2011 Acura MDX - Chassis
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 excellent noise, vibration and harshness (NVH) control.
In the early stages of MDX development, engineers tested European luxury SUV competitors including the BMW X5, Mercedes-Benz ML, 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 United States, the DNA of the MDX was largely formed in Europe.
One essential tool that Acura used was the famed 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" as 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 current MDX's wheelbase, tread width and suspension technology were changed from the first-generation MDX. Engineers changed the lift and squat characteristics of the suspension to provide a flatter handling. Thanks to the combination of its wheelbase and SH-AWD®, the 2011 MDX offers both exceptional high-speed stability and responsive handling.
The MDX's entire front chassis was tuned to provide the best balance of handling precision, road isolation and impact aftershake damping. MacPherson struts team with coil-over springs and lower control arms constructed of forged aluminum to reduce unsprung weight. The front subframe "floats" on rubber mounts to provide road and engine isolation, and the subframe assembly is affixed to the main body structure with heavy-duty braces. The front suspension also uses hydro-compliance bushings to provide excellent ride isolation as well as excellent 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 specially tuned to reduce impact harshness and improve ride comfort.
The MDX's rear suspension is a multi-link design mounted on a separate steel subframe. Squeeze-cast aluminum uprights reduce unsprung weight, while precise trailing-arm mounting points reduce body squat during hard acceleration. Constant rate coil springs, gas-charged dampers and a stabilizer bar are tuned for the optimum combination of ride and handling. Rear subframe mounts and lateral control link bushing rates 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 forcesteer properties were carefully developed for optimum response, stability and consistency.
ACTIVE DAMPER SYSTEM
Many luxury vehicles offer some type of body motion control system using air suspension, electronically controlled dampers or automatically adjustable stabilizer bars. Acura considered all of the available technologies before choosing electronically-controlled "semi active" Magneto-Rheological fluid-filled dampers as the ideal technology to maximize ride comfort and handling— all in one integrated system. Included within the available Advance Package, the Active Damper System gives the 2011 MDX an elevated level of handling precision designed to satisfy the most discriminating drivers, while maintaining a smooth and controlled ride.
The overall goal of the Active Damper System is to achieve an outstanding level of handling response while maintaining a refined level of ride comfort. This allows the driver intuitive vehicle control in a relaxed environment— free from a jarring ride, neck straining head toss and other sudden vehicle motions. This is made possible by the quick response of the dampers as controlled by computer algorithms that keep the dampers a step ahead of actual body movements.
Advantages of the Active Damper System include:
- Flat cornering feel with a reduced body roll— 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 "semi active" Magneto-Rheological fluid-filled 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 sec.). The ability to significantly change damping rate hinges on the principle of electromagnetic induction.
Each damper carries a field coil—a collar containing many windings of magnetic wire—that generates an electromagnetic field when current is passed through it. Special damper fluid contains microscopic (about 10 microns in diameter) ferric spheres that align when surrounded by an electromagnetic field, dramatically and instantly changing the effective fluid viscosity. Not only is the overall range of damping force significantly more than that of a conventional damper, but there are also hundreds of small steps available within that range.
An electronic control unit (ECU) determines the amount of electrical current that passes through the field coils via special algorithms which determine the best setting for the road conditions. The continually variable oil viscosity, combined with nearly instantaneous reaction time of the dampers, allows damping control to occur before the MDX's tires or body experience any extraneous motion.
The Active Damper System will deliver low damping when the road is smooth and the vehicle is being driven at a constant speed. Less damping reduces the amount of vibration and harshness that passes from the suspension to the body, quieting and improving the overall vehicle ride quality.
Using the dampers' 5-millisecond response time, the Active Damper System reacts to sudden changes in driver or road inputs. A high damping force achieved in this short time allows the dampers to quickly aid the coil 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 afforded by the MDX's Active Damper System is a noticeable reduction in the driver's workload. In short, the system is able to achieve the benefits of a sport-tuned suspension without the traditional ride comfort trade-off. The overall benefit is more driving enjoyment, better vehicle control and less passenger fatigue over a long drive.
The computer algorithms that control the Active Damper System were specially created for the MDX and offer a substantial difference between Comfort and Sport modes. Thus, drivers whose primary interest is comfort will be coddled over most road conditions, while those interested in a sport-driving experience will find the MDX ready to tackle challenging roads.
The Active Damper System does not require any special maintenance, has no scheduled replacement interval and 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 offered the best steering quality. Acura set out to exceed the BMW's combination of linearity, precise feel and security at high speed by using torque-sensing, power-assist rack-and-pinion steering.
Acura engineers spent considerable time studying steering "on-center" feel in an effort to elevate the quality of steering feedback the driver receives. Beyond offering the correct steering 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 2011 Acura MDX has standard 18x8-inch 5-spoke aluminum wheels finished in luxurious silver with a bright-machined face and a chromed Acura center cap. With the standard MDX, fitted to the 18-inch wheels are Bridgestone Dueler H/L 400 P255/55R18 104H M+S all-season tires that were specially developed by Bridgestone for the MDX. These tires offer a significant traction improvement in dry and wet conditions, while maintaining the previous MDX's level of snow/ice traction. The MDX with Technology Package uses the same 18x8-inch wheels, but makes use of Michelin Latitude Tour HP P255/55R18 M+S tires.
Available with the Advance Package are 19x8.5-inch 7-spoke aluminum wheels with Michelin Latitude Tour HP P255/50R19 all-season tires that were specially developed for the MDX by Michelin engineers. The tires offer outstanding traction in dry and wet conditions, while maintaining a high level of snow/ice traction. Of particular interest, the MDX's tires have been specially designed to deliver a 5-percent reduction in rolling resistance— thus helping to improve fuel economy.
4-WHEEL DISC BRAKES
The 4-wheel disc brakes on the 2011 MDX are the largest brake system ever featured on a production Acura vehicle, an honor shared with the ZDX. The brake system is engineered to provide best-in-class braking performance in dry conditions, as well as while driving in rain, snow or icy conditions.
Engineers benchmarked the Acura MDX against top competitors including the Porsche Cayenne and BMW X5 for braking performance and overall pedal feel. As a result, the MDX has minimal pedal travel and a firm pedal feel, in keeping with the sport-minded direct feel that enthusiast drivers desire. The ventilated front brake rotors measure 13.0-inches in diameter with a thickness of 1.1 inches, while the rear rotors are a 13.1-inch diameter solid design with a thickness of 0.43 inch. The front brake calipers are dual-piston design while single-piston calipers are used in the rear.
The MDX has an anti-lock braking system (ABS) with Electronic Brake Distribution (EBD), Brake Assist and Hill Start Assist as standard equipment. In addition, a Collision Mitigating Braking System™ (CMBS™) is available as part of the Advance Package.
VEHICLE STABILITY ASSIST™ (VSA®) WITH TRACTION CONTROL
Vehicle Stability Assist™ (VSA®) is included as standard equipment on the MDX. By continually monitoring the vehicle's operating parameters (such as road speed, throttle position, steering wheel angle, accelerating/braking/cornering loads), VSA® detects if the vehicle is experiencing oversteer or understeer. To help correct either of these situations, VSA® automatically reduces engine power output as needed, and applies one or more brakes individually to help stabilize the chassis. The VSA® response is so quick that the instability may be corrected even before the driver knows that it is occurring. Traction Control is integrated into VSA® and helps the MDX accelerate smoothly on low traction surfaces. As described in detail within the SH-AWD™ section, VSA® works with SH-AWD™ to adjust engine torque distribution— 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. Such pressure monitoring 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 over time could cause significant air loss, reduction of grip and potentially a blowout.
TPMS uses a small 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 that sends a unique signal for each TPMS sensor. The TPMS sensor then 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) screen. The TPMS system can display the air pressure in all four tires simultaneously via the MID, making it simple to locate a specific low-pressure tire. If there is a fault in the system, a "CHECK TPMS SYSTEM" alert appears.
At the driver's preference, the Multi-Information Display can continually show the air pressure in all four tires as the vehicle is driven. Regardless of whether the driver chooses to monitor the pressures, a warning will still occur if a tire drops a specified level.
Acura took thorough measures to ensure that off-road enthusiasts would be satisfied with the MDX. The 2011 MDX 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 MDX, including travel through standing water nearly 19-inches deep, generous approach and departure angles (22.2-degrees front and 23.8-degrees rear), and the ability to ascend off road and paved grades up to 60-percent. Components located underneath the MDX are tucked away to help avoid damage in off-highway driving, and the evaporative canister is protected by a high-strength steel guard.
The high-torque 3.7L V-6 engine, SH-AWD® system with hill logic and computer-controlled traction control capabilities help the MDX meet typical off-road challenges without the need for a low-range transfer case or special gears. Moreover, the 2011 MDX outperforms the first generation MDX without the need for a driver-selected 4WD "lock" mode.
SNOW AND ICE CAPABILITY
The same technologies that make the MDX capable off-road also enhance its performance in snow and ice. SH-AWD® with Grade Logic Control and computer-controlled traction control work together with the M+S tires to maximize power delivered to the road surface (and thus driver control) during winter driving conditions.
NOISE, VIBRATION & HARSHNESS (NVH)
The MDX uses an array of technologies to reduce noise, vibration and harshness (NVH) within 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 the 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 allow 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 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 shook the MDX body and measured vibration by scanning the floor with lasers to find any "hot spots". This scanning process 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 rated towing capacity of 5,000 pounds, the MDX can pull a boat or camper trailer with confidence. Standard towing equipment includes a high-capacity radiator with twin 160-watt electric cooling 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 Class III receiver assembly, hitch, ball and wiring harness.
One of Acura's durability tests involved towing a 5,000-pound trailer in Death Valley National Park during summer, including repeated climbs up Townes Pass and Daylight Pass with outside air temperatures between 113- and 120-degrees F. Testers also targeted the toughest boat ramp they could find - at Shasta Lake in Northern California - where the launch 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 2011 MDX comes standard with 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. Such oscillation may occur if the trailer is not loaded correctly, or during certain speed and road conditions. Using a combination of torque reduction and individual brake application, Trailer Stability Assist activates to stabilize the vehicle after sensing oscillations of the vehicle and trailer system.
The 2011 Acura MDX is assembled at Honda of Canada Manufacturing in Alliston, Ontario, Canada. A special dynamic quality-control process is employed to ensure the highest possible level of attention to detail for every MDX produced. After assembly is completed, each MDX enters a special Acura Quality line. The ability to immediately respond to the smallest issue allows the MDX to excel in quality— and in customer satisfaction.
For years, Acura vehicles have successfully met ever more stringent guidelines to reduce the content of harmful metals (such as chromium and lead) used, along with reducing volatile organic compounds (VOC) and polyvinyl chlorides (PVC). To reduce environmental impact at the factory, a special separation line isolates different manufacturing scrap materials for recycling. The latest MDX marks the first Acura to use a mercury-free Navigation display screen, which features LED backlighting instead of traditional cold cathode backlighting.