Almost 60 years ago, a French-made Ford Vedetta rolled off the assembly line as the first car equipped with a groundbreaking new suspension, invented by Earle S MacPherson. Since then, variations and evolutions of the MacPherson strut have been used on everything from Golfs to 911s, becoming the most common suspension for all types of cars. Originally designed as a way to emulate the performance of an unequal-length A-arm suspension in a smaller package, the current design is still similar to the original.
It consists of two moving members: a lower control arm and a strut assembly. The strut assembly contains the damping unit, using what would normally just be a shock absorber as a locating member of the suspension. The strut operates in a telescoping motion, linear to the movement of the strut. The top pivots in what is usually a flexible rubber bushing (known as a strut mount) while the bottom is either bolted to, or is integrated with the spindle, mounted with a ball joint at the outer end of the lower control arm. The inside pivot point of the lower control arm is mounted nearer to the center of the vehicle, either directly to the body or on a subframe. The most common set-up uses a coil spring over the strut, but cars such as the pre-1990 911 had a torsion bar, basically an uncoiled spring.
Enthusiasts have always liked the MacPherson design because of its simplicity and low weight. Higher performance dampers and a variety of spring rates make it easy to tune. One of the big drawbacks, however, is that it functions best within a specific range of operation. Drastically lowered cars will quite often handle worse than they did at stock height. For many years, this was something enthusiasts were willing to trade for a car that looked like it handled well.
Suspension tuning in the past consisted almost entirely of changing spring and damping rates. Higher spring rates by themselves make a car more resistant to roll, squat and dive, but will also create a less controlled, bouncy ride. Damping rates also need to be raised in order to control body movement and dissipate the extra energy from the stiffer springs.
In the early days, enthusiasts were forced to buy sport springs off the shelf and find a set of dampers that would work with them. Tuners then began making matched sets of shocks and springs that functioned well but didn't give the enthusiast much of a range of handling options. Adjustable dampers soon hit the market. These allowed enthusiasts to match damping rates to the particular spring they were using, as well as tuning handling characteristics to some extent. In recent years, enthusiasts have been leaning more towards adjustable coilovers.
Using a spring mount or perch, adjustable coilovers allow variations in ride height and, more importantly, facilitate corner-balancing. Ideally, a car's weight should be balanced equally at all four corners. Since cars are not symmetrical, either front to back or side to side, the weight is not spread evenly. By raising and lowering corners individually, the enthusiast (or more likely, the suspension tuner) can adjust weight balance with respect to each wheel.
Lowering the center of gravity will improve a car's handling, but most people don't understand why. The lower the center of gravity, the less impact it has on a vehicle's tendency to roll during cornering. It also affects the amount of weight transfer associated with this roll. The higher the center of gravity, the more weight is transferred from tire to tire for a set amount of body roll. The problem when lowering a MacPherson strut suspension is that it comes with a host of problems in the form of geometry changes.