2014 MDX re-engineered to pass IIHS test, reduce NVH

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The competitive marketplace would be enough of a reason for the 2014 Acura MDX, the third generation of the seven-passenger crossover, to be built on a new platform. But the U.S. engineering team for the Honda luxury model had a wide range of engineering objectives, led by the commitment to meet the small overlap barrier crash test of the IIHS (Insurance Institute for Highway Safety). Although the MDX first was revealed in March at the New York Auto Show (http://www.sae.org/mags/aei/11962), it is just going into production at the Lincoln, AL, plant, and will be on sale in July.

All other Acura models introduced in the past year have passed the test, but the MDX—the largest and heaviest vehicle in the lineup, with much more crash energy to safely disperse—was a greater challenge. It required going beyond just a later variant of the Honda light-truck platform and also dictated a new edition of Honda's ACE (Advanced Compatibility Engineering) approach to crash management.

Stiffener ring for door frame

On the MDX, the ACE structure includes an innovative "stiffener ring" at the front door frame to accept and help diffuse the small overlap test energy of a 40 mph (64 km/h) impact with a rigid barrier, such as a utility pole, by just 25% of the driver's side frontal area.

The front bumper beam is tied into each side rail and each body side, which includes the stiffener ring of the door frame and A-pillar so that the energy of the small overlap impact is distributed through both rail and body side. The stiffener ring is a tailor-welded single piece, with thicker blanks where needed. Then the assembled part is placed in an oven heated to 940°C (1725°F), where it is formed and quenched in the hot-stamping process, to complete it.

In addition to the energy management of the front-end and body-side design, the front wheels also participate. They are aluminum and, unlike steel wheels that would maintain approximate shape and be pushed back into the cabin, they simply collapse on the impact. Mark Pafumi, Chief Engineer, Honda R&D Americas, said its small overlap test results were so good, the front doors still could be opened. As a result, Honda has full confidence the MDX will get a top rating.

In a head-on collision (full frontal), the crash energy is transferred from the bumper beam into Y-branched rails at each side and a center Y that flares into each of them and also goes down the center of the underbody.

The MDX, at 193.6 in (4917 mm) overall, is 2.0 in (51 mm) longer than its predecessor. But even with the new ACE structure for the safety enhancements, weight reduction was possible. The 2014 Super-Handling All Wheel Drive model has a mass range of 4255-4332 lb (1930-1965 kg), an average of 275 lb (125 kg) less than the 2013 model.

The body, thanks to a 55% high-strength-steel content, contributes 123 lb (56 kg) to the reduction. Seats are 44 lb (20 kg) lighter, front and rear subframes and rear suspension combine to chip in with a 41 lb (18.6 kg) saving, and the aluminum steering hanger beam was replaced by magnesium for a 7.5 lb (3.4 kg) drop. There is also a front-drive-only edition of the MDX, to be marketed in southern, snow-free areas as a lower priced, more fuel-efficient alternative. It tips the scales at 4025-4103 lb (1826-1861 kg).

NVH changes throughout

Based on customer feedback, a major effort was made to lower NVH. The increase in body stiffness of the new platform itself certainly helped, but a lot more was done. A rigorous study of body sealing led to a two-thirds reduction in air leakage paths through the body, noted Pafumi. The door openings were fitted with triple sealing, which compared with double-sealing further reduces the turbulence from air blowing in and exciting the volume of air between doors and body, he added.

In addition, 22 custom-designed structural foam inserts are clipped on, inside the pillars, and when the body is baked in the paint shop the foam expands to fill. This is more efficient than foam injection, Pafumi told AEI.

Tire noise, he said, is a major issue, and it's both air and structure borne. Improved sealing of the wheelhouse openings helped, but engineered upgrades to stiffen the structure of the openings were even more important, Pafumi said.

All exterior glass, he said, contains a layer of PVB (polyvinyl butyral) acoustic material, including the windows, and in addition, the glass panels are thicker.

Active noise control is used, but the MDX also incorporates a number of components to reduce powertrain- and suspension-induced NVH, including 28-V vibration-canceling engine mounts for the new direct-injection 3.5-L V6.

The front suspension is on a lighter but more rigid subframe, and the shock absorbers have a triple-path mounting to better diffuse road surface inputs to the body. The steering shaft now is a single piece, providing a level of rigidity that raised the natural frequency of vibration into the body from 47 to 80 Hz, outside the sensing range. The damper coupling previously used was eliminated, as engineers found that the electric motor system provides a similar effect.

The rear suspension also makes a considerable contribution to reduced vibration into the body. The trailing arm with five mounting points per side was replaced by a multilink, with just three, but to locations that are 67% more rigid. Additionally, the subframe mounts' locations are much stiffer, and at two points incorporate liquid-filled (glycol) bushings. The rear subframe also incorporates stays (metal brackets) in four locations to reduce axle "float" during steering inputs, so the new MDX tracks better.

Further, the rear shock absorber mount was relocated from the relatively thin wheelhouse (even with a stiffener) to the very rigid frame.

Trim levels wouldn't appear to be a factor, but the Advanced level on the MDX gets such NVH lowering additions as a thicker rubber noise barrier layer in the carpeting and use of more noise-reducing insulation, including in the fender liner.

AWD upgrades

Acura's active Super-Handling AWD system actually can transfer torque side to side vs. the simpler, more limited effect from using the anti-lock brake system to reduce torque to one side. However, both will be used on the 2014 MDX, working in sequence.

SH-AWD is more effective on acceleration through a turn, but in normal driving, the vehicle first slows down as it enters a turn. So the MDX starts with torque vectoring by brake and maintains it up to the approximate midpoint of a turn and then changes to the SH system of active torque transfer as the vehicle begins to accelerate through it.

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