Suspension 101

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Suspension – What it is and What it Does

Webster’s Dictionary defines suspension as: n. Act of suspending; intermission; abeyance; deprivation of office or privileges for a time. In regards to your car, about the only thing in that definition you can apply is the “act of suspending”. The definition of suspend is: vt. To hang; to cause to cease for a time; to debar temporarily; to stay. Again, applied to your car, only the “to hang” section seems to fit. It seems, then, that the definition of suspension in automotive terms is “the act of hanging.” This slightly twisted definition is actually reasonably accurate, if you ask me, because, in my mind, suspension is what you use to hang the wheels on your car. It would also be accurate to state that suspension is used to hang the car on your wheels. While these last two statements seem to say the same thing a little differently, they actually have individual significance when looked at by themselves, and at some point in the future, I’ll explain why, but for now go ahead and file that bit of information in your gray matter somewhere.

So how do I define suspension? My definition is actually quite simple. Suspension is all the things that connect your car’s chassis to the wheels. Shock absorbers (more correctly called dampers), McPherson struts, coil springs, leaf springs, torsion bars, control arms, radius rods, spindles, knuckles, bushings, ball joints – the list is almost endless. I tend to lump everything into my definition, and then weed out what I don’t need at the moment. Even things like brakes and steering components might fit, depending on what I’m looking at. In some cases, I might even consider the tires and wheels part of the suspension, so my definition might actually be “all the things that ‘connect’ your car’s chassis to the ground.” (Hey, it’s my definition, and it may be a little vague, but I’m stickin’ with it!)

So now that we know what it is, or at least what I think it is, we can talk about what it does. I use two favorite words to describe what suspension does – handling and ride. In my book, handling is the most important function that the suspension has. Simply defined, handling is keeping the tires in contact with the ground. If the tires don’t have good, consistent contact, your car won’t accelerate, turn, or stop very well. On a smooth, straight road, the job is pretty simple. On a bumpy, twisting road, it can be a difficult task. The other job that the suspension has, ride, is the isolation of the chassis (including the driver, passengers, and cargo) from road inputs – by road inputs I mean anything that the tires may encounter - like bumps, dips, rocks, roadkill, etc… Both ride and handling have characteristics that can be measured, but ride is a more subjective thing. Some people may think your car rides too firmly to be comfortable, while some may think it’s too soft. Ride and handling are also usually related. Although not always the case, a car that rides more firmly usually handles better than the same car that has a softer suspension. The exception to this rule is usually at the extremes – a car that is too stiff will handle poorly, and a car that is too soft will ride poorly, in the sense that it makes you uncomfortable.

Measuring Handling

There are several ways to test handling, some more subjective than others. One of these tests, the skidpad, is a great place to measure ultimate “steady state” cornering ability. It’s a simple test, where you drive around a pre-measured circle at the very limits of traction. By steady state, we mean that we try to keep the conditions of the test constant, by keeping the throttle and steering wheel as steady as possible while trying to stay centered on the prescribed circle. Based on your elapsed time for one lap around the circle (with a running start), the lateral acceleration of the car can be determined. This quantity is expressed in terms of acceleration due to gravity, or “g”. One “g”, (1.0g) which is a benchmark of sorts, means that the car generates sideways grip that is equivalent to the weight of the car. I like to describe 1.0g by noting that if you were turning right at 1.0g with no seatbelt, the force pushing you against the driver side door would be enough to hold you up if the seat were removed from underneath you. Kind of like the amusement park ride that spins you around in a big drum and then drops the floor out from under you, leaving you pinned to the wall with no visible means of support.

Another test is the slalom, where cones are spaced at even intervals in a straight line. This is a great test of a car’s “transient” handling ability. By transient handling, we mean the ability of the car to transition, or change direction, in a controlled manner. The driver enters the slalom (again with a running start) and weaves back and forth through the cones as fast as possible without hitting cones or losing control, the time from the beginning to the end of the cones is measured, and an average mile per hour speed is determined.

A third test would be comparative lap times around a racetrack or other controlled course, which would create a combination of both steady state and transient handling conditions. The trick to getting good results in any of these tests, of course, is to have an experienced, consistent driver, otherwise, the results could be less than optimal.

Measuring Ride

Ride, being a more “subjective” subject, is a little more difficult to define, and each individual has his own perceptions. There are actually calculations that can be made, based on the weight of the car, the spring rates, and the shock absorber characteristics. The results of these calculations are compared to many years of data gathered by car manufacturers and testing facilities about what most people find to be comfortable or harsh. The most common calculation used to analyze ride is what’s referred to as ride frequency, which is actually a simple calculation that looks at the relationship between the spring rate and the vehicle weight. Ride frequency is usually expressed in terms of cycles per second (commonly known as Hertz, abbreviated Hz). If you've ever checked your shock absorbers using the “push down on the bumper and see if the car goes down and up more than once” test, you've taken a rudimentary step toward measuring your ride frequency. If the shocks are completely worn out, and the bumper goes up and down several times, it will do so at the ride frequency. This number is usually calculated mathematically, though, to eliminate other influencing factors.

While ride may be a personal preference issue, much has gone into the study of how the human body reacts to various inputs – certain parts of your body actually vibrate in harmony with certain frequencies that a car chassis can produce, sometimes with uncomfortable results. If you’ve ever been to a live concert and been close enough to the sound system that you really feel the music as much as you hear it, especially the heavy low notes that sometimes make your chest feel like it’s vibrating, you are experiencing the same kind of thing I’m talking about.

As always, I'll follow with another hub to continue. If you want some cylinder head modification basics, go to http://hubpages.com/hub/Cylinder-Head-Porting-101