2005 Acura MDX Chassis
8/31/2004 8:50:00 PM
The Acura MDX was the first product in what has proven to be a very popular portion of the market: a car-like, family-sized, luxury SUV. To venture beyond current competitive offerings with a no-compromise design, a new chassis was essential. Engineered to deliver the exhilarating driving experience and the outstanding vehicle dynamics expected of Acura products, MDX exceeds conventional SUV chassis design standards by a wide margin.
To provide the MDX with a smooth, refined, car-like ride, the MDX's suspension utilizes the following key design elements to meet its ambitious driving dynamics goals:
- Extra-wide track dimensions
- Short wheelbase for agility
- Rubber-isolated front and rear subframes
- Independent front and rear suspension systems with car-like geometry and long wheel travel for sporty handling and a comfortable ride
- Redesigned 17-inch cast-aluminum flangeless wheels (Touring Package)
- All-season, radial tires with silica tread compound for excellent traction and fuel Economy
- A Vehicle Stability Assist (VSA) system (which for 2005 has been enhanced with active yaw control) to integrate traction control, the VTM-4 system, Anti-lock braking, throttle control and stability control
- Four-wheel disc brakes with four-channel ABS, electronic brake distribution and new for 2005, Brake Assist
- Torque sensing rack-and-pinion power steering
POWER RACK-AND-PINION STEERING
MDX's steering system is tuned for quick, linear, car-like response and sensitive feel - and the torque sensing, power steering assist is high for parking maneuvers and low at highway speeds. As cornering forces rise, understeer is mild and linearly progressive to accentuate predictability.
This system has been tuned for optimal steering feel. The valve body incorporates a damper valve that improves steering feel at high speeds, and a kickback reduction valve that substantially reduces steering kickback when encountering bumps or objects in the road or trail. A roller bearing in the steering shaft makes steering easier in parking situations.
The front suspension provides an incredibly smooth ride without compromising the MDX's nimble handling. The shock absorber units feature NWS (New Wide Setting) damper valves, which are designed to reduce low speed compression damping 10 percent and rebound damping by 30 percent while increasing high speed damping by 30 percent.
MDX has a wide track - 66.3-inches front, 66.5-inches rear - to optimize handling precision, stability, and ride comfort. The strut-type front suspension provides a generous 7.3 inches of wheel travel (4.3-inches in compression, 3.0-inches in rebound) with a moderate spring rate and relatively firm damping. Separate load paths to the unit body are provided for the coil spring and the shock absorber to facilitate tuning out road noise. A solid 0.9-inch stabilizer bar is linked directly to the strut via ball-joint connections to resist body roll during cornering maneuvers. The small 0.04-inch scrub radius designed into the front suspension is unusual for SUVs but this feature gives the MDX car-like steering response and handling that's consistent and predictable in on- and off-road situations. Accelerating or hard braking with uneven traction underfoot does not cause MDX to drift off line as is the case with many SUVs.
A low roll center (6.2-inches above the ground) further improves MDX's handling precision. The lower control arm bushings are designed to provide a stabilizing toe-out steering effect when loaded during braking or cornering. The L-shaped arm allows a very tight steering lock for good low-speed maneuverability. In fact, the Acura MDX's turn-circle diameter is a modest 37.2 feet.
FRONT SUBFRAME AND MOUNTING SYSTEMS
The MDX's engine, transaxle, transfer case, steering gear, and front suspension are all supported by a welded-steel subframe secured to the unit body's longitudinal rails through four tuned rubber isolation mounts. The front of the subframe assembly is tubular for maximum stiffness with minimal weight. A stiffener located under each subframe attachment fastener helps stabilize the assembly, thereby sharpening handling and braking performance. A stiffener plate bolted across the subframe under the transfer case greatly increases the assembly's rigidity. Two dynamic dampers and one mass damper are positioned to counteract noise and vibration, while two fluid-filled engine mounts quell vibration at idle and isolate powertrain NVH from the passenger compartment. The steering gear mounts are made of heat-resistant rubber to provide good vibration isolation, the firm retention necessary for sensitive on-center-steering feel, and life-of-the-vehicle durability.
Like the front suspension, the rear suspension has also been tuned for a smooth, refined ride for each passenger position. The rear spring rate and damping were configured to reduce low speed compression damping and rebound damping while increasing high speed compression and rebound damping. To ensure more precise damping control throughout the entire range of the suspension system, the twin-tube, gas filled hydraulic shocks also feature New Wide Setting (NWS) damper valves.
The MDX's rear suspension is a compact, multi-link trailing arm layout for excellent ride and handling, light weight, and optimum packaging. Wheel travel is a generous 4.9 inches in compression and 3.3-inches in rebound. The three links that position each rear wheel laterally run between the knuckle assembly and the subframe. A trailing arm also runs from the unit body to each rear knuckle. Coil springs sit on the lowermost lateral link and anchor against the unit body directly behind each axle shaft. Shock absorbers, positioned ahead of the drive shafts, run from a low point on each knuckle to a secure attachment point on the unit body. Knuckles are an "in-wheel" design to optimize suspension geometry and packaging efficiency. Bushing compliance provides a modest toe-in effect in response to substantial cornering and braking loads to enhance dynamic stability. A solid 0.8-inch anti-roll bar helps keep the body relatively flat during hard cornering. The rear roll center is positioned at a 6.7-inch height to provide linear and predictable behavior at the limit of adhesion. The results speak for themselves, as MDX truly sets the standard for ride and handling in the luxury SUV category.
Packaging of major components at the rear of the MDX is a major engineering challenge. The rear subframe, which supports most of the rear suspension and the rear axle drive unit, is made of high-strength steel for high stiffness and minimal weight. A rear subframe stay helps maintain precise positioning of the rear suspension and provides improved handling precision. The shape of the rear subframe is very important since it must accommodate the drivetrain components for the VTM-4 four-wheel drive system and the multi-link rear suspension pieces while still enabling the versatility of the third-row seat and flat cargo floor. For excellent ride and handling characteristics, the subframe attaches to the unit body at four widely spaced, rubber-isolated mounting points. Rear-suspension components, especially the springs and shock absorbers, are as compact as possible to facilitate a wide, flat load floor and to leave room for both a spare tire and a full-size fuel tank. The rear axle drive unit is mounted to the subframe by means of rubber isolators to block road and powertrain noise and vibration from the passenger compartment. A tuned dynamic damper attached to the drive unit cancels propeller-shaft and drive-shaft vibration.
WHEELS AND TIRES
The MDX and the MDX with Touring Package are fitted with die-cast flangeless aluminum wheels. Both have 6.5-inch-wide by 17-inch diameter rims, a 45mm offset dimension, and a five-lug bolt pattern. The flangeless wheels provide a flush, integrated look. Large openings in the wheels provide ample brake-system ventilation. Tires supplied by Michelin and Goodyear are both sized P235/65R-17 and carry an M+S (mud and snow) label and a T (118 mph) speed rating. Both the tread and the internal construction were designed with computer-aided tools to achieve excellent ride comfort, low noise and wear, and best-in-class all-weather performance. Tread materials are compounded for excellent winter traction and low rolling resistance. During initial cold weather engineering tests, the MDX's tires consistently outperformed some of the most highly rated snow tires on the market.
The compact spare tire is carried under the load floor near the center of the vehicle and can be lowered by turning a hidden hex-head bolt with the provided lug-nut wrench. The hex-head bolt is located under a cover conveniently located in the rear hatch trim area. This arrangement guarantees the security of the spare and keeps it readily accessible while allowing enough space for the revised exhaust system. This configuration also allows access to the spare tire without disturbing luggage or cargo carried onboard. Room is provided to stow a flat or a full-size spare in the compact spare's location. Corrosion and failure of spare tire retention equipment, a common problem in some competitor SUVs, is avoided by use of stainless steel and polymer materials eliminating high-mileage failure.
TIRE PRESSURE MONITORING SYSTEM
The MDX features a direct Tire Pressure Monitoring System (TPMS). Using a series of four sensors and four initiators (one for each tire) and a receiver to capture and relay information, the system alerts the driver when it detects that tire pressure in one or more tires drops significantly below the recommended pressure.
The sensor transmitter is located inside the wheel. Should the tires be rotated around the vehicle due to normal maintenance or be replaced entirely, the new tires are "auto-located" by the system. If the system detects that tire pressure is significantly low (32 psi is recommended), a low tire pressure indicator located in the meter containing the fuel level and temperature gauge will come on while a schematic of the vehicle located in the center meter housing the speedometer will indicate which tires are low.
The MDX is equipped with state-of-the-art, four-wheel disc brakes and a four-channel Braking System. The brake system hardware is tuned for quick response, low effort, and short pedal travel. For optimum performance with widely varying loads - including the possibility of towing - the MDX features an innovative Electronic Brake Distribution system (EBD). New for 2005, Brake Assist has been added to assist with brake function during accident avoidance maneuvers. At the rear, a select-low braking strategy is used to preserve directional stability in slippery driving. In the event one rear wheel verges on lock-up, triggering a pressure modulation at that wheel, brake pressure is also diminished at the adjoining wheel to preserve lateral stability.
Extra-large brake rotors and calipers provide the capacity necessary for short stopping distances and excellent fade resistance - even with a heavy load in tow - and the precise pedal feel expected of an Acura. The vented front rotors are 11.8-inches in diameter and 1.1-inches thick. Solid drum-in-disc rear rotors are 12.3-inches in diameter and 0.43-inch thick. The MDX features dual-piston calipers on the front wheels for even more secure stopping. A drum-type parking brake mechanism is positioned within the inner portion of the rear rotor. Parking brakes are both set and released by stepping on a pedal, freeing space in the console area. The compact tandem-type vacuum booster consists of two 9-inch diameter booster chambers.
ANTI-LOCK BRAKING SYSTEM (ABS)
MDX brake components are large in capacity to help handle heavy loads - both those carried on board and those towed behind. Front-to-rear brake effort proportioning is electronically regulated. The Anti-lock Braking System (ABS) has four sensing and activating channels to detect a wheel on the verge of lock up. In the event this occurs, brake pressure is held, then reduced, to permit that wheel to regain traction before full braking resumes. A select-low strategy controls the braking effort at both rear wheels as soon as one nears lock up to safeguard lateral adhesion and to help avoid any tendency to spin or fishtail during hard braking.
New for 2005, Brake Assist helps drivers apply full braking pressure in an accident avoidance situation. To do so, a microprocessor continually analyzes and "learns" the driver's normal braking habits.
In an emergency braking situation when the brakes are suddenly applied but inadvertently released by the driver to avoid wheel lockup and then reapplied, brake assist helps the driver apply full braking force via the ABS actuator. A pressure sensor detects an increase in brake fluid pressure and activates the ABS system. Braking pressure is maintained momentarily after the driver's initial braking action to help the driver stop promptly.
Importantly, Brake Assist assists the driver in obtaining full braking performance
in an emergency. The Brake Assist system deactivates when the driver releases
pressure on the brake pedal.
VEHICLE STABILITY ASSIST (VSA)
For enhanced control during acceleration, cornering, and sudden collision-avoidance maneuvers, the MDX is equipped with a standard 4-channel Vehicle Stability Assist (VSA) system that works in conjunction with the VTM-4, drive-by-wire throttle and ABS systems. For 2005, this system was enhanced with new sensor logic to provide active yaw control to help provide even more secure footing. Active yaw control helps the MDX follow the cornering line intended (by the driver) more faithfully and naturally, greatly enhancing its stability.
The VSA system enhances vehicle stability by applying brake force to each of the MDX's four disc brakes independently while also managing the throttle and ignition systems. An additional benefit is the limited-slip differential effect the system provides for the front wheels by applying braking force to a slipping wheel thereby redirecting driving force to the wheel with more traction.
Analyzing data that is constantly received from eight vehicle sensors monitoring speed, steering input and lateral G forces, the VSA system compares the driver's control inputs with the vehicle's actual response. If the actual response is outside a predicted response range - as can happen when cornering forces exceed the tires' performance capabilities - VSA automatically intervenes with an appropriate corrective action. In the case of oversteer, VSA applies braking to the outside front and rear wheels to counter the unintended yawing effect. If understeer is detected, VSA applies braking to the inside front and rear wheels and reduces engine power to help bring the car back onto the driver's intended course.
The VSA system in the MDX has been carefully calibrated to add handling stability and predictability without intruding on the process of spirited driving. Whenever the system is actively enhancing the vehicle's stability, an indicator light flashes on the instrument panel. In addition, VSA's stability enhancement and traction control can be turned off with a cockpit switch while still leaving the Anti-lock Braking System fully functional.
For 2005, the MDX fuel tank was redesigned. The new tank reduces sloshing noise, increases capacity by 1.2 gallons, ups driving range by 23 miles and helps the MDX meet LEV-2 Evaporative emission standards. The tank is molded of high-density polyethylene for low weight, freedom from corrosion, and impact resistance. It's positioned immediately ahead of the rear wheels and over the propeller shaft to help guard against collision damage. Corners of the tank are rounded and the inside of the tank is baffled to diminish the likelihood of sloshing-fuel noise. The polyethylene filler pipe and fuel lines are light, not susceptible to corrosion, and resistant to fuel vapor losses. A high-efficiency fuel pump is housed inside the fuel tank. The fuel-filter is a lifetime design that never needs maintenance.
MDX complies with all evaporative emissions, on-board diagnostics, and refueling vapor recovery requirements. The fuel vapor canister and filter are rubber mounted for noise isolation and protected against rock and debris damage by a deflection shield.
The ability to haul pop-up campers, medium-sized boats, and recreational vehicle trailers is high on the priority list for many SUV owners. To understand this aspect of the MDX's makeup, MDX engineers polled focus groups and studied survey results that tapped 200,000 households. Their findings offered the insights needed to properly outfit the MDX to surpass the towing expectations of most customers. Engineers learned that roughly one third of the six-cylinder-powered-SUV owners expect to tow something at one time or another. In addition, 18-percent tow more than four times per year. Approximately 10 percent of the miles accumulated on six-cylinder SUVs are with a trailer in tow.
Another notable discovery was that many customers aren't particularly knowledgeable about towing technicalities. Terms like "gross axle weight" may be germane to the engineering process but such language can leave average customers in a state of bewilderment. This realization convinced engineers that customers' interests are best served by load ratings that are both realistic and easy to comprehend. Ultimately, the engineers concluded that a casual or weekend towing capability was most appropriate for MDX. Customer feedback helped set the towing limit at 4500 pounds for boats and 3500 pounds for other types of trailers. A heavier load is acceptable with boats because their pointed bow shapes impose less aerodynamic drag on the towing vehicle than a slab-faced, square-cornered trailer.
Industry practice is to boast a high maximum tow rating, even though some sacrifice of passengers and cargo may be necessary to suitably accommodate such a trailer load - in some cases limiting the vehicle to one passenger to accommodate the maximum specified towing capacity. The Acura MDX's 3500/4500-pound rating is calculated to include up to four passengers and their cargo.
To help ensure that customers will be able to move a maximum-rated load up a grade from rest (such as pulling a loaded boat trailer up a launch ramp), engineers sought out the most challenging entry roads and launch ramps in the country. Through testing, they verified that MDX proficiently handles the 17-degree (31-percent) grades on mountain roads approaching Lake Cumberland in southern Kentucky and the strenuous combination of 15-degree (27-percent) grades and 5280-foot elevation at Lake Tahoe. At sea level, MDX can move a 4500-pound boat and four passengers up an 18-degree (32-percent) slope. A note in the MDX owner's manual suggests reducing gross combined weight two-percent for every 1000 feet of elevation.
In support of safe towing, MDX's tow hitch and other hardware needed for the job are factory engineered for dealer installation. The dealer-installed trailer hitch is a Class III receiver-type design that bolts on with no drilling, cutting, or bumper-cover modifications. An external transmission cooler and a separate power-steering fluid cooler is also included along with a harness to provide electrical power to trailer lights plugs into a connector provided at the rear of the vehicle.