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How Suspension Works

Where comfort and control meet the road

Ask two drivers—a regular driver and a racer—to explain the purpose of suspension, and you may get two different answers. The regular Joe says it's easy: suspension keeps your teeth from rattling out over bumps. It's a comfort thing. The racer looks perplexed with that answer, and pipes up with the assertion that the job of suspension is to keep the wheels on the ground over bumps so you can drive faster. It's a control thing.

Surprise, surprise! They're both right.

In the early days of the automobile when speeds were low and engine power was decidedly limited, comfort was indeed the main point of designing mechanisms that allowed the wheels to bounce up and down furiously while the car's body rode along serenely above the commotion. By allowing the leaf springs that supported the weight of the body to flex freely, the harsh blows of early roads were softened substantially.

Seasickness was another issue, however, since springs rebound from bumps with nearly all of the enthusiasm of the original hit. Yes, the initial bump is more gentle, but it goes on and on. And on. The solution lies in some sort of mechanism that dampens out the bouncing motion. The design of this mechanism has evolved over the years, until we arrived at today's shock absorber. Though the design details vary considerably, the idea of a shock absorber is universal: It allows the suspension to move freely over bumps, but dampens out excessive suspension motion by using hydraulic resistance. This resistance is created by fluid friction inside the shock absorber. Friction makes heat in the shock absorber that is dissipated into the air.

Modern Tech

Today, the science of springs and shock absorbers is very sophisticated, with strategies like high-pressure nitrogen gas inside the damper for more constant performance and complex fluid controls to get just the right amount of resistance in every conceivable situation. Even some charmingly simple items like anti-roll bars (a.k.a. sway bars) live on in modern cars. These pivoting, flexible bars link the suspension on one side of the car to the suspension on the other side of the car to reduce body roll in corners. There are even active systems that replace the spring and damper with computer-controlled rams that lift the wheels precisely over bumps and set them back down seamlessly on the other side. All of these systems strive to accomplish the same basic thing, which is to let each of the vehicle's wheels absorb bump impact comfortably without causing any added handling upset to the vehicle.

There is more to suspension than springs and dampers, of course. The wheels can't be allowed to flail away crazily—they need some sort of structure to keep properly aligned with the chassis, and to answer the driver's steering commands even in the worst bumps. Even a fraction of a degree of momentary misalignment will steer a fast-moving car all over the road. The methods for accomplishing this precise alignment vary based on vehicle price and intended usage. The widely acknowledged best way to attach the wheels to the car is with double wishbones. Think Ferrari, Formula One, Corvette—you get the idea. Also known as upper and lower control arms, or upper and lower A-arms, this type of system has a pair of pivoting arms at each wheel that carry a spindle that supports the wheel. A spring and a damper control the motion of the arms. About the only bad thing anybody has been able to say about double wishbone suspension is that it takes up a great deal of room in the vehicle that might be otherwise put to use for people, cargo or other stuff, and it's complicated and expensive.

Alternative systems seek to address these issues. MacPherson strut suspension is the most commonly used these days. It has few parts, is relatively cheap to manufacture and takes up little space in a vehicle. The better versions of this system can be almost indistinguishable from double-wishbone suspension from the driver's seat. BMW uses MacPherson suspension for the front of its cars, and you don't hear many people complaining about THEIR ride and handling quality.

Since a car's rear wheels don't have to be steered by the driver, there is more variety in suspension design back there. Live-axle systems are descended from the dawn of the automobile; this Stone Age technology was everywhere up until a few decades ago and can still be found on cars like the Chevrolet Camaro and many trucks. It works, after a fashion, but with the rear wheels on opposite ends of a single axle, the downside is bumps on one side of the car also disturb the wheel on the other side. Cars with independently suspended wheels don't have this problem, and they ride and handle better as a result.

Allowing each wheel to follow road irregularities independently isn't just a comfort thing. Wheels that are airborne—even for an instant—can't do anything to make a vehicle accelerate, stop or turn. With modern engines, brakes and tires capable of imparting massive forces, the idea of individual wheel(s) checking out momentarily over pavement ripples is not an acceptable option. Our hypothetical racer would never stand for such an outrage. And come to think of it, neither would our Joe average driver when he's suddenly faced with a panic stop situation on a bunch of lumpy pavement patches. At moments like this, comfort ceases to be an issue, and suspension truly is all about control.