When people think of automobile performance, they normally think of horsepower, torque and zero-to-60 acceleration. But all of the power generated by a piston engine is useless if the driver can't control the car. That's why automobile engineers turned their attention to the suspension system almost as soon as they had mastered the four-stroke internal combustion engine.
The job of a car suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. In this article, we'll explore how car suspensions work, how they've evolved over the years and where the design of suspensions is headed in the future.
If a road was perfectly flat, with no irregularities, suspensions wouldn't be necessary. But roads are far from flat. Even freshly paved highways have subtle imperfections that can interact with the wheels of a car. It's these imperfections that apply forces to the wheels. According to Newton's Laws of Motion, all forces have both magnitude and direction. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The magnitude, of course, depends on whether the wheel is striking a giant bump or a tiny speck. Either way, the car wheel experiences a vertical acceleration as it passes over an imperfection.
A car's suspension, with its various components, provides all of the solutions described.
Let's look at the parts of a typical suspension, working from the bigger picture of the chassis down to the individual components that make up the suspension proper.
Car Suspension Parts
Another common dampening structure is the strut — basically a shock absorber mounted inside a coil spring. Struts perform two jobs: They provide a dampening function like shock absorbers, and they provide structural support for the vehicle suspension. That means struts deliver a bit more than shock absorbers, which don't support vehicle weight — they only control the speed at which weight is transferred in a car, not the weight itself.
Because shocks and struts have so much to do with the handling of a car, they can be considered critical safety features. Worn shocks and struts can allow excessive vehicle-weight transfer from side to side and front to back. This reduces the tire's ability to grip the road, as well as handling and braking performance.
Sway bars (also known as anti-roll bars) are used along with shock absorbers or struts to give a moving automobile additional stability. A sway bar is a metal rod that spans the entire axle and effectively joins each side of the suspension together.
When the suspension at one wheel moves up and down, the sway bar transfers movement to the other wheel. This creates a more level ride and reduces vehicle sway. In particular, it combats the roll of a car on its suspension as it corners. For this reason, almost all cars today are fitted with sway bars as standard equipment, although if they're not, kits make it easy to install the bars at any time.
Suspension Types: Front
So far, our discussions have focused on how springs and dampers function on any given wheel. But the four wheels of a car work together in two independent systems — the two wheels connected by the front axle and the two wheels connected by the rear axle. That means that a car can and usually does have a different type of suspension on the front and back.
Much is determined by whether a rigid axle binds the wheels or if the wheels are permitted to move independently. The former arrangement is known as a dependent system, while the latter arrangement is known as an independent system. In the following sections, we'll look at some of the common types of front and back suspensions typically used on mainstream cars.
Dependent front suspensions have a rigid front axle that connects the front wheels. Basically, this looks like a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. Common on trucks, dependent front suspensions haven't been used in mainstream cars for years.
In this setup, the front wheels are allowed to move independently. The MacPherson strut, developed by Earle S. MacPherson of General Motors in 1947, is the most widely used front suspension system, especially in cars of European origin.
The MacPherson strut combines a shock absorber and a coil spring into a single unit. This provides a more compact and lighter suspension system that can be used for front-wheel drive vehicles.
For the most part, this article has focused on the suspensions of mainstream front- and rear-wheel-drive cars — cars that drive on normal roads in normal driving conditions. But what about the suspensions of specialty cars, such as hot rods, racers or extreme off-road vehicles? Although the suspensions of specialty autos obey the same basic principles, they do provide additional benefits unique to the driving conditions they must navigate. What follows is a brief overview of how suspensions are designed for three types of specialty cars — Baja Bugs, Formula One racers and American-style hot rods.
The Volkswagen Beetle, or Bug, was destined to become a favorite among off-road enthusiasts. With a low center of gravity and engine placement over the rear axle, the two-wheel-drive Bug handles off-road conditions as well as some four-wheel-drive vehicles. Of course, the VW Bug isn't ready for off-road conditions with its factory equipment. Most Bugs require some modifications, or conversions, to get them ready for racing in harsh conditions like the deserts of Baja California.
One of the most important modifications takes place in the suspension. The torsion-bar suspension, standard equipment on the front and back of most Bugs between 1936 and 1977, can be raised to make room for heavy-duty, off-road wheels and tires. Longer shock absorbers replace the standard shocks to lift the body higher and to provide for maximum wheel travel. In some cases, Baja Bug converters remove the torsion bars entirely and replace them with multiple coil-over systems, an aftermarket item that combines both the spring and shock absorber in one adjustable unit. The result of these modifications is a vehicle that allows the wheels to travel vertically 20 inches (50 centimeters) or more at each end. Such a car can easily navigate rough terrain and often appears to "skip" over desert washboard like a stone over water.
Specialized Suspensions: Formula One Racers
The Formula One racing car represents the pinnacle of automobile innovation and evolution. Lightweight, composite bodies, powerful V10 engines and advanced aerodynamics have led to faster, safer and more reliable cars.
To elevate driver skill as the key differentiating factor in a race, stringent rules and requirements govern Formula One racecar design. For example, the rules regulating suspension design say that all Formula One racers must be conventionally sprung, but they don't allow computer-controlled, active suspensions. To accommodate this, the cars feature multi-link suspensions, which use a multi-rod mechanism equivalent to a double-wishbone system.
Recall that a double-wishbone design uses two wishbone-shaped control arms to guide each wheel's up-and-down motion. Each arm has three mounting positions — two at the frame and one at the wheel hub — and each joint is hinged to guide the wheel's motion. In all cars, the primary benefit of a double-wishbone suspension is control. The geometry of the arms and the elasticity of the joints give engineers ultimate control over the angle of the wheel and other vehicle dynamics, such as lift, squat and dive.
Unlike road cars, however, the shock absorbers and coil springs of a Formula One racecar don't mount directly to the control arms. Instead, they are oriented along the length of the car and are controlled remotely through a series of push and pull rods. They translate the up-and-down motions of the wheel to the back-and-forth movement of the spring-and-damper apparatus.
Specialized Suspensions: Hot Rods
The classic American hot rod era lasted from 1945 to about 1965. Like Baja Bugs, classic hot rods required significant modification by their owners. Unlike Bugs, however, which are built on Volkswagen chassis, hot rods were built on a variety of old, often historical, car models: Cars manufactured before 1945 were considered ideal fodder for hot rod transformations because their bodies and frames were often in good shape, while their engines and transmissions needed to be replaced completely. For hot rod enthusiasts, this was exactly what they wanted, for it allowed them to install more reliable and powerful engines, such as the flathead Ford V8 or the Chevrolet V8.
One popular hot rod was known as the T-bucket because it was based on the Ford Model T. The stock Ford suspension on the front of the Model T consisted of a solid I-beam front axle (a dependent suspension), a U-shaped buggy spring (leaf spring) and a wishbone-shaped radius rod with a ball at the rear end that pivoted in a cup attached to the transmission.
Ford's engineers built the Model T to ride high with a large amount of suspension movement, an ideal design for the rough, primitive roads of the 1930s. But after World War II, hot rodders began experimenting with larger Cadillac or Lincoln engines, which meant that the wishbone-shaped radius rod was no longer applicable. Instead, they removed the center ball and bolted the ends of the wishbone to the framerails. This "split wishbone" design lowered the front axle about 1 inch (2.5 centimeters) and improved vehicle handling.
Lowering the axle more than an inch required a brand-new design, which was supplied by a company known as Bell Auto. Throughout the 1940s and 1950s, Bell Auto offered dropped tube axles that lowered the car a full 5 inches (13 centimeters). Tube axles were built from smooth steel tubing and balanced strength with superb aerodynamics. The steel surface also accepted chrome plating better than the forged I-beam axles, so hot rodders often preferred them for their aesthetic qualities, as well.
Some hot rod enthusiasts, however, argued that the tube axle's rigidity and inability to flex compromised how it handled the stresses of driving. To accommodate this, hot rodders introduced the four-bar suspension, using two mounting points on the axle and two on the frame. At each mounting point, aircraft-style rod ends provided plenty of movement at all angles. The result? The four-bar system improved how the suspension worked in all sorts of driving conditions.
For more information on car suspensions and related topics, check out the links below.
