Vehicle Dynamics

A central page to host all of my posts around the subject of vehicle dynamics.

Ackermann Angle – The toe-out or toe-in of a vehicle with Ackermann steering when the wheels are positioned straight ahead.

Ackermann Deviation – The difference between the calculated ideal Ackermann angle and the actual outside wheel angle.

Ackermann steering

Ackermann Steering – A double-pivoting steering system where the outer ends of the steering arms are bent slightly inward so that when the vehicle is making a turn, the inside wheel will turn more sharply than the outer wheel.

Bottom Out – The point at which the suspension reaches its designed limits of travel, causing it to contact stops built into the suspension or the chassis.

Bump – The motion of the suspension as the strut is compressing.

Bump Steer – A generally undesirable condition in which a wheel steers slightly as its suspension compresses or extends.

Damper – Another name for a shock absorber, a device used to reduce unwanted vibrations of a system.

1960s Ford Differential

Differential – Geared device between the driven wheels to allow varying rotational speeds of the wheels around corners. We see differentials in everything from the Chinese cart that has an arrow that always points south, in steam traction engines through to complex electromechanical differentials in formula one racing cars.

Dashpot – A damping device, usually consisting of a cylinder and a piston in which relative motion of either displaces a fluid such as air or oil, resulting in friction.

Droop – Total downward movement of a wheel from a static ride height.

Full Wheel Travel – The wheel is turned as far as possible from full left turn to full right turn, at full jounce and full rebound.

Gross Vehicle Weight (GVW) – The maximum loaded weight, including the vehicle itself, passengers, and cargo, for which a vehicle is designed and as specified by the manufacturer. Often used as a criterion of vehicle size for the purpose of legislation.

Hotchkiss Suspension – A live-axle rear suspension in which leaf springs handle both the axle′s springing and its location.

Independent Suspension – A suspension system that allows each wheel on a vehicle to move up and down independently of the other wheels. Allows a vehicle to provide a more level and stable ride.

Jounce – Wheel travel upward from design load position.

Jounce Stop – A mechanical limit to vertical wheel travel.

Lower control Arm – The lower suspension link between the steering knuckle and the frame. The arm pivots where it attaches at the frame to allow vertical movement of the steering knuckle.

MacPherson Strut – Suspension system which uses a lower control arm and a damper/strut which is designed to take bending loads. MacPherson struts are used in most front-wheel drive vehicles for compact packaging. They also allow relatively long springs that can increase suspension travel and increase bump absorption capability. When used on the rear suspension this is known as a Chapman Strut.

Roll Axis – Longitudinal line around which a vehicle rolls (joins front and rear roll centres).

Roll Centre – Invisible moving point around which a vehicle is considered to rotate in a corner. This is the only point on the vehicle centreline that does not move up or down during a turn.

CAD image of a shock absorber

Shock Absorber – A mechanical device designed to smooth out or dampen a sudden shock impulse and dissipate kinetic energy. The shock absorber turns kinetic energy into thermal energy within the working fluids of system.

Solid Axle – Suspension where left and right wheels are rigidly connected except torsionally.

Sprung Weight – The weight supported by the suspension, including a proportion of the suspension arms.

Tyre – The rubber part of the wheel which contacts the ground. The construction can be bias-ply, bias-belted, or radial. The plies are made of rayon, nylon, and polyester. Belting can be fibreglass, steel, or kevlar. The rest of the tyre is hard rubber.

Tyre Envelope – The surface that fully encloses the tyre in all positions that it can occupy at full jounce and rebound, from extreme left to extreme right turn.

Unladen Weight – The actual weight of a vehicle or combination with no payload.

Unsprung Weight – All parts of the vehicle that are not supported by the suspension system, wheels, tyres, brakes, etc

Watts Link – Lateral locating device for a solid axle.

Wheel Hop – The vertical oscillatory movement of a wheel between the road surface and the sprung mass of the vehicle.