2012 BMW 328i: Suspension Walkaround
The 2012 BMW 328i is the first example of the all-new 6th-generation 3 Series sedan (code name: F30) we've had in the shop.
It's hard to tell with the naked eye, but the footprint of BMW's volume seller has grown significantly. Longer overall by 4.3 inches, it now rides on a wheelbase that's 1.9 inches longer. And even though its overall width has shrunk by a scant 0.2 inches, the U.S.-spec 2012 328's tires have been spread farther apart by 1.2 inches up front and 2.3 inches out back. Despite all this, the curb weight of a 328i manual only increases by 44 pounds.
In CAFE terms, the footprint has been increased 5 percent, from 44.8 square feet to 46.9 square feet. That's interesting because cars with bigger footprints have less aggressive fuel economy increase targets. And the new CAFE standards only regulate the footprints you build in a given year, not the footprints you built last time out. At least in part, this size growth may be an example of strategic upsizing for CAFE purposes.
But such regulatory talk is boring. Let's see the metal.
Along the way you might want to open my walkaround of a 2009 BMW 3 Series in another window. That one was an M3, so you'll see more aluminum, but you can still see how the basic architecture differs...or doesn't.
Even though the detail dimensions are different, the basic 3 Series suspension philosophy remains. This is still a strut-based front suspension.
As before, this is a "dual link" strut, in that it uses two distinct links (green), each with their own ball joint, instead of a single lower control arm and one lower ball joint.
The front stabilizer bar is direct-acting via an elongated stabilizer link (yellow) that connects directly to the strut housing.
The key advantage of this arrangement is the ability to move the virtual steering pivot point outboard to a spot that would be physically impossible with a single ball joint. This in turn allows the engineers to reduce the scrub radius and do other cool things with steering geometry. Of course the "instantaneous virtual" intersection point shown above is constantly moving, so figuring it out properly is a bit more complicated than just overlaying a couple of yellow lines.
Pretty neat, huh?
Among all that aluminum there's a front suspension height sensor (green) and we can just see the bottom of the twin-tube front strut (yellow).
One of the "tells" that gives away the twin-tube strut is this (green) flattened section for tire clearance. And this mechanism (yellow) shows that this 3 Series is equipped with the "Adaptive M Sport" suspension with electronically controlled dampers. The height sensor we saw previously is one of several inputs that feed into the controlling ECU.
The 328i's brakes, meanwhile, are comprised of single-piston floaters (green) and ventilated rotors. One of the openings in the front grille (yellow) brings in cool air.
Peel back the diagonal hood seals on either side and you'll find a strut tower brace (yellow) that ties into the firewall.
Hard to tell what's going on in back from this angle. Nicely finished wheel well, though.
The rear of the new 3 Series is propped up by a 5-link multilink setup that's similar, but not identical, to last year's. It's almost impossible to see all five links in one shot, but we'll eventually catch up with all of them.
As we've seen many times before, the two upper links (green) approximate an upper A-arm. The forward-most of the two lower links (yellow) is visible, but the rearmost one is not because it's behind the shock. The slender link at the rear (orange) is the toe link.
From this vantage point we can see the missing link (yellow), a massive affair that also carries the spring and shock absorber. The toe link (green) we saw in the last shot sits just above it. Both of them have eccentric cams built into their inner attachment points; the lower one is for camber and the upper one is for toe.
This head-on view of the same links shows the eccentric cams a bit more clearly. It also shows us the motion ratio of the spring to be about 0.60-to-1 and that of the shock to be about 0.85-to-1, not unlike the outgoing generation.
The underneath view (my new favorite) shows just how the two lower links work together. None of this was all that visible until I first removed an aerodynamic cover that fastens to four tiny bolts that surround the spring pocket.
A smallish (but big enough) rear stabilizer bar (green) snakes between the upper links on the way to meet its own drop link.
That link is made of high-grade plastic and has ball joints on both ends. It attaches directly to the knuckle at a 1-to-1 motion ratio.
Right next door stands another twin-tube shock with electronically adjustable damping.
Wherever there are computer-controlled shocks there will also be suspension position sensors.
This view from the front shows the axle and the rear diff housing.
All of the 2012 BMW 328i's rear links are steel, but don't expect that to remain the case when we eventually see the M3 version of the F30 3 Series, whenever that is.
Like the front, the rear end of the 2012 BMW 328i is hauled down by single-piston sliding calipers and ventilated rotors.
Any 2012 BMW 328i equipped with the optional Luxury Line, Modern Line, Sport Line or the M-Sport package will, among other things, do away with the base model's 17-by-7.5-inch wheels and 225/50R17 tires in favor of 18-by-8-inch wheels and 225/45R18 rubber. In accordance with BMW habit these days, the tires are run-flats.
As you can see, the mounted 18-inch setup weighs 53 pounds per corner. Compare that to the 2009 BMW M3, where larger 265/40R18 tires and wider 18-by-9.5-inch rims weighed just 52 pounds. What's the difference? The M3's tires were not run-flats, which made them lighter. And before you go there, no, I don't think our M3 had forged wheels; we had settled for the standard 18-inch configuration instead of the optional 19-inch combo. If true, the difference we're seeing here (or lack thereof) is mostly down to tire weight.
Even though run-flat tires are not the best choice for unsprung weight and have not been a fan favorite for ride or noise in the past, the lack of a spare nevertheless helps BMW reduce the 3 Series' dyno "test weight" for CAFE measurement and fuel economy labeling purposes.
Dang. We're back in the midst of that regulatory stuff again. Sorry.
Photos by Scott Jacobs
Animation by Mark Takahashi