geometry made opening the lid easy. Lifting the lid a nominal amount brought the counterbalance forced into effect. Reaching the full-open position from there required little or no effort. The hinges and props mounted completely outside the trunk opening to avoid intruding on luggage capacity when closed-a major improvement over prior models. Four-bar linkage-two pivoting links on each side of the trunk opening connecting the lid to the body-was more compact than the former goose neck hinges, while guiding the lid away from the body and providing ample room for loading. This hinge system was also strong and stable, providing long-term alignment accuracy and durability. The trunk was sealed by a full-perimeter tubular weatherstrip attached to a raised flange surrounding the opening. The raised flange prevented water from running into the opening when the lid was open.
Full-Coverage Wheelhouse Liners
Full coverage, molded-plastic front and rear wheelhouse liners protected the body structure from potentially corrosive road splash, and prevented noise due to stone impingement on the body shell.
High-impact, molded-plastic sill cladding resisted chipping better than painted sheet metal and did not rust if chipped. The Intrepid cladding aft of the front wheels was shaped as a stone chip protector. The cladding also covered the sill area in the door openings outboard of the body-mounted seals, carrying the line of the cladding into the door opening. Sill cladding was used in conjunction with sill construction having no appearance surface, thus providing maximum model differentiation flexibility at less cost than full sheet metal sills with add-on cladding.
One-piece, die-cast zinc side window opening moldings had better dimensional control than multi-piece stampings. One-piece construction provided a smooth appearance free of joint lines and ensured consistent gloss and color. Moldings were flush with the outer surfaces of the doors and formed the outer half of the glass channel, allowing the glass to more closely approach the surrounding sheet metal for a smooth, aerodynamic appearance and reduced wind noise. Using die-cast zinc, which was stronger than aluminum, provided the shallowest possible moldings and a high-quality surface finish. Side window opening moldings had a black, powder-coat paint finish with a satin finish. To prevent wind and other noises, a polyethylene foam backing was placed between the moldings and the door outer panels. "Flags" incorporated into each rear door molding aft of the window provided a smoothly curved continuation of the window opening.
Body-side moldings protected against parking lot damage. New injection-molded construction included preformed ends for a neat appearance. Inert-gas injection in the molding process provided a uniform outer surface and thin-wall construction that reduced weight compared to solidly molded parts. All moldings were painted body color.
Solar Control Glass was used for the windshields and rear windows to reduce the transmission of infrared and ultraviolet energy, to minimize interior heating and damage to organic materials from solar radiation.
The rear window molding was injection molded onto the glass, rather than being manually attached. It was dimensionally accurate and seam-free to provide a close fit to the body opening and a neat appearance. The molding included mounting clips and positioning spacers to ensure easier assembly and accurate alignment of the window on the body.
Front and rear bumper and fascia systems fit close to adjacent body panels while providing low-speed impact protection for safety-related equipment. The bumper systems exceeded the Canadian regulation for protection up to 5 mph (8 km/hr). Fascias were molded of either TPO (thermo-plastic olefin) or RRIM (reaction-injection molded urethane). Both materials were reformulated to increase abrasion resistance relative to their predecessors. New processes reduced fascia weight approximately 3 pounds (1.4 kg) per car compared to 1997.
New bumper systems continued to use a combination of high-density molded polypropylene-bead foam energy-absorbing material and light-weight, ultra-high-strength steel beams bolted to the body structure. The energy-absorbing foam conformed to both the interior shape of the fascia and the bumper beam. It cushioned low-speed impacts and restored to near its original shape after a low-speed impact to help maintain fascia appearance. Engineers used computer FEA models to design deeply curved bumper systems fulfilling appearance requirements that the paradigms of bumper design practice said were not feasible. New, denser foam enhanced the energy absorbing capabilities of the bumper systems. Shorter beams than in past practice terminated in lightweight, high-impact molded plastic inserts. These inserts also contributed to high-speed barrier impact energy management by extending the load path from the longitudinal rails of the body structure to the front of the bumper face bar. A patent was pending on this aspect of the bumper system.
New headlamps provided a broader and longer beam pattern on the road. Separate high- and low-beam units ensured optimum performance of each function. In high-beam mode, all four units were lit. The new high-beam units produced double the light output of their predecessors and reached 65 percent farther down the road. Low beams produced 50 percent more light. Computer-designed reflectors focused the light. Using the same bulbs as their predecessors, these reflectors were twice as efficient at projecting light down the road.
Headlamp units were mounted to the body structure, allowing body panels to fit closely around them. Aiming was done by moving the reflector within the lamp assembly. Each lamp included a bubble level and readily accessible adjustment screws for re-aiming the headlamps, if necessary. Because of their visual prominence, the headlamps included design features-decorated lenses, serrated textures on the outboard edges of the lenses, bright bezels beneath the lenses and styled interior surfaces that helped integrate them with the adjacent body panels.
New, more powerful fog lamps were integrated with the front fascias on Intrepid ES. Circular, computer-designed reflectors projected light on the road through clear, stone-chip resistant lenses. To increase their effectiveness as fog lamps, new switch logic allowed them to operate with parking lamps, but without headlamps. They turned off automatically when high beam headlamps were switched on.
Front doors used seven parts compared to 11 in 1997. Rear door part count was reduced even more, from 13 to six. Reducing the number of parts allowed a corresponding reduction in welding operations and the potential for variations resulting from each operation. Door inner panels were stamped from dual-thickness, laser-welded sheet metal to increase accuracy of the doors. Laser-welded inner panels eliminated the need for a separate welded-in reinforcement, the single largest cause of variation in door assembly. Instead of adding a reinforcement, the forward portion of the panel, to which the hinges were bolted, was nearly three times as thick as the remainder of the panel. The added thickness provided hinge mounting stability and contributed to a solid door closing sound. Furthermore, this inner panel construction was also lighter than a single-thickness panel with a welded reinforcement. To form the inner panel, sheets of dissimilar thickness steel were butted together and weld by a laser beam. The resulting weld joint was smooth and unobtrusive. Locating points in the inner panel stamping presses ensured that the weld seam was accurately aligned for proper sealing of the door in the door opening. One-piece aperture panels in the body sides facing the doors contributed to consistency of door fit.
Door hinge design and mounting provided more accurate door placement than the prior configuration. Hinge inner and outer halves were permanently assembled, reducing clearance required by the prior replaceable pivot pin system. Hinge attachment to the doors and pillars was controlled by alignment fixtures, eliminating the need for manual adjustment and its potential errors.
Door latches provided smoother and quieter operation. The new design provided 100 percent isolation against metal to metal contact between the latch pawl and the striker for quietness. Plastic and plastic-encapsulated steel components ensured quiet operation. Solid steel components were used only where required to assure strength and durability.
New 'door ajar' switches were integral with the door latch assembly. They operated both courtesy lamps and the door ajar indicator in the instrument cluster. This construction was more reliable and durable than previous stand-alone switches while being less susceptible to door adjustments, freezing, contamination and corrosion. New power lock motors were now virtually inaudible.
A body-mounted tubular weatherstrip encircled each door opening to provide primary sealing against wind noise and water leaks. A second body-mounted weatherstrip running up the windshield pillar and across the tops of the doors incorporated a trough to channel water away from the door opening. This sealing system prevented rain water from running into the passenger compartment when the doors were opened. A tubular-type weatherstrip attached to the leading edge of each rear door above the belt line prevented wind noise and acted as a sight shield. A tubular weatherstrip attached to the back of each door between the sill and the belt line also handled wind noise. Lip-type weatherstrips attached to the sill cladding sealed the gap between doors and sills to keep road splash and dust out of the door openings and block wind and road noise more effectively than door-mounted weatherstrips. These weather strips snapped into the sill cladding and also covered the cladding attachments.
Front door glass was cylindrical for fit and finish accuracy and easy window operation. Rear door glass was barrel shaped to conform to the compound curvature of the doors. As in the past, the rear windows lowered only partially dropping 9.4 inches (240 mm) into the doors. This permitted wider door openings than would have been possible with windows that dropped completely.
A vent-and-slide power sunroof was optional. It blocked out ultraviolet light and up to 81 percent of visible light, to minimize interior heating and damage to organic materials from solar radiation. The unit was thin- at least 0.25 in. (6 mm) thinner than competitive units-to minimize passenger compartment intrusion. An opening 33.2 in. (844 mm) wide provided the driver and front passenger with clear upward views.
Soft-touch switches labeled 'VENT, OPEN, and CLOSE' were placed between the courtesy lights in the overhead console. A rocker switch provided vent and slide functions. A push button closed the sunroof in either mode. Operation continued only as long as a switch was pressed to allow adjustment of panel position in vent and close modes. In vent mode, panel movement was slow for precise positioning. The new Concorde and Intrepid sunroof included an "express" open feature in the open mode. Pressing the 'OPEN' rocker switch caused the panel to move immediately to the full-travel position. Pressing the switch again before the panel reached the full-open position stops movement. An electronic control system used Hall-effect sensors rather than mechanical switches at the limits of panel movement.
The interior roof panel and headliner conformed to the sunroof opening for a finished appearance without an add-on welt or molding common to some competitive applications. A laminated plastic sunshade was covered with foam-backed headliner fabric. It slid manually using a recessed handle molded into the surface or slid back automatically when the sunroof slides back.
To minimize wind noise and buffeting, a curved air deflector pops up at the front of the roof opening and the panel stops short of the full-open position. The height of the air deflector and the open-position stopping point were optimized in proving grounds tests. Positive sealing was ensured by a cam system that moved the panel into the closed position from the top down. A compact electric motor at the rear of the structure moved the panel with cables that were enclosed for smooth and quiet operation. Tempered glass protected the occupants from injury in the event of breakage. When broken, it crumbled into small pieces without sharp edges. Urethane encapsulation of the glass provided a neat installation. The structure was aluminum and molded plastic for light weight and corrosion resistance.
Cross-functional coordination produced a windshield curvature and wiper blade design to facilitate uniform wiper blade pressure. Computer modeling of the wiper linkage determined appropriate wiper pivot locations and helped determine the windshield configuration required to maintain blade pressure. The computer models also aided in designing linkage with low load variation for quiet operation. Aerodynamic design of the hood and cowl screen smoothed air flow, helping to hold the wiper blades on the glass at the vehicles' highest attainable speeds without resorting to add-on air foils. This air flow pattern also caused water pushed down by the wipers to flow to the sides rather than running back up the glass. Aerodynamic design of the windshield pillar shape and molding guided water swept aside by the wipers upward, keeping the lower portions the side windows clear. Cold-weather wiper performance was enhanced by a new defroster system that effectively cleared frost from the glass and melted snow pushed down the base of the windshield by the wipers.
New wiper control logic in the BCM (Body Control Module) returned the blades to their parked position when the ignition was turned off, if the wipers were operating at that time. Continuing on the new Concorde and Intrepid was control logic that doubled the intermittent wipe delay time when the car was moving less than 10 mph (16 km/hr).
New wiper blades were three times as strong as current blades and provided more uniform pressure distribution. Aerodynamic configuration of the new wiper blades also helped hold them firmly on the glass at high speeds better than the added airfoils used previously. Bolt-on wiper arms were simpler to manufacture and more robust than the "latchlock" arms used previously.
A new, 30 psi (207 kPa) high-output windshield washer pump supplied two hood-mounted washer nozzle assemblies. Each nozzle assembly had three jets providing a triangular spray pattern that effectively covered the wiped area of the windshield. The spray nozzle design, pattern and pump pressure requirement were developed through extensive aerodynamic testing, which showed the individual jets to be more effective across the full vehicle speed range than other washer systems. A new washer fluid level sensor in the 110-ounce (3.25-liter) reservoir provided a more accurate indication of low fluid.
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