Ackerman steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii.
Steering Axis Inclination (SAI) is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. On a SHORT-LONG ARM (SLA) SUSPENSION the line runs through the upper and lower ball joints.
On a MacPherson strut suspension; the line runs through the lower ball joint and upper strut mount or bearing plate. This angle (SAI), when added to the camber to forms the included angle and causes the vehicle to lift slightly when the wheel is turned from a straight position. The vehicles weight pushes down and causes the steering wheel to return to the center when you let go of it after making a turn.
Like caster, it provides directional stability and also reduces steering effort by reducing the scrub radius.
If the Steering Axis Inclination (SAI) is different from side to side, it will cause a pull at very slow speeds. SAI is a nonadjustable angle, it is used with camber and the included angle to diagnose bent spindles, struts and mis-located cross-members.
The most likely cause for Steering Axis Inclination (SAI) being out of specification is bent parts, which has to be replaced to correct the condition. On older vehicles and trucks with king pins instead of ball joints, Steering Axis Inclination (SAI) is referred to as (KPI) King Pin Inclination.
A-arm: A triangular-shaped suspension control-arm device, with two points connected to the chassis and one to the wheel spindle. Control arms are sometimes called A-arms because from the top view, they are shaped like the letter A.
Active suspension system: Also known as computer-controlled suspension system, a computerized system able to control body roll, body pitch, brake dive, acceleration squat, and ride height. Suspension systems that are controlled by double-acting hydraulic cylinders or solenoids (actuators) mounted at each wheel. The actuators support the vehicle's weight, instead of conventional springs or air springs.
Adjustable shock absorber: A shock absorber having an external means of adjustment to calibrate it precisely for a specific operating condition.
Adjustable strut: A strut with a manually operated adjustment for strut firmness. The strut adjusting knob, usually accessible without raising the vehicle, varies the strut orifice opening.
Air jack: A device that uses compressed air to lift a vehicle. On some Indy and sports GT cars, the jacks are built into the chassis, permitting the whole car to be raised instantly. This enables the pit crew to change all four tires simultaneously.
Anti roll bar: Determines rate of lateral load transfer.
(Lateral load transfer = lateral acceleration x weight x CG / track width)
Anti-sway bar: A suspension component, often called a sway bar, intended to prevent side-to-side body movement in relation to the axles and wheels.
Apex: The innermost point of a turn or corner on a roadway or race course.
Ball joint: A suspension component that attaches the steering knuckle to either control arm featuring a ball-and-socket joint to allow pivoting in various directions. Also known as a spherical joint.
Body pitch: The tendency of a vehicle to dive or squat.
Body roll: A term often used for roll.
Bound: The inward travel of the piston rod in a shock absorber. Also known as compression.
Bump steer: Toe change as the suspension moves up & down. This self steering effect can create instability upon suspension movement (ie the steering veers suddenly in one direction when one or both of the front wheels strikes a bump). Audi uses bump steer effects to provide understeer under suspension compression.
Bump stop: A block, usually rubber, to limit suspension system deflection when a tire hits a bump.
Camber: The outward or inward tilt of the wheels, in degrees, on a vehicle as viewed from the rear or front.
Camber angle: The amount, measured in degrees from the vertical, that the top of a tire is tilted outward (positive) or inward (negative).
Camber angle: Expressed in degrees, it is how much the tire leans in or out. It
is considered positive when the wheel leans outward at the top and negative when it leans inward. Optimal camber while loaded optimizes the traction capabilities of the tire.
Caster angle: The forward inclination of the spindle or strut, like the forks on a bicycle. More specifically, the angle between the steering-spindle axis and the wheel vertical as viewed from the side. Provides camber change when the vehicles wheels are turned. It is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward. Large positive caster gains can limit wheel compliance and grip.
Center of gravity (CG): The exact point around which an object, such as a vehicle, is perfectly balanced in every direction. It is the center point of the vehicle's mass.
Coil: A term often used to describe a spring.
Coil bind: A condition where springs are compressed to the point that the coils touch.
Coilover shock: A suspension component that consists of a shock absorber inside/surrounded by a coil spring.
Coil spring: A spring-steel bar or rod that is wound into the shape of a coil to provide an up-down springing effect. Found on most vehicle suspensions, these springs are used to support the car's weight, maintain height, and correctly position all other suspension parts, but are little help in supporting side-to-side or lateral movement.
Compression; The relative displacement of sprung and unsprung masses in the suspension system in which the distance between the masses decreases from that at static condition. Compression damping is the primary factor in ride quality, road compliance and steering response.
Control arm: The main link between the vehicle frame and the wheels that acts as a hinge to allow the wheels to go up and down independently of the chassis.
Corner weighting: Optimizing the weight of the vehicle at each wheel to maximize the vehicles transitional response. Tire traction and suspension movements are determined by the force (weight) on each corner of the vehicle. Equal diagonal weighting provides the best transitional response while equal front weight provides the best braking response.
Damper: Shock/strut used to dampen energy of spring and control rate of load transfer. All hydraulic dampers (shock absorbers) work by the principle of converting kinetic energy (movement) into thermal energy (heat). For that purpose, fluid in the damper is forced to flow through restricted outlets and valve systems, thus generating hydraulic resistance. See also shock absorber.
Digressive damping: Digressive damping describes the style of damper valving. Digressive means that the compression or rebound force will change or digress from a given path at some point. In comparison, a linear rate damper follows the same increasing path.
Dive: The tendency of the front of a vehicle to press down on the front springs during heavy braking.
Double A-arm: A suspension system using two A-arms or A-frames to connect the chassis to the wheel spindle.
Double digressive damping: Double-digressive simply means that the damper valving is digressive on both the compression and the rebound side of the damper. (Digressive means that the compression or rebound force will change or digress from a given path at some point. In comparison, a linear rate damper follows the same increasing path.)
Double wishbone: A term used for double A-arm.
Externally adjustable damper (rebound only or rebound and compression adjustable):
These dampers are at the high-end of the spectrum. They can be adjusted while remaining mounted on the car so that damping levels can be adapted to personal requirements in a matter of seconds.
Front control arm: Horizontal arms that connect the front wheels to the car and that support the weight of the front of the car.
Full coil suspension: A vehicle suspension system in which all four wheels have their own coil spring.
Gas-filled shock absorber: A shock that uses nitrogen gas, at 25 times atmospheric pressure, to pressurize the fluid in the shock to reduce or prevent aeration or foaming. Also known as gas shock or gas damper.
Harshness: The high frequency (25-100 Hz) vibrations of the structure and/or components that are perceived tactually and/or audibly. (Thankfully STaSIS suspensions do not fall under this heading).
Heavy-duty shock absorber: Shock absorbers having improved seals, a single tube to reduce heat, and a rising rate valve for precise spring control.
Heim joint: A spherical-rod end joint
Helper spring: An additional spring device that permits a greater load on an axle.
High speed dampening: Generally associated with tire compliance, tire grip and driver comfort.
Independent front suspension (IFS): A suspension-system method of supporting the chassis on the wheels without the use of rigid axles where the movements of the two front wheels are not interdependent; one wheel does not force the other wheel to change its plane of rotation.
Independent rear suspension (IRS): A suspension system in which both rear wheels are free to move independently of the other, providing a reduction of unsprung weight as well as overall vehicle-weight reduction.
Independent suspension: A suspension system by which a wheel on one side of a vehicle can move vertically without affecting the wheel on the other side, and wheel jounce or rebound travel of one wheel does not directly affect the movement of the opposite wheel.
Jounce: The inward reaction of the spring and shock absorber when a wheel hits an obstruction.
Lateral link: A suspension component used to reduce side-to-side movement of a wheel.
Lift-throttle oversteer: A loss of grip on the drive wheels of a rear-drive vehicle when the throttle is lifted during fast cornering causing the rear of the vehicle to swing toward the outside of the turn.
Linear-rate coil spring: A coil spring with equal spacing between the coils, one basic shape, and constant wire diameter having a constant deflection rate regardless of load.
Low speed dampening: Generally associated with driver inputs, roll control, pitch control.
Lower A-arm: The lower member of a double A-arm suspension system.
Lower control arm: A front suspension component connected between the pivoting attachment point on the car frame and the lower ball joint, which is fastened to its outer end.
MacPherson strut: A type of front suspension having a shock absorber mounted directly below the coil spring.
MacPherson strut suspension: A front-end, independent suspension system in which the combined strut, steering knuckle, and spindle unit, supported by the coil spring at the top, is connected from the steering knuckle to an upper-strut mount.
Modified strut: A strut suspension where the coil spring is not part of the assembly and is independently located between the lower control arm and the frame.
Motion Ratio - Specifically we usually refer to the relationship between the motion of the wheel and the motion of the spring; i.e. If the spring is half the distance from the control arm pivot as the wheel is, the motion ratio relative to the wheel is .5 to 1.
Needle valve: The small, tapered male part of a needle and seat.
Neutral steer: The ideal balance when the front & rear tires gradually give up traction at an equal rate.
Opposite lock: Turning the steering wheel in the opposite direction of a turn to control or correct oversteer.
Orifice: A small hole or opening.
Oversteer: A condition when the rear tires lose traction first or when the rear wheels are carving a larger arc than the front wheels or the intended line of the turn. This is often described as a loose condition, as the car feels like it may swap ends. This condition can be caused by power oversteer, in which case you need to reduce power in order to bring the back end back into line.
Piston movement hydraulic drive: Moves more fluid per given suspension movement providing fast control of low speed suspension movements; difficult to isolate high speed suspension movement associated with ride quality and tire compliance.
Piston rod: A plated rod attached to the shock-absorber piston, usually extending from the top of the shock to provide attachment to the vehicle.
Piston-rod seal: A non-replaceable oil seal around a movable piston rod, located at the upper end of the hydraulic cylinder.
Polar moment of inertia:
1. The tendency of a body to resist angular or rotational acceleration.
2. The tendency of a vehicle to resist cornering.
Power oversteer: The loss of traction of the rear wheels while cornering and accelerating, causing the rear of the vehicle to swing toward the outside of the turn.
Power slide: A controlled four-wheel skid in dirt-track racing to maintain speed through a turn.
Progressive-rate spring: A spring used in a vehicle that stiffens under load.
Push: A condition in cornering when the slip angle of the front tires is greater than the slip angle of the rear tires and the front end of the vehicle tends to break loose and slide toward the outside of the turn.
Rake: A suspension or structural design to lower one end of the vehicle in relation to the other end.
Rebound: The relative displacement of the sprung and unsprung masses in a suspension system in which the distance between the masses increases from that at static condition. More simply, it is the outward extension of the springs and shocks in a vehicle suspension system. The core functions of rebound in a damper are to provide the roll control of the vehicle and to control the spring rate on the car.
Rebound valve: A calibrated piston valve mounted on the shock piston that provides variable resistance to fluid flow during rebound.
Ride height: The distance between the road and the bottom of a vehicle.
Rod-end bearing: The spherical bearing found at the end of a suspension arm or rod.
Roll axis: The longitudinal axis of a vehicle defined by an imaginary line running through the front and rear roll centers.
Roll bind: Any binding of suspension components that occurs as the body of the car leans over in a turn
Roll center: The points at the front and rear about which the vehicle's sprung mass will roll/rotate in a turn. The attachment points of the suspension components determine the roll center.
Roll steer: The direction and amount that the rear axle may cause the vehicle to steer as it moves through its travel when the body rolls during cornering requiring the driver to oversteer or understeer to compensate for the problem. This is caused by the rear control arms pivoting around their forward mounting point, drawing the axle forward as the arm moves up or down.
Roll stiffness: The resistance, measured in pounds per inch of spring travel, of a suspension system to the rolling of the vehicle's mass.
Schrader valve: A spring-loaded valve, similar to a tire valve, located inside the service-valve fitting and some control devices to hold vapor or fluid in the system. It requires special adapters for access to the system.
Scrub radius: The distance between the centerline of the ball joints and the centerline of the tire at the point when the tire contacts the road surface.
Set screw: A type of screw having a point that fits into the matching recess of a shaft to secure a component.
Shaft displacement hydraulic drive: Provides moderate fluid transfer per given suspension movement; good for high speed shaft control and overall vehicle platform control.
Shock: A term used for shock absorber.
Shock absorber: A hydraulic device used at each wheel of the suspension system to help control the up, down, and rolling motion of a car body by dampening the oscillations or jounce of the springs when the car goes over bumps, thereby contributing to vehicle safety and passenger comfort. Also referred to as shock or damper.
Shock-absorber function: A typical shock absorber has three functions to dampen the effect of spring oscillation in order to control the ride stabilization of a vehicle, to control body sway, and to reduce the tendency of a tire tread to lift off the road surface (a problem often caused by static unbalance).
Shock fluid: Specially formulated hydraulic fluid used inside of shock absorbers.
Shock foaming: The mixing of air and shock fluid due to rapid movement of fluid between the chambers, causing the shock absorber to develop a lag because the piston is moving through an air pocket that offers up resistance. A gas-filled shock absorber is designed to reduce oil foaming.
Shock piston: The component attached to the bottom of the piston rod containing the rebound valve that moves back and forth inside the inner cylinder.
Shock rebound: The rebound travel when the shock absorber is in its lengthened position, which occurs when the suspension or spring moves downward.
Slip angle: The difference in the path the wheels follow during a turn compared to the actual direction they are pointing, caused by centrifugal force at higher speeds.
Spherical joint: A term used for ball joint.
Spin: To skid out of control by 180 degrees or more.
Spring: A steel or composite elastic coil-like device that compresses as it absorbs energy and returns to its original position when it releases that energy. Itâ€™s primary function is to control the roll of the vehicle.
Spring bind: A term used for coil bind.
Spring perch: A platform, usually on the damper, on which the spring sits or rests.
Spring rate: The relationship of spring deflection to load applied, such as the amount of weight, in pounds per inch (or newtons per millimeter), required to deflect the spring. Spring rate determines the rate of load transfer. It is the force necessary to compress the spring, i.e. a 200 lb/in spring requires 200 lbs to compress it 1", 400 lbs to compress it 2", etc.
Spring sag: The loss of spring load due to overloading and/or metal fatigue.
Spring seat: The recess in a chassis where a coil spring is mounted.
Sprung weight: The mass of the vehicle that is supported by the springs; including the body, engine, and transmission.
Squat: The tendency of the rear end of a vehicle to press down on its springs during hard acceleration.
Stabilize bar link: A device that connects the lower control arm to the stabilizer bar.
Stabilizer bar: A long, spring-steel bar attached to the cross member and interconnects the lower control arm that twists like a torsion bar during turns to transmit cornering forces from one side of the vehicle to the other to help equalize wheel loads and prevent excessive leaning.
Steering geometry: The relationship of the steering linkage and the wheels to the road affected by caster, camber, scrub radius, steering offset, toe in, and toe out.
Steering wander: The tendency of the steering to pull to the right or left when the vehicle is driven straight ahead on a smooth road surface that may be caused by improper caster adjustment.
Strut: Components connected from the top of the steering knuckle to the upper strut mount that maintain the knuckle position and act as shock absorbers to control spring action in a vehicle's suspension system. This is used on most front wheel drive cars and some rear wheel drive cars.
Strut cartridge: A self-contained unit with a pressure tube and a piston rod assembly, factory sealed and calibrated.
Suspension: The system that supports the weight of the vehicle and provides for a comfortable and safe ride for the driver and passengers.
Suspension system: Components that support the total vehicle, including front and rear suspensions, springs, shock absorbers, torsion bars, axles, MacPherson strut system, and connecting linkages.
Sway bar: A bar on the suspension system that connects the two sides together. It is designed so that during cornering, forces on one wheel are shared by the other.
Sway-bar link: A connector from the lower control arm to the sway bar.
Toe: The difference in the distance between the leading and trailing edge of the tires. Inboard toe provides stability while the outboard toe provides response and possibly grip.
Toe in: The amount by which the front of a front wheel points inward.
Toe out: The amount by which the front of a front wheel points outward.
Trailing arm: A suspension arm that attaches to the chassis ahead of the wheel, generally for the rear wheels on a front-wheel drive vehicle.
Understeer; A condition when the front tires lose traction first or when the front wheels are carving a larger arc than the rear wheels. This is often described as push or pushing, as the front end feels like it is plowing off of a corner. Further acceleration only compounds the push, as weight shifts back to the rear drive wheels off of the front turning wheels, leading to a further lessening of the car's ability to turn in. Understeer can be remedied by slight modulation in throttle to transfer weight forward to the front wheels, aiding their traction and ability to carve the turn.
Unsprung weight: The vehicle weight not supported by the springs (suspension), but is supported directly by the tire or wheel and considered to move with it, including the steering knuckle, brake assembly, tire and wheel.
Upper control arm: A front-suspension component bolted to the frame between the pivoting attachment point on the vehicle-frame cross-member and the upper ball joint, which is fastened to its outer end having bushings that are attached to the control arm.
Valving: The way fluid is routed inside the damper which effects the ride of the vehicle. Common terms; orifice, shim stack, needle valve, barrel valve, preloaded spring stack.
Wheel rate: The combined effect of spring rate, motion ratio, friction and/or binding of other suspension components measured at the wheel.
Yaw: The rotation of a vehicle structure around a vertical axis.
Zero toe: Adjusting the wheels so they point straight ahead.