2007 BMW 3-series

Driving and chassis

Improving on tradition: all-new chassis engineering

In concert with the more powerful engines, the chassis now has the benefits of BMW’s double-pivot front suspension to the 3 Series for the first time. The 5-link rear suspension further perfects the 3 Series’ unique blend of agility and stability. Chassis/body rigidity has been further improved and run-flat tires are standard equipment. Four-wheel ventilated disc brakes have been refined extensively and a new-generation Dynamic Stability Control system adds an array of new functions.

Double-pivot front suspension. Though familiar from BMW’s larger models (5,6 and 7 Series as well as the X3 and X5 Sports Activity Vehicles) the double-pivot front suspension concept is new to the 3 Series. It is a more elaborate system than that of the previous 3 Series models, featuring two lower arms (hence the name "double-pivot") that work in concert with the spring/shock absorber strut. The system provides:

• Small positive steering offset, for best steering feel and control under all road conditions. The two arms don’t actually intersect; if you visually extend their axes to a point where they do intersect, you find a "virtual pivot point" that is ideal for achieving this result. (Steering offset is the "lever arm when" through which road forces act on the suspension system.)
• Large steering caster, for outstanding stability in straight-line driving and excellent steering return action coming out of curves.
• Space for large brakes, by virtue of the two lower arms’ arrangement.
• Further advantages are found in the details:
•  The trailing (forward) lower arm has a rubber/hydraulic cushion, which provides highly effective "compliance" for enhancing ride comfort.
•  The transverse (rearward) lower arm is cushioned by a finely tuned rubber element that promotes direct and precise steering response in corners and curves.
•  Extensive aluminum componentry in RWD models for low unsprung weight. This improves the suspension’s response to bumps and other road irregularities; it can markedly improve riding comfort and, on any irregular road surface, handling as well. In these models, aluminum components include: both lower arms, steering knuckle, brake calipers and the brake shields.

In addition, the subframe that carries the front suspension of RWD models is made of aluminum, as is the steering rack. These components are not unsprung but do contribute to overall weight reduction and, being up front, also to the typical BMW near 50/50 weight distribution. The new-design subframe adds rigidity, in enhances steering precision and is also an integral element in the vehicles management of energy in a frontal crash.

Because their front suspension handles drive forces as well as those of steering, cornering and braking, the AWD "xi" models have mostly steel components in this area.

5-link rear suspension
In its concept, the 3 Series’ new 5-link rear suspension be described as a double-A-arm with an additional lateral track rod; the upper and lower A-arms are actually two links each, their vertical positions differing. This is analogous to the front suspension’s dual lower arms.

The advantages of this rear suspension system are numerous:

•  As at the front, there is a virtual pivot point for each pair of links, giving the engineers similar freedom in optimizing the system’s geometry for best handling. Also as at the front, the axis connecting these virtual points is configured to ensure that driving, braking and road forces all act effectively on short leverage. The result is very precise handling, especially insensitive to road disturbances.
•  Under cornering forces the system controls geometry in such a way as to achieve contradictory qualities; the degree to which they are mutually achieved is a measure of a suspension’s excellence.
•  Contributing to the outstanding geometry is the wide and rigid basis on which the rear tires are "planted" on the road. The lateral links are extremely rigid also, as is the subframe.
•  All suspension links connect to the subframe; no longer does any link pivot directly from the body structure. This further reduces the effects of road irregularities on riding comfort, and improves handling precision as well.
• The large and elaborate subframe enhances energy management in a crash, including the more severe rear-end impact that the new models are designed to withstand.

As always with BMWs, the final drive (differential) it is also mounted to the sub frame through rubber, creating acoustic decoupling that minimizes the transmission of driveline noises into the body. Widely spread mounts to the body, in an area where the body structure is particularly rigid, further help optimize the combination of precision and ride comfort. The rear suspension system, sub frame and brake calipers are all of steel.

Steering: evolution standard; revolution optional

The 3 Series’ engine-speed-sensitive variable-assist power rack-and-pinion steering system has generated universal praise. An auto magazine once described it as "nearly telepathic." This standard steering system continues essentially unchanged, although with a somewhat less direct overall ratio of 16.0:1. Yet numbers don’t always tell the entire story, for the new suspension and sub frame have already sharpened the 3 Series models’ reaction to steering inputs.

The "revolution" here is the newly optional Active Steering for RWD 3 Series models, previously available on the 5 and 6 Series.

xDrive: BMW’s "intelligent" all-wheel drive system

The 328xi Sedan and the 328xi Sports Wagon incorporate BMW’s advanced xDrive AWD and traction. xDrive operates as follows:

•  Driving torque is always transmitted to the rear wheels, and most of the time, to all 4 wheels.
•  The portion of the torque transmitted to the front wheels is controlled by a multi-disc clutch that can be fully open, fully engaged or at any degree of partial engagement in between. The torque split between rear and front wheels is steplessly variable.
•  Engagement pressure on the multi-disc clutch is directed by an electronic control system in response to actual road and driving conditions.

xDrive doesn’t just optimize traction; it can also enhance both agility and stability on grippy as well as slippery road surfaces. Via the same type of logic that Dynamic Stability Control employs to recognize and correct for excessive over-and understeer, xDrive adjust the front/rear torque split to avoid these tendencies. If undesirable oversteer is sensed, the multi-disc clutch is completely closed, sending the maximum possible torque to the front wheels. If excess understeer is detected, xDrive opens the clutch lines completely, sending no driving torque to the front wheels. In the BMW tradition, the driver enjoys optimum vehicle dynamics under a wide range of driving and road conditions.

Germany’s authoritative auto motor und sport (March 2, ’05) described and commented on xDrive in a new 5 Series Sedan this way: “Normally, the 530xi runs with the accent on rear-wheel drive: 60% of torque to the rear wheels, 40% to the front, but pure rear-wheel drive at speeds of 180 km/h [112 mph] or more. When traction is lacking or the vehicle tends toward instability, the torque split changes lightning-fast.

&ldquo

;The driver doesn’t notice any of this going on – but he or she does realize that on a slick road, the 530xi is very easy to drive. It has a high degree of stability."
Such comments would of course apply equally to a 3 Series model with this remarkable AWD system.

Engines

Safety