Acura MDX Chassis
CHASSIS DEVELOPMENT
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.
Front suspension
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.
REAR SUSPENSION
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.
OFF-ROAD CAPABILITY
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
TOWING
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.
