From the June 2017 issue
How, you may wonder, do dynamics engineers determine what kind of hardware to use in a suspension? Here’s the short version: Other people tell them. Engineers must work within the constraints of the basic suspension arrangement dictated by packaging requirements, budget, and the vehicle architecture. But there’s plenty of tweaking to be done: After collecting benchmark data from the kinematics-and-compliance machine, engineers define a set of K&C targets for their car. They then collaborate with suspension designers to create front and rear geometries that mimic those attributes, altering mounting points, bushing stiffness, link and arm design, and other variables. Here are the five most common suspension configurations found in today’s vehicles:
Using the axle housing to locate the wheels is as durable as the idea itself, which is why this ox-cart technology persists in off-roaders, pickups, and commercial vehicles. The obvious flaw: a bump at one wheel also excites the opposite wheel. When a solid axle connects two driven wheels—also known as a “live axle”—the axle shafts, differential, and housing all contribute to unsprung weight, affecting ride quality and aggravating axle hop under acceleration and braking, particularly in high-torque vehicles.
A pair of lateral arms, sometimes called double wishbones or A-arms, offers better control over the kinematics than a strut-type arrangement. Among the benefits: an upper arm that’s shorter than the lower arm to optimize the orientation of the tire contact patch as the body rolls, increasing lateral grip. Control arms also require less height than a strut suspension—the better to slip under the low hoods of sports cars such as the Acura NSX and the Chevrolet Corvette.
The greatest sophistication and tuning flexibility comes from using a combination of links and arms, or just five individual links. One common arrangement includes three lateral links for side-to-side wheel location, one longitudinal link for fore-aft restraint, and a toe-control link that effectively makes minute steering adjustments as the suspension strokes. The multilink approach allows for higher lateral stiffness and the desired toe change with appropriate vertical and longitudinal compliance. Multilink setups can also be designed to better resist dive and squat under braking and acceleration, respectively. Put simply, multilink suspensions offer the most separation between handling and ride-quality attributes to reduce compromises.
Frequently found at the rear of economy cars, this arrangement uses trailing arms integrated with a crossmember that is designed to twist as the wheels move. While the torsion beam or “twist beam” isn’t as compromised as a solid axle, neither is it a truly independent suspension. Stiffer bushings can compensate for the torsion beam’s inherent side-to-side compliance, but that comes with a toll of greater impact harshness. (Some vehicles use a Watt’s linkage or a Panhard rod to improve lateral stiffness without compromising ride quality.) Low mounting points along with springs and dampers that are mounted farther outboard than in other arrangements create more interior and cargo space.
In this, the most popular front-suspension setup, a beefed-up damper (typically with a concentric coil spring) serves double duty, acting as a locating device while calming vertical movement. Strut configurations are commonly chosen for their simplicity and cost, and they’re narrower than control-arm and multilink arrangements, making them ideal for transverse-engine cars. However, the strut design limits an engineer’s options to optimize camber as the wheel moves vertically.
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