The most common mistake I see when modifying a car is lowering the car too much. We all agree that cars look better lowered, but if they are lowered too much, it adversely affects the handling of the car which is the reason we lowered it in the first place.
Many bad things happen when you lower a car too much:
Travel: The worst thing is that you run out of suspension travel. This causes poor handling and poor ride quality.
Geometry: Lowering the car changes the height of the roll center and the sprung mass. Sometimes the roll center is moved so much that the car rolls more after it has been lowered than before it was lowered. In addition, sometimes the roll center becomes unstable, resulting in jacking forces that make the handling unpredictable. This is especially true on MacPherson strut cars.
Bump Rubbers: The bump rubbers not only protect the suspension and the car as it bottoms out over a large bump, they are also a progressive spring. They are carefully calculated to allow you to get a good ride under normal driving, and better handling when you go faster. It accomplishes this by having around (.5” – .75”) of travel when cruising around slowly. When the progressive spring is not in contact, the spring rate is softer, giving you a better ride. When you corner hard, the suspension uses up travel as the car rolls, engaging the bump rubber and stiffening the spring rate thereby giving you sporty handling. When you lower the car, this messes this relationship up. Often times suspension manufacturers will tell you to cut the bump stops shorter. This does not work, because if you cut the stiff end of the progressive spring, there is not enough stiffness and the car will get very soft and actually roll more. In addition, it will bottom out much harder because it will blow through the bump stop when you hit a large bump. If you cut the soft end, the bump stop will be less progressive, and you will feel it contact which upsets the handling of the car and hurts ride quality. The correct solution is to provide new bump stops with the suspension kit. Many manufacturers do this correctly, however, you have limits. This is because it takes travel for the bump stop to be progressive, so you can usually only shorten them around .5” and still have a nice progressive spring. Depending on the motion ratio, you can usually only lower a car (.5” – 75”) and still maintain good handling and ride quality.
Below is a bump rubber stiffness chart from a test we conducted during suspension development. Note the increase in stiffness as suspension travel increases.