Test Related Terms

Accelerometer – Sensor that measures linear accelerations (3-axis, typically) of an object

Ackerman - Steering kinematic characteristic where inside front wheel turns more than outside front wheel. 0% Ackerman is parallel steering; 100% Ackerman defines both wheels pointing at a common center; Anti-Ackerman is where the outside front wheel turns more than inside front wheel.

Anti-Dive – Front suspension kinematic characteristic which reduces the amount of pitch under braking forces; 100% anti-dive refers to no change in front suspension height under braking.

Anti-Lift – Rear suspension kinematic characteristic which reduces the amount of pitch under braking forces; 100% anti-lift refers to no change in rear suspension height under braking.

Anti-Roll Bar (aka Stabilizer Bar, Sway Bar) – Suspension device, usually a round bar with lever arms at the ends, which acts to reduce the roll angle of the vehicle under cornering loads. Important for adjusting the balance of the car

Anti-Squat – Rear suspension kinematic characteristic (on RWD vehicles) which reduces the amount of pitch under acceleration.

Bump (aka Jounce) – Upward movement of the tires with respect to the chassis

Camber Angle – Wheel inclination angle relative to vertical in front/rear view. Negative camber is defined as the tops of the wheels closer together than the bottoms.

Camber Compliance – Portion of camber change due to forces and moments on a tire (other than normal load)

Camber Gain – The amount of camber change induced by vertical movement of a tire

Caster Angle – The angle of the kingpin axis (line through the upper and lower ball joints) from vertical in side view. Provides stability, improves camber angle of outside wheel at high steering angles, and can affect corner weights as the wheel is turned.

Center of Gravity (aka Center of Mass, CG) - Location on the vehicle or sprung mass where all mass may be assumed to be located. Longitudinal position defines front/rear weight distribution, and Height defines lateral and longitudinal load transfer.

Coil Bind – When the coils in a spring make contact due to excessive compression

Critical Damping – The minimum required damping to eliminate overshoot and oscillation

Cross-Weight (aka Cross, Wedge) – Percent of vehicle mass supported by opposing corners (typically left rear and right front). Can be used for tuning cornering balance in oval tracks; 50% is desirable for road-race track configurations.

Droop (aka Rebound) – Downward movement of the tires with respect to the chassis

G's – Acceleration represented as a multiple of the acceleration due to gravity

Hysteresis – Term describing the tendency of measurements to change when made from opposing directions. For instance, friction in suspension bushings causes hysteresis in force measurements.

Loose – Slang term for oversteer

Mechanical Trail – Longitudinal distance between the centerline of wheel and the intersection of the kingpin axis and the ground. Pneumatic trail and mechanical trail are the largest components of steering effort.

Moment of Inertia (in Yaw, Pitch, and Roll) – Term analogous to mass except for rotation about vertical and horizontal axes; quantifies the resistance of a body to rotate. Moving redistributing mass toward vehicle center of gravity will lower the yaw moment of inertia make the vehicle more responsive to steering inputs.

Natural Frequency – The frequency of oscillation of a mass when not subjected to a continuous or repeated external force

Neutral – Handling characteristic between understeer and oversteer. This is usually the goal of chassis tuning, with a slight bias towards oversteer or understeer depending on conditions and driver preference.

Normal Load – The force on a tire exerted perpendicular to the road surface

Oversteer – Handling characteristic describing cornering instability due to a lack of grip from the rear tires; can happen in corner entry, mid-corner, or corner exit, and each situation warrants a different response from setup engineers

Pitch – Rotation about a horizontal axis perpendicular to the center-plane of a car; pitch happens during braking and acceleration

Pneumatic Trail – Longitudinal distance between the center of pressure of the tire contact patch and wheel centerline; results in a small self-centering effect

Potentiometer (Linear or Rotational) – Sensor that measures position via electrical resistance; used to measure suspension travel, throttle position, steering wheel position, etc.

Pushing (aka Tight) – Slang term for understeer

Ride Rate (Front or Rear) – The stiffness of a vehicle suspension in pure bump and rebound; higher ride rate results in higher natural frequency.

Roll – Rotation about a horizontal axis parallel with the center-plane of a car during cornering

Roll Camber – The amount of tire inclination induced by chassis roll

Roll Center (front, rear, or Roll Axis) – The point or line about which the chassis rotates when a roll moment is applied

Roll Couple Distribution (RCD) – See Weight Transfer Distribution

Roll Rate (aka Roll Flexibility) – Expressed as degrees of chassis roll per G of lateral acceleration; aka "degrees per G".

Slip Angle – The angle between the direction a tire is moving and the direction it is pointing. “Slip” is a misnomer because the difference in angles is caused mainly by deformation of the tire while rotating; low slip angles do not result in actual sliding.

Sprung Mass – The mass of the chassis of a vehicle that is isolated from the unsprung mass and road surface by the suspension system. Sprung mass is considered the mass of the vehicle chassis plus approximately half of the mass of the control arms, links, springs, shocks.

Steady State (Cornering or Acceleration) – For a given constant corner radius, vehicle speed and yaw rate are constant. Pitch and roll are constant, and there are no transient effects on the vehicle

Strain Gauge (or Strain Gage) – Sensor adhered to a component to measure load via the elastic deformation of the material. Load cells contain strain gauges.

Tire Contact Patch – The portion of a tire that is in contact with the road surface; reducing the load variation in the contact patch increases grip and handling stability

Toe Angle – Angle of wheels in plan view. Toe-in refers to the front edges of the wheels closer together than the rear edges. For front wheels, toe-out is stable; for rear wheels, toe-in is stable.

Toe Compliance – Portion of toe change due to forces and moments on a tire (other than normal load)

Toe Gain (aka Bump Steer) – Change in toe angle under bump/droop with no change in steering wheel position

Track Width (Front and Rear) – Lateral distance between tires, measured to the tire centers

Understeer - Handling characteristic describing cornering instability due to a lack of grip from the front tires; can happen in corner entry, mid-corner, or corner exit, and each situation warrants a different response from setup engineers

Unsprung Mass – Mass of components that are not isolated from the road surface by the suspension system: the wheel, tire, upright or strut, brake components, and approximately half of the mass of the control arms, links, springs, and shocks. Reduction in unsprung mass results in less variation in load on the tire contact patch, and therefore more consistent grip.

Weight Transfer (per G) – Force that is distributed to the outside tires during a corner

Weight Transfer Distribution – The ratio of weight transfer of the front and rear wheels, usually expressed as a percentage.  E.g., "60% front" means 60% of the total weight transfer of the sprung mass is transferred through the front suspension, and 40% through the rear. This is one of the major factors affecting balance, and is easily varied for tuning with anti-roll bars.  More front weight transfer distribution means more understeer; more rear means more oversteer.

Wheelbase – Longitudinal distance between the front and rear axles, measured at the tire center

Yaw – Rotation of a vehicle about a vertical axis

 

Motorsports Testing Glossary:
Amos Hine, Test Engineer
GEO Systems Technologies LLC.

References:
Malicky, David. "Suspension and Chassis Geometry"
Milliken, Douglas and William Milliken. Race Car Vehicle Dynamics