Tech Center

Suspension II: Still Rocking After All These Years

In the first installment of our series on suspension we looked at the basics of suspension design. We discussed the differences between solid axle and independent suspension systems, and we reviewed shocks and springs.

This time around we'll take a closer look at shock absorbers and spring arrangements. In addition, we'll examine the other key components that make up a suspension system.

As you may recall, shocks and springs form the bedrock of suspension design. These two components do almost all the work. Like most of us, they're overlabored, and hear only from the other components when something goes wrong.

Let's zoom in on springs for a moment.

More on Springs

Most passenger vehicles use, either singly or in combination, one of four spring systems: coil springs, leaf springs, air springs, and torsion bars. Let's look at coil springs first.

As we mentioned in the first article, coil springs are by far the most commonly used spring arrangement in passenger vehicles. Coil springs not only absorb road shock, they perform other duties as well.

For instance, coil springs support the car's weight (this is the real "suspend" in suspension), lifting the vehicle's body and frame above the wheels, keeping the two separated. When doing this, they also maintain the vehicle's height, or "stance," on the road, keeping the proper distance between the upper and lower halves. They also hold the other suspension components -- tires, shocks, ball joints, control arms -- in proper position, allowing the other parts to function properly. If a spring should go out of tolerance it will affect the performance of -- and eventually, the wear and tear on -- all the other components.

Coil springs can be cylindrical or barrel-shaped. Their advantages include low weight, moderate cost, and limited space needs. Another big plus: they require no maintenance. On the downside, they have limited load-bearing capabilities and require additional components (sway bars, control arms, bushings, shims, ball joints) to control wheel travel.

Leaf springs are generally found in the rear of large cars and trucks. The most common leaf-spring setup consists of a series of flat steel leaves bolted together to form a single unit. This design is called a semi-elliptical leaf spring. A U-bolt attaches the springs to the rear axle, and the two ends of the leaves are bolted to the bottom of the frame using spring shackles. The shackles allow the springs to "travel" in response to the car's motion.

Some cars utilize a variation on the traditional leaf-spring design. GM's Corvette, for instance, uses transverse leaf-spring suspension. In this arrangement, a single, high-performance leaf spring runs parallel to the rear axle, attaching to the wheel hubs at each end.

Leaf springs have excellent load-carrying traits, making them ideal for trucks and other heavy-duty vehicles. They also are better than coil springs at transferring forces from the road to the frame. One disadvantage: leaf springs sometimes require maintenance, as interleaf friction can cause noise. Plastic inserts will generally fix this.

Torsion bars represent a third kind of vehicle suspension. In this design, a steel bar takes the place of a leaf or a coil. As with the other spring setups, torsion bars operate by resisting motion; however, in this case, the resistance comes from a steel bar's opposition to twisting. One end of the bar is anchored to the vehicle's body, while the other is free to turn. A lever arm attaches to the free end of the bar, and then to a calibrated suspension device. When the vehicle strikes a bump, the bar's resistance acts like a spring.

Torsion bars are usually mounted longitudinally (lengthwise) under the vehicle, but are sometimes mounted transversely (crosswise). They have the advantage of working where a traditional coil spring or MacPherson strut may not fit, and also can be used to adjust a vehicle's height. They're cheap to manufacture, and maintenance-free. On the downside, torsion bars do not produce the smoothest ride.

In the last decade or so automakers have begun using air springs, our fourth kind of spring, on their higher-end vehicles. While this system retains shock absorbers, traditional springs have been replaced by four air springs, one on each wheel. An on-board air compressor, fed by sensors, continually adjusts the amount of air in the springs. As you can imagine, this can make for a very smooth ride, like you're floating on air.

Shock Absorbers, the Sequel

Shocks are a little less involved than springs. Because they have a very specific job to do, their design parameters are fairly narrow. Most shocks do the same thing, in the same way, only with different fluids. Aside from traditional shocks, we have MacPherson struts, gas shocks, and air shocks.

A shock absorber is designed to do two things: prevent excessive car body roll and eliminate spring oscillation.

It works like this. When a vehicle hits a bump, it compresses the spring. The spring's resistance keeps the axle from hitting the bottom of the frame. But now the spring wants to recoil -- to return to its original position. This "return" motion sets up something called an oscillation. (Think of the waves created when you rustle your hand in a pool of calm water.) The resistance -- there's that word again -- inside the shock absorber works to deaden, or "dampen" the waves of motion and return the spring to its starting point, in preparation for another bump.

The inside of a shock absorber resembles a cylinder and piston arrangement in an internal combustion engine. With one exception: a shock absorber piston has holes in it, called valves, through which oil flows from one side to the other. Because the valves are small in diameter, the oil can travel only so fast. Whichever direction the piston travels, in compression or extension, the fluid must pass from one side of the piston to the other. This resistance causes a deadening effect, which in turn kills spring oscillation. This is how a traditional shock absorber works.

MacPherson struts operate in a similar manner, except that the shock, or strut, has been placed inside a coil spring. As we've touched on earlier, this reduces space and makes for a very effective coupling between shock and spring.

Gas-filled shocks work like regular shock absorbers, except that the air inside the chamber has been replaced by a pressurized gas. The gas reduces aeration -- the tendency of the oil inside the shock to "foam" or bubble while squirting through the narrow valve. Aeration causes the shock to lag. Since a gas shock has less aeration -- fewer air bubbles -- it works better, producing a smoother ride.

Air shocks have a separate, sealed air chamber inside the shock absorber. Because this chamber is sealed, it is not subject to aeration. This produces an even better ride than gas shocks. But air shocks serve another, equally important purpose: they can be used for load leveling.

Here's how it works. The air chamber has a valve coming off it, just like a bicycle tire. It can be pumped up or deflated, depending on the condition. If, say, you own a Ford Explorer and you're planning on taking your ski boat out over the weekend, add some air to the rear shocks. This will help level the load and make for better steering control. When you return, deflate the shocks slightly, returning them to their previous position. (FYI: Most SUVs do not come with air shocks as standard equipment; it's typically an aftermarket add-on.)

Some vehicles, such as Lincoln Navigator, come with Automatic Level Control, a system of sensors and air shocks with an on-board air compressor. Similar in operation to the air springs discussed above, the system automatically senses the need to make an adjustment and inflates or deflates the air shocks accordingly. These sorts of systems are also widely available in the aftermarket. To investigate further, surf the Net under "air shocks," or pick up an RV magazine. Something to consider if you tow a boat or a trailer.

In Conclusion

You now know the basics of suspension design, including solid axle vs. independent suspension, as well the major kinds of springs and shocks. In fact, you now know everything we know. Hey, no fair!

In our next installment we'll take a close look at active suspension systems, a technology that utilizes air shocks and air springs and many of the things we've discussed here.

Incidentally, in preparation for Part Three, you may want to click on the link below. One of our editors took a two-day test drive of the then-new Mercedes-Benz CL 500 (it's a tough job, but somebody has to do it) and wrote a "First Drive." He came back fully stoked, shouting the praises of their ABC (Active Body Control) suspension system.