To get the most grip from a tire, its contact patch must be held flat to the road or performance is diminished. At first, this doesn't seem hard to do. Keep the tire perpendicular to the road as the suspension moves. But as the car rolls, the tire will lean in the opposite direction. This is known as the camber curve-the angle the tire maintains during a suspension's range of motion.
A wheel doesn't travel in a constant arc as it moves up and down. A suspension is composed of several members that pivot at both ends. Its range of motion is constrained by several different arcs. The MacPherson strut, in particular, has two moveable members that operate in two arcs. When a vehicle is lowered, the suspension is moved into an area the engineer did not intend. By placing the tire in the wrong section of the camber curve, there may not be the amount of camber gain needed as the car's body rolls. In the case of a car lowered by an extreme amount, it may actually gain positive camber.
One misconception is that positive camber occurs when the outer ends of the suspension arms are higher than the inner. In actuality, it is the angle that occurs between the suspension arm and the struts' operating motion that matters. When the angle between these two members exceeds 90 degrees, the wheel starts to gain positive camber.
Another problem with lowering a vehicle comes in the form of bumpsteer. Since a tie-rod also swings in an arc about a pivot on the steering rack, the tire tends to be steered through the range of motion. Lowering the vehicle may place the tie-rod at an angle that will exaggerate bumpsteer and make the car unstable on rough roads, as well as changing steering angle as the body rolls.
Lastly, something we need to think about is the vehicle's roll couple. The roll couple refers to the distance between a car's center of gravity and the suspension's instantaneous roll center. Using basic geometry, you can plot the axis about which the car's body will roll during cornering. Unfortunately with MacPherson strut set-ups, this point moves as the suspension moves. During cornering, the roll center is in a different location than when the car is static. Consequently, the roll center also changes with ride height.
When a MacPherson strut-equipped vehicle is lowered, the roll center will generally drop at a greater rate than the center of gravity, thereby increasing the roll couple. The effect of this is that the CG has a greater lever to act on rolling the car's body. This means either increasing spring rates, or anti-roll bar size, or both, to achieve even the same amount of roll as the factory set-up.
There are really only two solutions to the above problems. One is not lowering a car to the point where it affects the above constraints negatively. Adjustable ride height coilovers are ideal in this instance, providing all the performance advantage of stiffer spring and damping rates while allowing the owner to specify ride height.
The second option has been expensive historically, involving custom fabrication to modify the strut body and/or spindle. H2Sport has changed this for Volkswagen owners, but the company seems to be alone in the tuning world. With the popularity of the MacPherson set-up, more products of this kind should be available. Dampers and springs are still the first steps in changing a car's handling, then correcting geometry. Perhaps H2Sport will pioneer a whole new avenue.