2009 BMW 750i: Suspension Walkaround

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Today we're looking at the BMW 750i, this week's Car of the Week. Some of the front suspension might look similar to what we saw with the BMW M3, but a lot of it is radically different, including the entire rear suspension.

Here we're looking at the front suspension from the front. We can see that the lower end has the dual-pivot arrangement favored by BMW and an increasing number of others. It's therefore no surprise we see two aluminum lower links (yellow). The steering tie-rod (green) acts ahead of the axle centerline for enhanced steering precision, and the front stabilizer bar (orange) sweeps in underneath it all.

But this car doesn't have a strut suspension like you'd find in a 1-series or a 3-series. The 750i uses an upper wishbone (white) and a coil-over shock instead.

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Like other cars that use the dual-pivot, dual ball-joint design, the actual pivot is a virtual point in space that lies at the intersection of the axis of the two links.

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Here we're looking at the front suspension from behind, making it easy to see the gracefully curving aluminum alloy knuckle (yellow). It's also apparent the the coil-over shock mounts some ways inboard of the ball joint, meaning that motion ratio is less than 1:1. My eyeball estimate is about 0.7:1.

And like last week's M3, we can see that the 750i's big front brake calipers are in fact single-piston sliding (aka floating) calipers.

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The word "alloy" is thrown around too liberally, especially when it comes to auto parts. Most people mean "aluminum alloy" when they say or write it, but steel wheels are alloy wheels too, as steel is an alloy of iron and carbon (with other elements thrown in to tweak the material properties).

The particular aluminum alloy used in the steering knuckle is actually cast right into it for all to see. That's Aluminum, Magnesium, Silicon and Copper for those of you who forgot your Periodic Table.

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The amount of anti-dive geometry in the upper arm (represented by the inclination of the yellow line) isn't as pronounced as what we've seen in some of the Nissan's we've looked at. But the 750i is a luxury car with a longer wheelbase than those cars.

The 750i also has a die-cast aluminum upper structure that supports the upper shock mount, similar to what we saw in the Nissan GT-R.

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This close up of the upper arm pivot shows a sensor that monitors the position of the suspension at all times. All four wheels have one just like it.

A lot of the sophisticated systems this car has could not get along without theses babies. The front sensor's output is compared to the rear to constantly adjust the levelling of the HID headlights. Pitch and roll amounts can be inferred by looking at all four at once, and this is used by the central computer (along with other inputs, such as a steering angle sensor) to calculate the proper response of the computer-controlled adaptive dampers, active stabilizer bars and the rear-wheel steering.

They may even be wired into the anti-theft system. I forgot to unlock the car before jacking it up for some of these photos, and the alarm went off before the tire came off the ground.

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This close-up of the shock shows where the stabilizer bar's link attaches to the shock body (yellow). It may look like a direct-acting mount, but remember that the shock mounts inboard of the knuckle with a 0.7:1 motion ratio, so the stab bar's efficiency can be no better than that.

The white arrows point to an accelerometer pack (left) mounted directly to the shock body to monitor road inputs and a damping adjustment mechanism (right) that continuously alters the damping force based on the computed results from that input, other sensors and the position of the cockpit mounted switch used by the driver to specify the level of comfort or sportiness he or she wants from the suspension.

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This angle shows the various electronic aspects of the front suspension in one go: the suspension position sensor (yellow), the accelerometer pack (green) and a second (there are two per shock) damping adjustment mechanism.

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Here's another view of the massive single-piston sliding front brake calipers. Like the M3, two-piece rotors are used. But here the friction surface is pinned (white) to the aluminum hub in a slightly different way. Cooling air comes in from the front bumper through a generously-sized duct (orange).

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On to the rear suspension. And while it fits into the broad definition of multi-link, we haven't seen anything like it in previous walkarounds.

There are only three locating pieces: a broad h-shaped lower arm (yellow), a single upper link (white) and an active toe control link (green). Active, you say? H-arm? Read on.

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An H-shaped arm such as this should be able to fully locate the lower end of the hub, carrying lateral loads and acceleration loads while it defines the toe-in setting and the caster angle. Only one upper link would be needed to set the camber angle, and there should be no reason for a toe-control link at all. But this one is not a typical H-arm. For one, it only attaches directly to the knuckle in only one place (yellow). The other leg of the H connects instead to a short "torque link" (white) before attaching to the knuckle. Why, you ask? To facilitate the active toe control link. The 750i has rear-wheel steering, you see, and this little link allows some slight steering angle to occur. But the link keeps the knuckle from winding up when you gun the throttle or mash the brakes. Still don't follow me? You're not alone. All you need to know is that's it's very clever--and expensive. Maybe the next couple of photos will help it sink in.

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Here's another perspective that shows how the torque link attaches. That H-arm is hollow, by the way. $$$

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Another one of those suspension position sensors (yellow) is mounted to the upper surface of the H-arm. The single upper link (green) holds the wheel at the proper camber angle and acts as a conduit for myriad wires.

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Here we finally see that active toe-control link (white) that facilitates rear-wheel steering. It connects to the aluminum knuckle with a ball joint (orange) for the precise control needed to make this feature pay off. The rear shock mounts directly to the knuckle (yellow) for a 1:1 motion ratio, even though the chunky shape of the rear knuckle makes it appear otherwise. But the stabilizer bar mount isn't so lucky. Its mount isn't even halfway out along the lower H-arm, so it's motion ratio is a paltry 0.4:1 or so. The 750i gets away with it, for reasons we'll soon see. Our first hint is the massive and highly accurate stabilizer bar bushing (green).

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An electronic steering rack (white) shifts the steering arms in and out at the whim of the central computer. *Added June 26: The rear wheels steer no more than 3 degrees. At low speeds the rear wheels steer opposite the fronts to aid tight cornering into things like driveways and parking spaces. They steer in the same direction at high speed to add stability to maneuvers like freeway lane changes.

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Here is another view of the computer-controlled steering rack (white). But there's more. Those super-accurate stabilizer bar bushings we saw before support a hydraulically-driven active stabilizer bar. The hydraulic motor within can either de-couple or over-drive the bar, depending on what the computer decides after deciphering what the sensors say about driving agressiveness, road conditions and the driver's cockpit choice.

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Simple coil-over steel springs support the rear. Yawn.

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The lower end of the rear shock is a lot more exciting. Like the front, each rear shock also wears an accelerometer pack (yellow), and here we can see the two damping adjustment mechanisms. *Added June 26: Shocks always generate different levels of damping force whether they are compressing or extending (aka compression and rebound). Why? Imagine you just hit a bump. The spring itself helps to slow the upward motion of the wheel during suspension compression, so lower damping forces from the shock are sufficient to keep things in check. But the spring's unleashed stored energy wants to slam the wheel back down hard after passing over the bump, so higher damping forces are needed here to control the spring's influence and produce a soft landing. That's why the rebound forces generated by a shock or strut are typically 2 to 3 times greater than the compression forces they generate. But the amount of force needed varies greatly from bump to bump, as does the optimum ratio of compression to rebound damping force. That's where computer-controlled damping comes in. And in order to get the maximum benefit from computer-controlled damping, you need to adjust rebound and compression independently of one another. That's why we see two adjusters here--one for each.

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The 750i's rear brakes are comprised of single-piston sliding calipers that squeeze one-piece ventilated rotors.

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Finally we arrive at the tires, Goodyear Excellence run-flats. A few people swear by run-flats, but more and more are swearing at them, especially among the 3-series crowd. That's because they only do the run-flat trick because they have hyper-stiff sidewalls, and that can harm ride comfort and make a car feel skittish in corners, especially if those corners feature ripples, seams or cracks. But we're not seeing 3-series levels of that here with our 750i, in large part because of the sophistication of the adpative dampers and the active stabilizer bars we've just seen. For the record, the 245/45R19 front tires weigh 58 pounds when mounted on their rims, while the rear 275/40R19 tire and wheel assemblies weigh 63 pounds.

Dan Edmunds, Director of Vehicle Testing @ 11,234 miles