Why is my car so bouncy when I drive?

A vehicle that sways and bounces excessively while in motion is disconcerting for the driver, and also very dangerous.

Common reasons for this to happen:

Wheels are not aligned

Wheel alignment is one of the most important facets of automobile maintenance. The purpose of wheel alignment is to set the angle of the wheels so that your car is optimally set for the road. If one or more of your wheels is misaligned, your wheels will be angled incorrectly, and your car will have a hard time driving in a straight line. It will sway in one direction, and when you try and straighten the wheel, you’ll experience bounce in the drive.

Excessive and uneven wear on the tires:

As your tires wear, they change their size, shape, and angle. It’s relatively common for tires to wear unevenly on the inside of the tire, meaning that the tread on the outside of the tire is much thicker than that of the tread on the inside. This results in a tire that is tilted instead of standing up straight. This uneven wear has a similar effect to misaligned wheels, and results in a car that sways when it drives.

Loose steering linkage:

Your steering system contains a lot of elements. The rack and pinion, which exists in almost all modern vehicles, is the most comment element of the steering linkage to cause excessive bounce and sway in your car. If the rack bushings become either too loose or too stiff (such as if they get stuck), your car will sway or bounce. Other damage to the steering linkage can cause this problem as well, though the rack bushings are the most common culprit.

Damaged struts:

The struts are a vital part of your vehicle’s suspension, and connect the wheels to the body of the car. They can be attached to both the front and the rear wheels, and they feature shock absorbers and coil springs. The shock absorbers are cylinders filled with gas or liquid, and a piston. When the wheels experience impact from the road, the shock that the car feels is limited by the shock absorbers, as the piston moves in the cylinder, and is met with resistance by the liquid or gas. The coil springs further serve to reduce the amount of shock that the body of the car is subject to. When the struts are damaged, loose, or stuck, your car is subject to the full force and impact of the road. This causes the car to bounce with every imperfection in the road, and sway as it loses contact with the road.

Source: General Automotive