InsideLine: WalkAround
05-28-2010, 02:54 PM
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Safety Car
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InsideLine: WalkAround
I know what you're thinking. "Acura ZDX? Didn't you just do a walkaround on the Honda Accord Crosstour, Honda's other sloped-back hatchback crossover thingamabob? Aren't they badge-engineered twins?"
I know this because I assumed it myself when I first saw the ZDX some months back. I know this because a Honda employee I know asked approximately the same question just two days ago. OK, he works for the motorcycle division, but still...
It's an easy mistake to make. I mean, who would go to the expense of engineering two totally different but similar-looking versions of a car for a segment that doesn't yet exist? No one really knows if people will buy five-door hatchback crossovers with all-wheel drive because the jury isn't merely out, it hasn't even been empanelled yet.
Nevertheless, Honda came out with two cars with this odd form-factor at about the same time, and it turns out they couldn't be more different under the skin.
Let's have a look, already.
Right off the bat, we're seeing a huge difference. The Accord Crosstour uses double-wishbone front suspension, but the ZDX's front end is suspended by MacPherson struts.
In addition to the strut, there's an aluminum lower control arm and a 2-piston sliding brake caliper. The stabilizer bar and its long, slender link (yellow) attach to the strut body in a direct-acting fashion.
Just like every other strut that came before it, the upper end of the ZDX's coil-over strut bolts to reinforced "shock towers" in the unibody via a Lazy-Susan top mount above the spring so the whole assembly can rotate as the wheels are turned.
Here's a close-up of the L-shaped aluminum control arm. The main component of most road impacts is vertical, and these loads generally go straight up into the spring and damper without much effect on the arm. But the fore-aft load component, which can be significant, goes straight into these bushings.
The forward bush and its heavily-reinforced mounting pocket are designed to tackle the fore-aft component, head on. But since the source of the load is far outboard at the ball joint, this bush also acts as a pivot around which the arm tries to twist back. Enter the rear bush, which is optimized to absorb the resulting in-out loads.
Because the ZDX has a transverse-mounted engine, the steering rack (green) does its work from behind the front axle centerline. It would take fewer words to say "rear-steer" but some of you might start thinking about forklifts. We can't have that.
Meanwhile, the front stabilizer bar (yellow) sneaks behind the lower control arm on its way to its connecting link.
Obvious high-pressure hydraulic lines (yellow) are a hallmark of hydraulically-assisted rack-and-pinion steering. But this low-pressure line (green) is far less universal. So, where does it go?
It goes to a power-steering cooler. There isn't a whole lot of heat to get rid of, so a single-pass heat exchanger (yellow) is enough to do the trick.
Here's another look at the 2-piston sliding brake caliper and the ventilated one-piece brake rotor it squeezes.
Moving to the rear, we once again see that the Acura ZDX shares nothing whatsoever with its similar-looking crossover-hatchback cousin, the Honda Accord Crosstour. If you remember, the Crosstour used a Honda Accord multilink setup featuring three lower links, an upper wishbone and a coil-over shock.
We don't see any of that here. The ZDX instead borrows from the Honda Pilot SUV, and as such its multilink setup uses a trailing arm (yellow), a pair of lower links (white and orange), and a single upper camber link (green). There's no coil-over spring in sight.
Instead of a coil-over, the ZDX's spring operates below the body, where it's cradled about halfway along the swollen lower link at something like a 0.5:1 motion ratio. The shock and stabilizer bar attach at points further out. In our Crosstour, all of these attached directly to the outboard knuckle for a 1:1 ratio, a move that allowed them to be lighter. Not so here.
The plus side of this arrangement is less intrusion into the cargo area. But the ZDX's Pilot roots show up in the ride quality -- the suspension doesn't "breathe" nearly as much as the Crosstour. This could be a result of an intentional tuning strategy designed to give the ZDX a sportier feel. Whatever the reason, the Acura has a more stiff-legged ride than the Crosstour.
Here's another view of the ZDX's rear spring and the urethane bump-stop contained within its coils.
Let's see those links and arms again. There's the main trailing arm (yellow) that locates the wheel in the fore-aft direction. Down below we have two lateral links, a larger and longer main one (orange) and a shorter toe-link (white). Up top, the camber link (green) defines the camber axis at the top end.
The trailing arm is bolted rigidly to the knuckle, though the nuts themselves are hidden on the back side.
The ZDX has single-piston sliding calipers, solid-disc rear rotors and a drum-in-hat parking brake.
Compared to the Crosstour and its 225/60R18 rubber, the Acura ZDX rides on significantly larger P255/50R19 Michelin Latitude Tour all-season tires. They weigh 62.5 pounds when mounted, some 8 pounds apiece heavier than the Crosstour's shoes.
This transfer-case-that-looks-like-a-PTO is the first ZDX component that looks remotely similar to the corresponding Crosstour piece.
Once we get to the business end at the back, however, the similarities end.
Instead of the Crosstour's single hydraulically-engaged clutch on the input end that turns rear-wheel drive on and off based on front/rear axle speed differential, the Acura ZDX has permanent all-wheel drive regulated by two computer-controlled electromagnetic wet clutches, one each on the left and right sides of the differential.
That PTO we saw before overdrives the rear axle so it spins 1.7-percent faster than the front, and that means there's always a little bit of slip in these clutches. But this offset also allows the clutches, when fully engaged in unison, to deliver significantly more than 50-percent of the overall drive torque to the rear wheels. Fine control of the left-right clutch offset is used to create a limited-slip effect through the otherwise open differential, but the system also can create large left-right offsets to send as much as 100% of the rear-wheel torque to the outside wheel during aggressive cornering, something we call torque-vectoring but Acura calls Super-Handling All-wheel Drive.
Actually, they call the whole thing SH-AWD, but the torque vectoring trick is the "Super-Handling" part.
So, there you have it. Even though it looks like a badge-engineered brother to the Crosstour, the 2010 Acura ZDX is another animal entirely.
But did the market really need two of these (three, if you include the BMW X6)? Honda saw fit to develop two entirely different cars around the concept, so we can only assume they recorded some slam-dunk focus group results in product development clinics before they green-lighted these projects.
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