Suspension Kinematics and Vehicle Dynamics
Suspension kinematics describes how each wheel moves and changes orientation as the suspension travels or the steering turns. Vehicle dynamics describes how forces, mass, tires, and those motions combine while the vehicle accelerates, brakes, corners, and crosses uneven surfaces.
Geometry and force are different views
A kinematic analysis starts with hard points, links, joints, and their permitted motion. It can calculate wheel travel, camber, toe, steering-axis position, motion ratio, and the geometric roll center without first assuming a particular spring or damper force.
A dynamic analysis adds mass, inertia, tire forces, springs, dampers, anti-roll devices, compliant bushings, actuators, and time. The result depends on speed and load as well as geometry. Two suspensions with similar static alignment can therefore respond differently because their wheel rates, damping, compliance, unsprung mass, or control systems differ.
For the physical layouts that create these motions, see Suspension Construction.
Wheel travel and alignment
Bump, also called jounce or compression, is suspension motion that reduces the distance between the wheel and body. Rebound, or droop, increases it. As a wheel travels, its camber and toe may change. Steering motion also changes the wheel plane and the tire contact patch relative to the vehicle.
Engineers choose these curves rather than judging only the alignment at normal ride height. The useful range must account for body roll, pitch, payload, braking, drive torque, road inputs, and the available clearance before a bump or rebound stop is reached.
Springs, dampers, and motion ratios
The wheel rarely moves the spring or damper by exactly the same distance. Installation position and linkage geometry create a motion ratio, so component stiffness and damping cannot be read directly as wheel stiffness and damping. The ratio can also change through the travel.
Springs carry load and set much of the ride-height and wheel-rate relationship. Dampers generate velocity-dependent force to control oscillation and transient movement. Anti-roll bars and active systems add forces that couple or independently control wheel and body motion.
See Suspension Springs, Suspension Dampers, and Active Suspension for the component-level mechanisms.
Body motion and load transfer
Heave is vertical body motion, pitch is rotation about the vehicle's lateral axis, and roll is rotation about its longitudinal axis. Suspension geometry and force elements control how the body responds, but they do not eliminate the load transfer created by acceleration and the height of the center of mass.
Roll centers, anti-dive, and anti-squat describe geometric force paths. They can change how much of a body response is carried through linkage forces rather than spring deflection. They do not remove the underlying longitudinal or lateral load transfer.
Compliance and real hardware
Bushings, mounts, links, subframes, and the body deform under force. Elastokinematics describes the alignment and position changes caused by that compliance. Compliance can be tuned to filter harshness or produce a helpful steering response, but excessive wear or damage can create unintended toe, camber, noise, and instability.
The complete suspension must therefore be evaluated across travel, steering, load, temperature, tolerance, and time. A single static dimension or peak force cannot describe its behavior.