2008 Acura MDX - Chassis
9/12/2007 7:47:00 PM
From its inception the MDX targeted the dynamics and driving feel of its strongest European competitors. The essential elements of this engineering goal included steering, suspension, braking and overall dynamic performance, as well as noise, vibration and harshness (NVH) control.
At the beginning of MDX development, engineers test European luxury SUV competitors including the BMW X5, Mercedes-Benz ML350, Porsche Cayenne and Volvo XC90. These experiences provided real-world understanding of how these luxury SUVs functioned in everyday driving on their "home turf." Although later development testing was conducted in the U.S., the DNA of the MDX was largely formed in Europe.
One essential tool that Acura used was the Nürburgring Nördschleife road circuit in Germany. This 13-mile course contains over 170 corners, numerous altitude changes and is considered the toughest natural road course in the world. Engineers used the Nürburgring as a "weakness finder" because it quickly exposes problems with a vehicle's dynamics, making them easier to identify and fix.
In order to build in a combination of increased handling agility, increased stability and comfort, the vehicle's wheelbase, tread width, and suspension technology were all changed from the first-generation MDX. Engineers changed the lift and squat characteristics of the suspension to provide a flatter takeoff. Thanks to its extended wheelbase together with SH-AWD™ (see Powertrain section), the MDX offers both exceptional high-speed stability and responsive handling.
The entire front chassis area was tuned to provide the best balance of handling precision, road isolation, and impact after-shake damping. MacPherson struts with inversely wound coil-over springs and forged-aluminum lower control arms reduce unsprung weight, while the front subframe "floats" on rubber mounts to provide road and engine isolation. The front suspension also uses newly developed hydro-compliance bushings to provide superior ride isolation as well as chassis vibration control. Suspension and engine mounts are specifically tuned to provide minimum vibration transfer to the occupants, and the front struts employ internal rebound springs to improve ride and handling. Anti-dive properties were revised to improve impact harshness and ride comfort.
The MDX's rear suspension is a multi-link design mounted on a separate steel subframe. Squeeze-cast aluminum uprights (or "knuckles") reduce unsprung weight for improved response, while revised trailing-arm mounting points (compared to the previous MDX) reduce body squat during acceleration. Coil springs, dampers and an anti-roll bar are tuned for the optimum combination of ride and handling. Rear subframe mounts and lateral control link bushing rates were significantly revised to create a discrete level of lateral force steer at the rear axle. Since the SH-AWD™ system constantly varies drive torque at the rear axle, the longitudinal and lateral force steer properties were carefully developed for optimum response, stability and consistency.
ACTIVE DAMPER SYSTEM
Included with the optional Sport Package, the Active Damper System gives the MDX an elevated level of handling precision designed to satisfy the most discriminating drivers, while maintaining a smooth and controlled ride. Acura considered all of the available technologies before choosing electronically controlled, fast-acting Magneto-Rheological fluid dampers as the ideal technology to maximize ride comfort and handling in one integrated system.
The overall goal of the Active Damper System is to achieve outstanding levels of handling response while maintaining refined levels of ride comfort. This allows the driver intuitive vehicle control in a relaxed environment - free from neck straining head toss and other sudden vehicle motions. This is made possible by the quick response of the dampers and by the computer algorithms that keep the dampers a step ahead of the actual body movements.
Advantages of the Active Damper System include:
- Flat cornering feel with minimal body roll or pitch even during spirited driving
- Responsive, precise and secure steering feel
- Elimination of unwanted under-damped body heave, roll and pitch motions
- Improved tire adhesion, vehicle stability and road isolation
Real-Time Damper Control
The secret to the MDX's Active Damper System is its fast-acting shock absorbers. These dampers are built and tuned specifically for the MDX and offer the ability to individually adjust from minimum to maximum damping force in as little as five milliseconds (0.005 seconds). This ability hinges on the principle of electromagnetic induction. Each damper carries a unique coil that generates an electromagnetic field when current is passed through it. Proprietary damper fluid contains microscopic (on the order of 10 microns in diameter) ferric spheres that align when surrounded by the electromagnetic field, dramatically and instantly changing the effective fluid viscosity. Not only is the overall range of damping force available significantly more than that of a conventional damper, but there are hundreds of steps within that range.
The current that passes through the field coils is controlled by an electronic control unit (ECU) that uses special algorithms to determine the best setting for the road conditions. This combined with nearly instantaneous reaction time of the dampers allows damping control to occur before the vehicle's tires or body are allowed to have any extraneous motion. Active Dampers can run with low damping when the road is smooth and level and the vehicle is being driven at a constant speed. This further reduces the amount of vibration and harshness that passes from the suspension to the body thus quieting and improving the vehicle's ride quality.
Using the dampers' 5-millisecond response time, the Active Damper System reacts to sudden changes in driver or road inputs. The high damping force achieved in this short time allows the dampers to aid the springs and stabilizer bars in roll and pitch control, greatly improving the handling of the MDX.
The result of the improved transient handling, road isolation, and body control is that the MDX with the Active Damping System noticeably reduces the driver's workload when compared to other performance-oriented SUVs. The system is able to achieve the benefits of a sporty suspension without the traditional ride comfort tradeoffs. The overall benefit is more enjoyment, better control and less fatigue over the long haul.
SPORT OR COMFORT MODE
The Active Damper System provides two driver-selectable settings - Sport and Comfort. Sport mode prioritizes handling response, vehicle body control and tire adhesion to allow for spirited driving with high levels of precision and composure. Comfort mode allows for a more relaxed driving experience by prioritizing road isolation and reduced passenger fatigue caused by road inputs. Either Sport or Comfort mode can be used in any type of operating conditions as preferred by the driver.
The Active Damper System does not require any special maintenance and has no scheduled replacement interval. The Active Damper system maintains the same quality, durability and performance as all other Acura components.
POWER RACK & PINION STEERING
Of the luxury SUVs that Acura targeted in developing the MDX, the BMW X5 3.0i offered the best steering quality. Acura set out to exceed the BMW's combination of linearity, precise feel and security at high speed with torque-sensing, variable power-assist rack-and-pinion steering.
Acura engineers spent considerable energy studying steering on-center feel in an effort to elevate the quality of steering feedback the driver receives. Beyond offering the correct effort level, the MDX provides subtle transient steering feedback that is an integral part of its subjective handling quality. Notable geometry changes include increasing the rake and trail over the previous MDX to provide a stronger steering feel.
WHEELS AND TIRES
The 2008 Acura MDX has standard 18x8-inch 5-spoke aluminum alloy wheels and P255/55R18 104H M+S all-season tires. The tires were specially developed for the MDX by Bridgestone and Michelin and offer a significant traction improvement in dry and wet conditions, while maintaining the previous MDX's level of snow/ice traction. Every MDX trim level except base features P255/55R18 104H M+S all-season tires, and Michelin P275/45R19 summer tires are also available with the Acura accessory 19-inch aluminum alloy wheels.
Acura designers purposefully moved the wheels and tires to the high end of the competitive set. In 2008, the MDX has bigger tires than all of its 4WD base competitors, and is the only vehicle in the group with standard 18-inch tires.
4-WHEEL DISC BRAKES
The 4-wheel disc brakes on the 2008 MDX are the largest brake system ever featured on a production Acura vehicle. The brake system is engineered to provide best-in-class braking capability in dry conditions, as well as while driving in rain, snow or icy conditions.
Engineers benchmarked the existing MDX as well as the top competitors including BMW X5 and Porsche Cayenne for braking performance and pedal feel. The 2008 model has short pedal travel and firm pedal feel, in keeping with the direct feel that enthusiast drivers want. The ventilated front brake rotors are 13-inches in diameter with a thickness of 1.1-inches, while the solid rear rotors are 13.2-inch diameter units with a thickness of 0.433-inches. Dual-piston cast iron brake rotors are used in front and single-piston cast iron calipers are fit in back.
The 2008 MDX brake design is validated by a severe fade test utilized by the German enthusiast magazine Auto Motor und Sport and by rigorous testing at the Nürburgring.
VEHICLE STABILITY ASSIST™ (VSA®) WITH TRACTION CONTROL
Vehicle Stability Assist™ (VSA®) is included as standard equipment. By continually monitoring the vehicle's operating parameters (such as road speed, throttle position, steering wheel position along with acceleration, braking and cornering loads), VSA® anticipates if the MDX is approaching oversteer or understeer. To help correct either of these situations, VSA® first acts to transfer torque bias to the axle with the most traction. This preventative measure significantly reduces the number of brake and throttle interventions. The response is so quick that the instability may be corrected even before the driver knows it is occurring. Traction Control is integrated into the VSA®, and it helps the MDX accelerate smoothly on slippery surfaces. As stated in the SH-AWD™ section, the VSA® can request the SH-AWD™ to adjust its torque level - thus allowing Direct Yaw Control of the MDX to maximize stability under all driving conditions.
TIRE PRESSURE MONITORING SYSTEM (TPMS)
An onboard Tire Pressure Monitoring System (TPMS) warns the driver if the air pressure decreases significantly in any tire. The system helps reduce the chance of damaging a tire or losing control of the vehicle due to low air pressure. This is important because a slow leak, such as one that might be caused by a nail or road debris, might not be easily detectable but could cause loss of grip and potentially a blowout.
TPMS uses a sensor mounted on each wheel that continually monitors tire pressure and sends a coded signal to the TPMS electronic control unit (ECU). The process begins with a signal initiator located inboard of each tire on the chassis. The initiator sends a unique signal for each TPMS sensor. Then the TPMS sensor sends tire pressure data to the TPMS ECU, which compares the values for each tire's pressure against an acceptable range of pressures.
When a tire's pressure drops significantly below the proper level, a "check tire pressure" alert will appear on the Multi-Information Display (MID) alert screen. The system can display the air pressure in all four tires simultaneously via the MID, making it simple to locate a low-pressure tire.
At the driver's preference, the MID can continually show the air pressure in all four tires. Regardless of whether the driver chooses to monitor the pressures, a warning will still occur if a tire drops below specification.
Acura took thorough measures to ensure that off-road enthusiasts would be satisfied with the MDX. The vehicle meets design criteria developed for the original MDX, which was based on research at different off-road areas across the country. This research was the foundation for specific minimum performance criteria for the new MDX, including travel through standing water nearly 19-inches deep, steep approach and departure angles (28º front and 23º rear, respectively), rocks, off road and paved grades up to 60-percent. Components located underneath the vehicle are tucked away to avoid damage in off-highway driving and the evaporative canister is protected by a high-strength steel guard.
The high-torque engine, SH-AWD™ system with hill logic and computer-controlled traction capabilities help the MDX meet typical off-road challenges without the need for low-range gears. The MDX outperforms its predecessor without the need for a driver-selected 4WD "lock" mode. Generous front approach, break-over and rear departure angles enable the MDX to meet the rough terrain needs of its target buyers.
SNOW AND ICE CAPABILITY
The same technologies that make the MDX capable off-road also enhance its performance in snow and ice. The SH-AWD™ system with grade logic and computer-controlled traction capabilities work together with the standard M+S tires to maximize grip and driver control in winter driving conditions.
NOISE, VIBRATION & HARSHNESS (NVH)
The MDX uses an array of technologies to reduce noise, vibration and harshness (NVH) in the passenger cabin. Acura began by attacking vibration in the body during the engineering phase instead of simply adding materials to the body to damp it out later. Two examples of this process are new rear suspension mounting points, which are now attached to the unit body for greater rigidity and reduced vibration, and the stiffening of the front and rear subframes, which allowed the mounting bushings to be made softer.
Engineers paid strict attention to addressing two specific issues - concrete road noise and drumming - neither of which is acceptable to luxury customers. By nature SUVs are susceptible to drumming noises in the cabin interior because of their large open area. Where necessary, meltsheets are used in critical areas on the front bulkhead and floor, and lighter insulators are used on the bodysides, doors and roof. A new hybrid carpet and padding system further reduces noise intrusion by balancing the transmission loss and absorption properties of the carpet.
To verify that the results were on target, engineers dynamically shook the body and measured vibration by scanning with lasers the MDX floor to find the "hot spots". This provided exact information on where to focus efforts. As a result, the MDX is 20-percent stiffer and quieter than the previous version, registering just 64 dBA at 60 mph
With a towing capacity of 5000 pounds the MDX can tow a boat or camper trailer with equal confidence. Standard towing equipment includes a high-capacity radiator with twin 160-watt fans, an automatic transmission fluid cooler, a heavy-duty power-steering cooler and pre-wiring for an electric brake controller. A Towing Package is available as dealer-installed accessory and includes a hitch, receiver, ball and wiring harness.
One of Acura's durability tests was towing a 5000-pound trailer in Death Valley National Park during summer, including climbs up steep Townes Pass and Daylight Pass with outside air temperatures between 113º and 120º F. Testers also targeted the toughest boat ramp they could find - at Shasta Lake in Northern California. The ramp is extremely steep, and its length varies depending on the lake level. They also proved the MDX trailer-pulling skills elsewhere in California as well as in Kentucky and West Virginia.
Trailer STABILITY Assist
Every 2008 MDX comes with standard Vehicle Stability Assist™ (VSA®) that includes special Trailer Stability Assist algorithms. Trailer Stability Assist interprets oscillation, or wag, in the vehicle/trailer combination while driving. This may occur if the trailer is not loaded correctly, or under certain speed and road conditions. Using a combination of torque reduction and individual brakes, Trailer Stability Assist activates to stabilize the vehicle after sensing oscillations of the vehicle and trailer system.