Jeff Lacina
October 8, 2008
Photos By: Team MM&FF

By now you should know that your muscle Mustang or fast Ford is more than a simple assortment of parts tossed under some shiny paint. Your entire car, from top to bottom, has been systematically designed to provide miles and miles of driving enjoyment. That is, until you get bit by the "I can make it better" bug that is embedded into most enthusiast's DNA.

Yes, most of us want to make our cars "our "cars, put our own performance tune under the hood and under the fenders. It doesn't matter if you're into road course driving, drag racing or simply carving up your favorite section of two-lane twisty, there are plenty of performance-enhancing parts available to us.

But, what you must remember before making any changes is that each and every one of your new part will have an affect on another part of your vehicle--this is especially true when it comes to the suspension. Everything must work together, modifying one part of the system without taking the other components into consideration can be a recipe for less-than-expected or desired results.

Your car's suspension has three basic functions; 1) Maximize the friction between the tires and the road surface; 2) Provide steering stability with acceptable levels of vehicle dynamics (acceleration, braking, direction stability, etc); and 3) Ensure the comfort of the passengers.

If a road were perfectly flat, with no irregularities, suspensions wouldn't really be necessary. But, we all know that roads are far from flat and there are plenty of irregularities to deal with, from small bumps, to speed bumps and potholes. Even freshly paved highways have subtle imperfections that interact with the tires of your 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 the wheel and suspension movement depends on whether the wheel is striking a giant bump or a small seam or crack. Either way, the wheel experiences a vertical acceleration as it passes over an imperfection.

Without an intervening structure (the vehicle's suspension), all of the wheel's vertical energy would be transferred to the frame or unibody, which would move in the same direction. In such a situation, the wheels would lose contact with the road completely. Then, under the downward force of gravity, the wheels would slam back into the road surface. This would certainly be uncomfortable, not to mention dangerous. What we need is a vehicle system that will absorb the energy of the vertically accelerated wheel, thereby allowing the frame and body to ride undisturbed while the wheels follow bumps in the road. This is one part of a suspension's job under your car-to maintain compliance between the tires and the road so you can maintain control of the vehicle.

Our engineer friends call the study of the forces at work on a moving car, Vehicle Dynamics. We need to have a basic understanding of these concepts in order to have an appreciation of why a vehicle's suspension system is designed and built the way it is. And this understanding is also important if you plan to modify (read: improve) the suspension and handling.

Automotive engineers consider the dynamics of a moving car from two perspectives: ride, which is a car's ability to smooth out a bumpy road, and handling, which encompasses a car's ability to safely accelerate, brake and corner. These two characteristics can be further described in three important principles: road isolation, road holding and cornering.

Let's take a quick look at these principles and how engineers attempt to solve the challenges unique to each.

Road isolation is defined as the vehicle's ability to absorb or isolate road shock from the passenger compartment. The goal is to allow the vehicle body to ride undisturbed while traveling over rough roads. The engineering solution is to absorb energy from road bumps and dissipate it without causing undue oscillation in the vehicle.

Next is road holding, which is defined as the degree to which a car maintains contact with the road surface in various types of directional changes and in a straight line. OK, now in English: the weight of a car will shift from the rear tires to the front tires during braking. Because the nose of the car dips toward the road, this type of motion is commonly called dive. The opposite effect is called squat and occurs during acceleration, which shifts the weight of the car from the front tires to the back. Here, the goal is to keep the tires in contact with the surface, because it's the friction between the tires and the road that affects a vehicle's ability to steer, brake and accelerate. The solution is to minimize the transfer of vehicle weight from side to side and front to back, as this weight transfer reduces the tire's ability to grip the road.

Last, but not least is cornering, which is the vehicles ability to travel a curved path. Here, the goal is to minimize body roll which happens as centrifugal force pushes outward on a car's center of gravity while cornering, causing one side of the car to raise, and the other lower. Our engineer friends try to transfer some of the vehicle's weight from the high side to the low side during corning maneuvers.

Whew... ya still with me? We now it's a lot so get out there and drive with your enhanced understanding of what is keeping you and your vehicle on the road.