What's so special about the suspension on the 2011 Infiniti QX56? A lot, actually. Some of it's weird, some of it's brilliant. On the weird side, it has two rear springs per side. Brilliance comes in the form of a hydraulic body motion control system that replaces the stabilizer bars, just like we saw in the McLaren MP4-12C, but in this application there's one additional advantage.
The 2011 Infiniti QX56 has a few more tricks up its sleeve because it's no longer an SUV that's been adapted from the Titan pickup platform. It now rides on the home-market Nissan Patrol, the flagship SUV in Nissan's international lineup. That alone frees up development cash for more interesting mechanical bits.
On with the show...
Up front, the 2011 QX rides on double-wishbone suspension. The lower a-arm (white) is beefy and made of welded steel. A lighter stamped steel upper arm (yellow) can be used because its high-mount location reduces the magnitude of the loads transmitted to the upper ball joint.
A coil-over front spring and shock (white) is used, and the hydraulic rack and pinion steering acts in front of the axle centerline. What's missing? We can't see any sign of a stabilizer bar or its drop link.
That's because this particular 2011 Infiniti QX56 is fitted with the Deluxe Touring Package ($5,800) which includes, among other things, Hydraulic Body Motion Control. In english that translates to "a series of interconnected hydraulic lines to generate roll stiffness instead of stabilizer bars".
It's almost exactly the same as the admittedly prettier stainless steel setup we saw a few weeks ago on the McLaren MP4-12C. Like the McLaren, a pair of hoses (white, yellow) sprouts from the top of the shock absorbers: one for rebound and one for compression. It's not quite clear which is which from outside, but my money says the white one is the rebound one.
It doesn't really matter though because, like the McLaren, the compression hose here will span across the car to the rebound side of the shock on the other side, and vice-versa. This "cross-wiring" allows the natural pressure increase on the lengthening rebound side of the inner shock to "prop up" the shortening outer shock's compression side to quell that very motion (and vice-versa).
But here's the trick that makes this system even better. The front and rear circuits are hydraulically compared to one another in a junction box (hidden, unfortunately). If the front and rear circuit pressures follow the same trend, the system knows the car is cornering with the axles and body rolling in-phase with one another, and it allows the rising pressure to quell body roll as described above.
But if the front and rear axle pressure trends are going in opposing directions, the system knows you are off-road in a frame-twist wheel articulation mode. In this case the pressure build-up is vented off to accumulators so there is instead zero hydraulic roll stiffness, a move that allows maximum "opposite phase" wheel articulation. This is a trick the MP4-12C has no use for because, well, it's no off-roader.
The ability to disconnect stabilizer bars, hydraulic or steel alike, has another benefit. The engineers don't have to deal with the roll stiffness trade-off that all off-road machines must face. A disconnect system allows the fitment of much stiffer stabilizer bars (or roll stiffness via hydraulics) for cornering than would be possible on a "normal" SUV that doesn't have such a disconnect system.
And so this QX56 corners much, much flatter than a 3-ton SUV has a right to. It's pretty damn nimble, in fact. It does not wallow. None of my passengers got the least bit carsick.
The arrows show where the two hoses make a turn and begin their journey to the other side of the car. One of them (yellow) is going into a T-junction, one leg of which runs to the back of the car.
The two hoses that cross the car are protected by this rock guard (yellow). Meanwhile, the T-junction and the pipe that runs to the back of the car (white) are easier to see here.
There's nothing odd about the brakes, though. Twin-piston (yellow) sliding calipers, albeit large ones, handle the stopping chores, and they do it well.
Here's another view of the brakes. But there's a piece missing from the axle nut, and I only just noticed it as I sat down to do this. There's supposed to be a serrated "castle" retainer and a cotter pin to catch things if the axle nut starts to loosen for some reason. This one is missing for some unknown reason. It's a backup system, and the axle nut shouldn't loosen if it's properly torqued, by the cotter pin assembly should be there nevertheless. Ooops.
Even though there's a lot to take in, the rear suspension is, in fact, a double wishbone arrangement. There's a lower wishbone (yellow), and upper wishbone (green) and ... wait ... two springs? And what's that other link (black) for, the one with the coil spring?
Yep, there are two springs. The coil spring is the primary one, but the rubber bellows that surrounds the shock tells us this is an air spring. Because it's not the only spring, it's there for load leveling, either on account of passengers and cargo or the considerable tongue weights that accompany an 8,500-lb tow rating. The use of a supplemental air spring means the main coil can be softer for decent unloaded ride comfort.
This view makes it easier to see the upper and lower (black) wishbones as wishbones. The upper one (white) is particularly difficult to see because it wraps around behind the supplemental air spring/shock assembly.
Here's yet another look at the upper wishbone (white), the lower one (yellow) and the air spring (green).
Our lower wishbone is a wishbone because it has just one lower ball joint.
With one upper and one lower ball joint, that means the massive "link" that carries the coil spring must be a toe-control link. Yep. Weird, huh? Weird or not, it seems to work.
Here's another look at it. Note how the outside pivot of the toe-control link (yellow) is close to the same elevation as the axle nut. Note also that this axle nut has the little castle-thingy and cotter pin that were missing from the front.
A rubber bump stop (white) keeps the suspension from stroking too far: X marks the spot where it makes contact with the upper wishbone (yellow).
Like the front, the rear shocks feature a pair of hydraulic hoses (white, yellow) that are cross-linked to the other side of the rear axle. Again, rebound on this side (white) goes to compression on the other side, and compression on this side (yellow) is linked to rebound on the other side.
Meanwhile, the smaller hose (green) brings pressurized air in from the compressor to the donut-shaped air spring chamber that encircles the lower half of the shock.
The compressor sits next to the spare tire and has two outlet hoses; one for each side.
Height sensors are attached to the rear toe-links to monitor the position of the rear suspension. If a computer feels than rear end is compressed too much, it adds air and increases the ride height. Air is bled out once the suspension position changes to indicate that people got out or the trailer was disconnected.
Rear braking chores are aptly handled by single-piston sliding calipers. The protruding "hat" section of the rotor is a sign of a drum parking brake within.
Infiniti is so convinced that QX56 owners in the USA will never take this beast off road that they paired these redonkulous P275/50R22 low-profile tires with that trick Hydraulic Body Motion Control system in the Deluxe Touring Package. Yes, such rubber optimizes handling and the low roll angles generated by the hydraulic stabilizer bars. But they're not something you'd ever want to risk off-road, where sidewall is king, so the off-road benefits of the hydraulic stabilizer bars in "disconnect mode" will be more or less invisible.
Off-roaders can of course rectify this by minus-sizing down to 19- or 20-inch rubber, because the brakes aren't filling all the space available within the dubby-two inch rims. It might even be possible for 18-inch off-road tires to fit over the calipers, but don't quote me on that.
None of this matters now, because Infiniti is probably right about the off-road thing for the near term. But in 5 or 10 years, when these beasts drop in value enough for real off-roaders to consider them, the minus-size strategy will be alive and well.
Oh, and these 22 rims and tires weigh in at 83 pounds apiece. That and their low profile doesn't do cracked-road ride comfort any favors. The QX56 doesn't ride harshly, but there is a slight edge to, well, edges. Too bad you can't get HBMC with the stock 20-inch rubber.