Degrees of Freedom

Every object in 3D space has six "degrees of freedom." Three of them are translational:  forwards/backwards (x-direction), side-to-side (y-direction), and up/down (z-direction). The other three are rotational: roll (rotation about x-axis), pitch (rotation about y-axis), and yaw (rotation about z-axis).  When it comes to vehicles, it makes sense to have the origin of the axis system be a the vehicle's center of gravity. 

• The engine and brakes control the vehicle's movement in the x-direction

• Steering controls the vehicle's movement in the y-direction, as well as yaw

• Springs are used to control the vertical motion of the car

• Pitch stiffness is used to control the pitch of the vehicle

• Roll stiffness is used to control the roll of the vehicle

As suspension designers, we are concerned with constraining all degrees of freedom except the longitudinal direction. We'll let other people worry about that one. 

The concept of degrees of freedom can also be applied to the wheel and tire. 

When designing your suspension, it is important to keep in mind how you want to constrain the wheel's degrees of freedom. This video explains how to do that with suspension links.