Definitions – Part 1

Force – an influence on an object, capable of producing or tending to produce a change in movement.

Suspension Bump – Occurs when either physical track changes, such as bumps or body roll due to cornering causes the suspension to travel upward from the set ride height.

Suspension Droop (or Sag) –Occurs when either physical track changes, such as holes, grooves or body roll due to cornering causes the suspension to travel downward from the set ride height.

Suspension Travel – This is the total distance that the suspension moves from the full height of Suspension Bump to the full drop of Suspension Droop.

Jounce Bumper – This is a rubber, or even micro-cellular, bump stop installed typically on the lower control arm to prevent metal to metal contact of the suspension components with the vehicle frame. These are sometimes part of the active suspension system by progressively controlling the travel into full compression. These usually only come into effect after the coil springs have been compressed to a point that the suspension nearly reaches its full compression amount.

Unsprung Weight – This is the weight of the vehicle that is not supported by the suspension. An example of what would be considered part of the unsprung weight would be the wheel, tire, brake caliper, brake rotor, spindle, lower control arm etc., and half of the weight of the spring and shock.

Sprung Weight – This is the weight of the vehicle, which is supported by the suspension. This would include the chassis, upper control arm, body, driver, passenger(s), engine, transmission, interior components, fuel, ballast and any other weight that would be supported by the suspension.

Center of Gravity – This is the point within the vehicle where all of the weight may be considered to be concentrated.  This would be the point that if you were to pick it up, it would not want to tip forward, backward or to either side. It would be perfectly balanced in all directions.

Roll Center (or Moment Center) – In a 2 dimensional example this is the point which the vehicle wants to roll, or in other words the “pivot point” of the vehicle created by the location and angles of the suspension components. This can be determined by the angles of the suspension components. It is also to be noted that this point will move relative to the movement of the combined angles of the suspension components. The front suspension roll center, and rear suspension roll center have separate locations which a line between the front roll center and rear roll center will determine the vehicles overall roll center axis.

Instant Center – This is the point in a 2 dimensional example where a line drawn through the center of the ball joint and pivot point on the upper control arm and a line drawn through the center of the ball joint and pivot point on the lower control arm intersect. This represents a theoretical pivot point for that suspension section at that wheel of the vehicle only.

Roll Angle – This is the amount of roll, measured in degrees, which the vehicle moves as the weight is shifted during cornering.

Swing Arm Length – This is the distance between the Instant Center point and the center line of the tire contact patch associated with that suspension section of that wheel. A longer swing arm length results in less camber gain of that wheel during suspension travel. A shorter length results in more camber gain.

Camber – If you are looking at the front of the car, camber is the amount, in degrees, that the top of the tire/wheel is tilted either in or out from tCamber Diagramhe centerline of the vehicle. This effects how much of the tire is in contact with the track/road surface. If the tire/wheel is tilted in at the top toward the centerline of the vehicle, then it is said to have negative (-) camber and the inside of the tire has more contact area (or patch) than the outside . If the tire/wheel is tilted out at the top, away from the centerline of the vehicle, then it is said to have positive (+) camber, and the outside of the tire has more contact area (or patch) than the inside.  On street cars, it is preferable to have the tires/wheels not tilt at all and be perfectly perpendicular to the road surface, to help prevent uneven tire wear. This is neutral camber. Camber is typically adjusted by using shims on the upper control arm mounting holes.

 

Toe – This is the amount of angle (if looking from above) that the front wheels/tires are either point away from each other (toe-out), pointed toward each other (toe-in), or parallel with each other (zero-toe). Obviously if there is toe-in, the distance between the front edges of the front tires will be less than the back edges of the front tires. Toe is typically adjusted and controlled by the length of the tie-rods on the steering system.

Toe In
Toe In

 

Toe Out
Toe Out

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All Rights Reserved.

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