Suspension and Chassis Glossary

Abbreviations to the term are listed in (parentheses) and some related words are listed in

[brackets].

Common abbreviations used in the text:

aka: also known as

CL centerline

f/r: front / rear

IC instant center

i/s: inside

K__: spring stiffness, e.g., Kbar

o/s: outside

RWD: rear wheel drive

SLA short long arm

Ackerman: Steering kinematic characteristic where inside front wheel turns more than outside

front wheel. 0% Ackerman is parallel steering; 100% Ackerman defines all four wheels

pointing at a common center; Anti-Ackerman has o/s front wheel turn more than i/s front

wheel. Street cars have 40-80% Ackerman; race cars have up to 20%, sometimes anti-

Ackerman. [front steer, rear steer, tie rod, upright].

Aerodynamic Load: The load on a wheel, wheel pair, or entire vehicle due to the vertical forces

exerted on it by its passage through the air.

Alignment: Angles of the tire and suspension axes relative to each other and the ground: caster,

camber, and toe. Also, the adjustment of components to bring them into a predetermined

position for the most efficient operation of wheel and vehicle for proper even tire wear.

Anti-Roll Bar: aka: stabilizer bar, stabar, sway bar. [balance, oversteer, understeer, neutral, roll

stiffness] (acronym: ARB) Suspension device, usually a round bar with lever arms at ends,

which acts to reduce the roll angle of the vehicle under cornering loads. Also important for

adjusting the balance of the car. A steel torsion bar, linking the left and right side of a

suspension. It comes into play during cornering. As the car leans in a turn, the anti-roll bar

resists this leaning by transferring more weight to the outside tire. It provides a means to

achieve good handling from stiff roll resistance while maintaining a comfortable ride

through soft springs.

Anti-Dive: Front suspension kinematic characteristic which reduces the amount of pitch under

braking forces. 100% antidive refers to no change in front suspension height under

braking. Achieved kinematically by precessing the wheel in bump. If excessive, will tend

to bind up suspension under bumps. Street cars have 20-40% antidive, race cars 0-30%.

[precession, recession, pitch]

Anti-Lift: Rear suspension kinematic characteristic which reduces the amount of pitch under

braking forces. Street cars may have up to 150% antilift for driver comfort. [anti-dive,

pitch]

Anti-Squat: Rear suspension (on RWD vehicles) kinematic characteristic which reduces the

amount of pitch under acceleration. Achieved with recession. Street cars have up to 50%

antisquat, race cars up to 30%. [anti-dive]

Axle Ratio: (aka: ‘differential ratio’ or ‘rear end ratio’) is the number of revolutions required by

the transmission’s output shaft to make the drive wheels revolve once.

Backspacing: The measurement from the back of the bolt pad to the back edge of the rim; used

to calculate offset and determine where the back of the bolt pad is located in relation to

the rim width, sometimes referred to as rear-spacing.

Balance: Qualitative description of the handling characteristics of a vehicle, describing

understeer, neutral, or oversteer.

Ball Joint: Spherical bearings that connect the control arms to the steering knuckles. They are

used on virtually every automobile made and work similar to the ball and socket design of

the human hip joint. A ball joint consists of a bearing stud and socket enclosed in a casing;

all these parts are made of steel. The bearing stud is tapered and threaded, and fits into a

tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into

the joint assembly. Usually, this is a rubber-like boot that allows movement and expansion

of lubricant. Motion control ball joints tend to be retained with an internal spring, which

helps to prevent vibration problems in the linkage.

Ballast: Weight added to a vehicle usually to bring it to a minimum weight requirement-useful for

adjusting balance.

Bar Rate (Kbar): Torsional stiffness of the antiroll bar, either at the drop links or at the wheel's

contact patch. Usually expressed in lb/in.

Bell Crank: Linkage component with a central pivot and multiple link attachment points. Usually

used with inboard suspensions to transfer loads from the pull/push rod to the

spring/damper. Technically, a "rocker" uses the same pivot for pull-rod and damper, while

a bell crank has different attachments for each. [rocker]

Body Roll: see Roll Angle.

Brake Bias: The distribution of braking between front and rear wheels. Typically set so the front

wheels lock slightly before the rears.

Brake Caliper: Component using 1-6 pistons to compress pads onto disk-May be floating or

fixed to upright. May be inboard (common on rear of race cars) or outboard (universal on

front).

Brazing: Method of joining metal by melting a bronze/silver filler rod with a high temperature

flame (oxy-acetylene). If properly done, the metal reaches a lower peak temperatures than

TIG or MIG welding (e.g., 1200°F). For a heat treated metal, this can be valuable as much

of the heat treatment is retained. Bond strength is higher than tube strength for most thin

wall tubes. Overall, brazing is somewhat easier to perform than TIG, though precise

miters are still required. [MIG, TIG, space frame]

Breakaway: The point at which tire cornering traction is lost.

Bump: aka jounce, bound, or compression of the suspension. Vertical movement of wheel

towards chassis. Bump ‘stiffness’ is the measure of rate at which a shock absorber

compresses. In racing, the terms "bump and droop" are used together to define suspension

travel; in industry, "jounce and rebound". [droop, rebound]

Bump Steer: Change in toe angle under bump/droop. More critical in rear suspension than front.

Typically, slight toe-in with bump is used in rear and slight toe-out with bump is used in

front to aid stability.

Bump Stop: A cushioning device, usually rubber, that limits the upward movement of the wheels

and suspension to prevent metal-to-metal contact that could lead to suspension damage or

failure. Also called "jounce bumpers."

Camber Angle: Wheel angle relative to ground in front/rear view. Negative camber is defined as

the tops of the wheels closer together than the bottoms. Lateral grip of an outside tire is

optimized with -1° to -2° camber. 0° camber is optimum for acceleration and braking.

Excessive negative camber wears i/s of tire under accel/braking and reduces grip. Not

enough negative camber wears o/s of tire under cornering and reduces grip. Performance

cars run ‘negative’ camber (the tops of the wheels are tilted in).

Camber Thrust: The cornering force developed by a tire due to its camber. This force is in the

same direction as the leaning of the tire.

Caster Angle: The angle between upper and lower ball joints in side view, about which the wheel

turns. Provides stability, improves camber angle of o/s wheel at high steering angles and

can affect corner weights as the wheel is turned. Street and race cars have caster angles of

3° to 6°. [kingpin, upright]

Center of Gravity {Height} (CG, {CGH}): Location on the vehicle or sprung mass where all

mass may be assumed to be located. Longitudinal position defines f/r weight distribution.

Height affects roll moment. CGH are typically 9-11" for FSAE, and 20-25" for street.

Centrifugal Force: The force that tends to throw a vehicle away from the center of rotation

during cornering; Measured in lateral force, or lateral G’s.

Chapman Strut: MacPherson strut used in a rear suspension.

Chromoly: Chromium-Molybdenum alloy steel. Usually refers to 4130; May also be 4340.

Typical material used in space frames. Lighter, stronger, and more expensive than

‘normal’ steel.

Clutch-Pack: Type of limited slip differential with compressed clutch plates limiting wheel spin.

Differential is "locked" until a preset torque is reached, then it is open with that torque

difference between the wheels. e.g., Positraction, Salisbury. [torque sensing, viscous

coupling]

Coefficient of friction: (grip value between tires and road surface). The amount of force a tire

can generate, measured in g’s is directly affected by the coefficient of friction. This

includes cornering force (lateral acceleration), straight line acceleration, and braking.

Coil Over Shock: Standard assembly of spring and damper, where spring perches are at each end

of damper. The defining characteristic of a coilover is usually the adjustability of both

bump and rebound settings of the shock absorber component.

Coil Springs: Suspension components of coiled spring steel that compress and expand in

response to road pressure, permitting the up-and-down movement of a vehicle (and

driver) as it goes over the irregularities of a road surface.

Compliance Steer: A steering effect caused by the deflection or compliance of rubber bushings,

joints, and other suspension components under loads and forces.

Constant Velocity Joint (CV): Similar to a universal joint, except that rotational speeds are

maintained under angular misalignments during suspension travel. Usually capable of

operating at a maximum of 7° misalignment for long periods; May or may not accept

plunge.

Contact Patch: The area of the tire that is in direct contact with the road surface.

Control Arm: Usually an upper or lower member of a double A Arm suspension; May also attach

to spring, damper, anti-roll bar, pull/push rod. Acts like a hinge, used to connect the un-

sprung components of a suspension to the sprung chassis, which allows suspension travel.

Corner Weights: Weight distribution on all four wheels. Refers to more information than just f/r

weight distribution. Asymmetric corner weights are used extensively for tuning on oval

race tracks, but are usually avoided on road courses unless the track has many more left

turns than right, or vice versa.

Cornering Force: The amount of force that a tire can generate in the lateral sense. Used to

measure lateral acceleration.

Curb Weight: The total weight of a vehicle with no passengers and full of fuel/fluids.

Damper: Proper name for a "shock absorber". Usually consists of oil forced through a valved

piston/cylinder under bump/droop. External or internal gas pressurization limits foaming

of oil under heavy use ("gas shocks"). Amount of damping may be varied at low, medium

and high velocities. External adjustments are usually overall or high speed. Amount of

damping is used extensively in road racing for transient handling tuning. [bump, droop,

jacking down]

Deflection: The moving or bending of suspension parts. Usually results in camber loss during

cornering.

Degrees per G: See roll flexibility.

Differential: Geared device between the driven wheels to allow varying rotational speeds of the

wheels around corners. Limited slip feature may be added. [Torsen, Quaife, posi, clutch-

pack]

Dive: The weight transfer of the sprung mass of a car from back to front under braking. More

simply put, when you drive your car and hit the brakes hard, the feeling of the car (and

your body) lunging forward is ‘dive’. Vehicle pitch under braking. (aka: nose-dive).

Downforce: Vertical aerodynamic loads increasing normal force and thus cornering force of the

tires. (aka: negative lift). [lift]

Droop: Vertical movement of wheel down from chassis. (aka: rebound). In racing, the terms

'bump and droop" are used together to define suspension travel; in industry, "jounce and

rebound". [bump, jounce]

Drop Links: Similar to tie rods in construction, but used to attach antiroll bar to suspension arms

or upright. Name reflects the typical configuration, where the link drops down from the

antiroll bar to the lower arm.

Dynamic Balance: Balance in motion. The balance of a wheel while it is rotating; a condition in

which a tire and wheel assembly has weight distributed equally on both sides of the wheel's

axis of rotation.

Dynamic Camber: Camper Angle during the suspension motion. Camber angle change that is a

result of suspension travel.

Dynamic Load Transfer: The shift in vehicle weight from one tire to another when the vehicle is

accelerated in one sense or another. Vehicle gross weight is not affected, only its

distribution.

Dynamic Weight Distribution: Weight distribution under transient handling or aerodynamic

forces.

Dual/Double A-Arm (AA Arm): Suspension configuration with a short upper and long lower

arm connecting chassis to upright. Each arm is in the form of an A or V when viewed

from above. Very common in racing; allows precise control of camber and roll center.

[double wishbone, short long arm, MacPherson strut, solid axle]

Dual/Double Wishbone: (aka: double A-arm) [short long arm]

Fore and Aft: (Longitudinal Load Transfer) A factor where load is transferred from the front

tires to the rear tires during acceleration and from the rear to the front tires during

braking.

Frame Stiffness: Usually the torsional stiffness of the chassis between the front and rear spring

mounts. FSAE cars typically are 1000 ft-lb/deg to 3000 ft-lb/deg. If insufficient (< 1500-

2000), antiroll bars may become unable to transfer weight as expected and wheel camber

may be compromised under heavy loadings.

Friction Circle: Graphical tool to show the tradeoff between lateral and longitudinal cornering

force of a tire/vehicle. A circle is drawn representing the maximum grip of the tire/vehicle,

with top representing acceleration, bottom braking, etc. The coordinates of a point on the

circle represent the lateral and longitudinal acceleration. The distance from the point to the

center represents the grip/acceleration at that time.

Front Steer: Steering system with the rack and tie rods in front of the wheel. Often tends

towards anti-Ackerman at large steering angles. [Ackerman, rear steer, steering arm]

G's (A): (aka: G-force) A unit of acceleration. "g" is a symbol that represents the acceleration of

gravity. Acceleration at 1g equals 32 ft/sec2 or 9.8 m/sec

2.

Grip: The coefficient of friction between a tire and the road surface.

Inboard: 1) Suspension spring system where the spring/damper is mounted near or within the

chassis via rocker and pull/push rod system. Principal advantage is removing the

spring/damper from the airstream, improving drag and downforce (on ground effects

cars). 2) Brake system where the disk and caliper are mounted on the sprung mass, via

axles and CV joints. Principal advantage is reducing unsprung mass.

Independent Suspension: Any suspension which is not a solid axle type. e.g., double A Arm,

MacPherson Strut, multilink. A suspension system in which the front or rear pair of wheels

of a car are independently connected to the frame or underbody. In this system, deflection

of the wheel on one side will not affect the wheel on the other side.

Inertia: The tendency of any mass at rest to stay motionless, or any mass which is moving to

remain moving. Inertia is the measurement of the mass’s resistance to said changes in

movement. Inertia is also a measurement of a vehicle moving in a straight line’s resistance

to turning.

Instant Center (IC): Instantaneous point about which the wheel rotates. In a rear view of a

double A-arm, the IC is an imaginary point located at the intersections of the lines passing

through the upper and lower arms.

Jacking: Vertical force on chassis under cornering caused by a roll center above ground. Can

lead to catastrophic "tuck-under" for swing axle suspensions (early Corvair and Beetle).

For most suspensions, will cause a slight increase in ride height and slight loss of camber

while cornering.

Jacking Down: Temporary lowering of chassis on bumpy surface due to excessive rebound

damping [damper, bump]

Jounce: see bump. [bump, droop, rebound]

Jounce Bumper: see Bump Stop

Kinematic: Describes the motion characteristics of a suspension, as opposed to the force

characteristics. [instant center, roll center, motion ratios]

Kingpin: Angle formed between vertical and the line joining the upper and lower ball joints.

Term arose from solid axle front suspensions where the front wheels were steered by

pivoting about a long pin. Positive kingpin is defined with the upper ball joint closer to the

chassis than the lower. Kingpin is useful for reducing scrub radius, but excessive kingpin

and high steering angles leads to positive camber change and corner weight variations.

Street cars have 7°-15° kingpin, Race cars have 0° to 5° kingpin. (aka: Steering Inclination

Angle)

Laden: (Wheel) – The term ‘laden wheel’ refers to the outer wheel of a pair that bears increased

lateral loading during a turn.

Lateral Load Transfer: A factor in cornering where load is transferred from the inside tires to

the outside (laden) tires.

Lift: Opposite of downforce.

Limited Slip Differential: Differential with mechanism to control wheel spin of unladen wheel.

E.g., Positraction, Detroit Locker. Technically different than a Torque-Sensing differential

in that it is an open differential with an added component. [differential]

Load Index: A numerical code which specifies the maximum load a tire can carry at the speed

indicated by its speed symbol, at a specified tire inflation pressure.

Loading: (aka: Vertical Load) The amount of weight put on tires. Increased load will increase

cornering force to a limited extent, as indicated by a tire’s performance curve. The amount

of traction available from any given tire is dependent upon how much weight is on it.

Longitudinal Load Transfer: Transfer of load fore and aft (forward and back) as a result of the

forces of acceleration and braking. (See: ‘Dive’ and ‘Squat’)

Loose: Slang for oversteering.

MacPherson Strut: Suspension system which uses a lower control arm (or 3 lower links) and a

damper/strut which is designed to take bending loads. Typical on street cars for its low

cost and reasonable performance. Camber and roll center control are not nearly as good as

with a double A-Arm. aka Chapman strut (if in a rear suspension).

MIG welding: Metal Inert Gas welding. Uses the filler rod as a consumable electrode and the rod

is automatically fed. Extremely easy to perform, it is well suited for mass production.

Weld quality is not as high as TIG. Requires .060 metal thickness or greater for easy

welding; possible but difficult with .035 wall. [space frame, TIG, brazing]

Monocoque: Vehicle chassis consisting of stressed skin/panels and bulkheads. (Contrast with a

space frame).

Mono-shock: Inboard suspension system which actuates both left and right wheels to a single

spring/damper. All roll resistance is then carried by the antiroll bar. In F1, allows better

aerodynamics when used on the front. For FSAE, saves some cost and weight compared

to a standard 2 shock inboard system. Roll damping is assumed to be unimportant or

carried by the opposite end of the car and/or tires.

Motion Ratio (MR): Ratio between spring/damper displacement and wheel travel. Typically,

front outboard suspensions have MRs around .75, and rear outboard suspensions are

around 0.9. Inboard MRs are as desired. A high MR (>0.6) is desired to keep wheel forces

from multiplying excessively at the spring/damper. [rising rate, wheel rate]

Multilink: Suspension system, usually Short-Long-Arm, which uses multiple links instead of

upper and lower arms.

Negative Camber: Occurs when the top of the tire is tilted inward from perpendicular to the

road surface.

Negative Caster: A setting where the steering axis is inclined forward at the top as viewed from

the side; a condition which tends to cause instability.

Negative Offset: (Wheels) When the back of the bolt pad is closer to the inside of the wheel;

when the mounting face in inboard of the rim centerline; extends assembly away from

vehicle resulting in wider tracking

Neutral: Handling characteristic between understeer and oversteer. This is usually the goal of

chassis tuning, with a slight bias towards over or under-steer depending on conditions and

driver preference.

Normal Force (N): The vertical force between the tire and the ground. A tire's grip is reduced at

higher normal forces, but, of course, its maximum lateral force continues to increase.

Offset: Lateral distance between a wheel's vertical centerline and hub mounting face. Positive

offset has wheel CL further outboard than hub face. A high offset rim is useful for

reducing scrub radius by allowing ball joints to be placed deeper within the rim. Wheels

can be offset both inside and outside of this centerline.

Open Differential: Standard differential with no limited slip or torque sensing features--allows

unlimited wheel spin. Equivalent to a "0% locked" clutch-pack. An open differential

vehicle with high roll stiffness at the driven wheels will spin the inside wheel upon corner

exit. Common on street cars; rare on race cars. [limited slip, Torsen]

Outboard Suspension: Conventional suspension springing system where the spring/damper is

mounted directly between the chassis and lower/upper arm or upright. In front

suspensions, the spring is mounted to the lower arm; in most rears, the upright. [inboard,

motion ratio].

Overhang: Longitudinal distance from wheel CL and the nearest end of the car.

Oversteer: Vehicle handling characteristic where the rear wheels tend to lose traction before the

front. Technically, it is where the rear wheel slip angles are greater than the front.

Moderate oversteer may be advantageous on slow, tight corners, but any oversteer is

usually difficult to manage on high speed corners. [loose, understeer]

Percent (%) Slip: Amount of longitudinal (accel. & braking) slip between the tire and the road.

Expressed as the ratio of the velocity of slipping to the CL wheel velocity. As with slip

angle, there is an optimum percent slip for maximum grip, beyond which the tire spins.

Pitch: Viewed from the side, the change in vehicle angle with respect to the ground. For example,

in braking, weight is transferred from the rear to the front, causing unloading of rear

springs and additional loadings of front, resulting in dive. The effect may be reduced or

eliminated with anti-pitch suspension geometries, lower CGH, longer wheelbase or stiffer

springs. [dive, squat, anti-]

Plowing: The loss of cornering capability of the font of the vehicle. The car tends to go straighter

than the desired driving line through a turn: understeer.

Pneumatic Trail: Difference between actual center of tire contact patch and wheel centerline.

Results in a small self-centering effect.

Positive Camber: Occurs when the top of the tire is tilted outward from perpendicular to the

road surface.

Positive Caster: A setting where the steering axis is inclined rearward at the top, as viewed from

the side; makes possible the self-centering force that tends to return the wheel to the

direction the vehicle is traveling.

Positive Offset: When the back of the bolt pad is closer to the street side of the wheel; when the

mounting face is outboard of the rim centerline, draws assembly towards the vehicle

resulting in narrower tracking.

Precession: Suspension kinematic characteristic where the wheel (normally for front suspension)

moves forward in bump and rearward in droop. The amount of precession directly affects

the amount of anti-dive. [anti squat, antidive, recession, SVIC]

Polar Moment of Inertia: The M*R^2 of all masses on the vehicle, about the CG of the car. A

high polar moment provides stability; a low polar moment improves maneuverability. A

low polar moment is achieved by concentrating the masses around the CG (engine, driver,

battery...).

Pull-Rod: Inboard suspension springing component which normally attaches near the upper ball

joint and pulls on the rocker in bump. Pull-rod designs often have somewhat lower and

more non-linear motion ratios due to packaging of the rocker next to the ground.

[inboard, pushrod]

Push: Slang for understeer.

Push-Rod: Inboard suspension springing component which normally attaches near the lower ball

joint and pushes on the rocker in bump. Push-rods may be preferred to pull-rods for higher

and more linear motion ratios, but may introduce buckling concerns. [pull-rod, inboard]

Pyrometer: Gauge to measure tire temperature. Tire temperatures are an indication of the

loading between tires and across each tire--information valuable for tuning.

Quaife: Brand name of a torque sensing differential. Not quite as good as a Torsen for the

maximum torque differential between wheels. [Torsen]

Rack and Pinion: Steering component to change rotational steering wheel input into

translational motion to the tie rods.

Rebound: aka: extension; the movement of the suspension as the springs expand or stretch.

Rebound ‘stiffness’ is the measure of rate at which a shock absorber stretches up or

‘rebounds’ from a compressed state. (see droop).

Recession: Suspension kinematic characteristic where the wheel (normally for rear suspension)

moves rearward in bump and forward in droop. The amount of recession directly affects

the amount of anti-squat. [anti squat, antidive, recession, SVIC]

Rear Steer: Steering system with the rack and tie rods behind the wheel. Tends towards

Ackerman at large steering angles. NOT related to rear-wheel steering. [Ackerman, front

steer]

Ride Frequency: The undamped natural frequency of the body in ride. The higher the frequency,

the stiffer the ride.

Based on the application, there are ballpark numbers to consider.

· 0.5 - 1.5 Hz for passenger cars

· 1.5 - 2.0 Hz for high-performance sports cars

· 3.0 - 5.0+ Hz for high downforce race cars

It is common to run a spring frequency higher in the rear than the front. The idea is to

have the oscillation of the front suspension finish at the same time as the rear. Since the

delay between when the front suspension hits a bump and the rear suspension hits that

bump varies according to vehicle speed, the spring frequency increase in the rear also

varies according to what speed the one wants to optimize for. This calculator will

calculate the required rear spring for a "flat" ride based on your selection of front spring

rate.

Ride Height: The distance between the frame of the vehicle and the road.

Rising Rate: Suspension motion ratio which increases with jounce. Useful for reducing

suspension travel at high wheel loadings, but may introduce significant roll-couple

distribution changes (oversteer and understeer).

Rocker: Inboard suspension component used to transfer wheel loads to spring. The rocker may

be either an entire suspension control arm (usually the upper arm) or a component

between the pull/push rod and spring.

Rod End Bearing: Spherical ball joint with stud integral with case. Usually referred to simply as

a "rod end", though technically, a "rod end" is a simpler part and has no ball joint.

Roll Angle: The lateral load transfer from a car’s sprung mass, from side to side during cornering

(turning). Body roll is generally measured in degrees.

Roll Axis: A theoretical line through the front and rear roll centers, about which the car body

rolls.

Roll Center {Height} (RC {RCH}): Instantaneous point in the front or rear view of the chassis

about which the chassis rolls--also the point through which all forces are transferred

between sprung and unsprung masses. A high roll center reduces the roll moment; a low

roll center reduces jacking. Most SLA suspended cars have roll center heights between 0

and 2", while solid axle suspensions are usually be higher (3" to 10") due to no camber

change under jacking and packaging.

Roll Couple Distribution (RCD): 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 RCD means more understeer; more rear RCD means more

oversteer. Street cars have RCDs between 55% and 65% front, most race cars are

between 45% and 55% front, and autocross cars may be as low as 30% front to reduce

understeer. Infinite chassis stiffness is usually assumed; excess chassis flexibility will not

allow large deviations between RCD and static weight distribution. E.g., a car with "30%

front RCD" and "45% front weight distribution" will not behave as expected if the chassis

is too flexible--it will tend towards 45% front RCD.

Roll Flexibility: Expressed as degrees of chassis roll per G of lateral acceleration. aka "degrees

per G". Equal to (roll moment)/(roll stiffness). Street cars range between 2.5° and 6°.

Most race cars 0.3° - 0.8° (ground effects cars essentially do not roll -- below 0.1° is

common). Excessive roll stiffness degrades handling on bumpy circuits. Insufficient roll

stiffness reduces transient response and tire grip.

Roll Moment: The overturning moment caused by lateral acceleration of the CG. Equal to

(sprung mass)*(lateral acceleration)*(center of gravity height - roll center height).

Together with the roll stiffness, this factor determines the roll flexibility.

Roll Steer: Bump steer during roll, usually used when referring to a solid axle suspension.

Roll Stiffness (RS): The elastic resistance to chassis roll from the springs, anti-roll bars or both.

May be of front or rear suspension, or both. Together with the roll moment, the total roll

stiffness of the vehicle affects the roll flexibility. [roll couple distribution]

Rolling Radius: Distance between tire CL and ground under load.

Scrub: Lateral displacement of the wheel CL in bump/droop due to an IC above or below

ground. Suspension design should keep scrub below 3-5mm to reduce unwanted side

loads generated during a bump. Scrub is not the same as "scrub radius".

Scrub Radius: Lateral distance between the CL of wheel and the intersection of the kingpin axis

and the ground. The wheel will turn about the contact patch with a scrub radius of 0.

Scrub radius should be minimized; acceptable scrub radii are thought to be those smaller

than 25% of the tread width.

Short-Long-Arm (SLA): Any suspension system with a short upper arm(s) and long lower

arm(s). e.g., Double A-arm, multi-link. The term "SLA" is usually used in industry, but it

is a useful term because it covers the essential features of a double A-arm and many other

similar suspensions.

Side View Instant Center (SVIC): Instantaneous point about which the wheel rotates in the side

view. In a double A-arm, the SVIC is an imaginary point located at the intersections of the

lines passing through the inner pivots of the upper and lower arms. This parameter

determines the precession/recession characteristics, and thus anti-dive or squat.

Side View Swing Arm (SVSA): Imaginary swing arm between the SVIC (Side View Instant

Center) and the wheel.

Single Wheel Bump Rate (SWBR): Vertical wheel travel stiffness when only one wheel hits a

bump. Quasistatically, this parameter takes into account stiction and the coupling of the

left and right wheels through the anti-roll bar--higher "roll stiffness due to bar" will

increase the SWBR in comparison to the normal wheel rate. Dynamically, the SWBR may

take into account the inertial affects of unsprung mass, damper forces and/or friction.

Skidpad: A perfectly circular continuous turn used for tuning and testing ultimate chassis grip.

Slip Angle: In plan view, during cornering, angle between the direction of the wheel and that of

the tire tread. Tire grip increases with slip angle until a certain point, at which the tire

slides.

Solid Axle: Suspension where left and right wheels are rigidly connected (except torsionally)-

Used in NASCAR, TransAm, F440, Dirt. Lateral locating device (e.g., Watts link,

Panhard rod) may be independent from longitudinal links. e.g. Hotchkiss, 3-link, 4-link.

Not related to a "spool", though the terms are sometimes confused.

Spool: Rear axle where left and right wheels rotate at the same speed. A non-differential

(sometimes called a "welded diff" for obvious reasons). Usually introduces corner entry

understeer which may be difficult to tune out. Often requires driving style changes, e.g.,

pitching the car into the turn.

Space Frame: Chassis typically made up of welded or brazed tubing-not fully triangulated. Tubes

undergo bending, torsion, and tension/compression.

Space Truss: Fully triangulated space frame. Tubes only undergo tension/compression and joints

carry no moments. Extremely efficient in torsion, but difficult and costly to achieve on a

race car. A space truss is automatically achieved if all exterior surfaces consist of triangles

with no open faces. Open faces (e.g., driver bay) require additional tubing or

reinforcements to compensate.

Spring Rate (Ksp): Spring stiffness, usually in lb/in. The measurement of a spring’s stiffness,

measured in inch/pounds (in/lbs). For example, a (non-variable rated) spring rated at 300

in/lbs means that 300 lbs of force will be required to compress the spring 1 inch. As such,

600 lbs of force would be required to compress the spring 2 inches. [wheel rate]

Sprung Mass (M): Mass of all non-suspension components, plus approximately half the mass of

the spring, antiroll bar, and control arms. [roll moment, unsprung mass]. Sprung mass is

"better" than unsprung mass, since it may be isolated from the road via the suspension.

i.e., unsprung mass behaves like a go-kart in that it is accelerated directly by bumps.

Squat: Vehicle pitch under acceleration. The load transfer of the sprung mass of a car rearward

under acceleration. More simply put, when you’re at a stop light and hit the accelerator,

the sensation you feel of leaning back as you speed up is ‘squat’. [anti-squat, pitch]

Stabilizer Bar (Stabar): see anti-roll bar.

Static Weight Distribution: Front and rear tire loads with car at rest and with full fluids, usually

with driver.

Steering Arm: Lever arm rigidly attached to upright which is connected to the tie rod. Steering

arm and tie rod placement define the Ackerman of the steering system.

Steering Inclination Angle (SAI): see Kingpin

Steering Knuckle: That part which contains the wheel hub or spindle, and attaches to the

suspension components. It is variously called a steering knuckle, spindle, upright or hub,

as well.

Steering Ratio: The ratio between the turn angle of the steering wheel (in degrees), and the turn

angle of the wheels (in degrees). The steering ratio is the amount of degrees you have to

turn the steering wheel, for the wheels to turn an amount of degrees. In most passenger

cars, the ratio is between 12:1 and 20:1. Example: If one complete turn of the steering

wheel, 360 degrees, causes the wheels to turn 24 degrees, the ratio is then 360:24 = 15:1

(360/24=15).

Steering Response: The reaction time between driver input at the steering wheel and the

directional change of the vehicle.

Stiction: Static friction, usually referring to suspension. Higher stiction in a suspension causes

higher SWBRs. If excessive, may lead to unpredictable chassis heights and poor handling.

Sway Bar: Archaic term. See anti-roll bar.

Swing Arm: 1) Simple independent suspension of 1 lateral link rigidly connected to wheel (a la

VW bug). 2) short for "virtual swing arm" on any suspension.

Tie Rod: Steering component between rack and upright usually consisting of a bar with a rod end

bearing on each end. [steering arm, rack]

TIG welding: Tungsten Inert Gas welding. Filler rod is fed by hand, independent of the non-

consumable tungsten electrode. Argon is used to shield the weld from oxygen in the air.

Considered the strongest and most precise form of welding, but also the most difficult and

expensive. Capable of joining metals as thin as .010. Requires precise tube miters,

especially with thin wall tubing. Normalizing is usually done after welding to reduce heat

stresses and improve ductility. [MIG, space frame]

Tight: slang for understeer.

Tire Aspect Ratio: Ratio of a tire's section width to section height, in %. e.g., 60% = 60 series.

Tire Compound: Hardness of rubber-softer is stickier, harder lasts longer.

Tire Section Height: Radial distance from wheel rim to unloaded tread surface. A low section

height improves transient response; a high section height improves ride and decreases tire

spring rate.

Tire Section Width: Maximum lateral dimension of unloaded tire. Wider is normally better for

grip, though excess width may result in insufficient tire temperatures and unnecessary

unsprung weight.

Tire Temperature Distribution: Temperature readings across the tire tread, indicating the

distribution of loading on the tread. Ideally, the inside will be 10° to 20° F hotter than the

outside, and the middle of the tread will be exactly between the inside and outside

temperatures. [pyrometer]

Tire Tread Width: Width of tire tread; not the section width.

Toe Angle: Angle of wheels in plan view. Toe-in refers to the front side of the wheels closer

together than the rear side. Toe-out is opposite. For front wheels, toe-out is stable; for

rear wheels, toe-in is stable. Toe-out in the rear will be undriveable (oversteer) except for

very tight corners.

Top Hat: Brake component between disk and hub.

Torsen: Brand name of a TORque SENsing differential made by Gleason Gear Works. Capable

of a higher torque split than a Quaife.

Torque Sensing Differential: Special type of differential to apply tractive torque to the driven

wheels in proportion to their capacity to handle it. E.g., Torsen, Quaife. Ideally, these

operate anywhere between 0% locked (open) and 100% locked (spool), though the actual

torque split range is somewhere between 20% and 80% locked.

Track (t): Dimension between centers of left and right tire contact patches. Race cars usually

have wider front tracks than rear for improved turn in. Wider track reduces weight

transfer but degrades maneuverability. (aka: tread).

Traction Capacity: The amount of force a tire can generate in the longitudinal sense. Used to

measure straight line acceleration.

Transients: Usually refers to dynamic handling maneuvers.

Tread: 1) Tire tread; 2) track

Triangulation {Full}: Configuration of chassis tube members such that all members are in pure

tension and compression, and joints carry no moments. [space truss]

Tuning: Suspension and chassis adjustments to optimize handling.

Tub: Slang for a monocoque chassis

Understeer: Vehicle handling characteristic where the front wheels tend to lose traction before

the rear. Technically, it is where the front wheel slip angles are greater than the rear.

Understeer is more common on slow, tight corners. Understeer is sometimes desired for

high speed corners because it is safer. (aka: push). [oversteer]

Unsprung Mass (m): Mass of all suspension components outboard of the upper and lower ball

joints, plus approximately half the mass of the spring, antiroll bar, and control arms.

Unsprung mass should be minimized since it behaves like a go-kart in that it is accelerated

directly by bumps. A vehicle with high unsprung mass will tend to skitter on bumpy

corners. At high wheel frequencies (>5 Hz), vehicle becomes more like a go-kart and the

importance of unsprung mass is reduced. [roll moment, sprung mass].

Upright: Suspension component between upper and lower ball joints, which carries the hub and

steering arm. May also carry brake caliper and spring mount.

Variable-Ratio Steering: A system that uses different ratios on the rack, in a rack and pinion

steering system. At the center of the rack, the space between the teeth are smaller and the

space becomes larger as the pinion moves down the rack. In the middle of the rack you'll

have a higher ratio and the ratio becomes lower as you turn the steering wheel towards

lock. This makes the steering less sensitive, when the steering wheel is close to its center

position and makes it harder for the driver to oversteer at high speeds. As you turn the

steering wheel towards lock, the wheels begins to react more to your steering input.

Vertical Load: The amount of force applied to an individual tire in the direction perpendicular to

the road surface. (aka: Load).

Virtual Center: See instant center.

Virtual Swing Arm (VSA): Imaginary swing arm between instant and wheel.

Viscous Coupled Differential: Limited slip device which uses sheared fluid between two plates

to limit wheel spin. Behaves as an open differential until a certain wheel spin is reached,

then limits any further wheel spin. [clutch-pack, torque sensing]

Watts Link: Lateral locating device for a solid axle-common in Trans Am ("flat Watts link").

Weight Distribution: See static weight distribution

Wheels: Rims (without, and not to be included with tires). Generally, wheels and tires are not

used interchangeably or in combination. Tuners like to keep the terms separate to maintain

the distinction between both components.

Wheel Frequency: Square root of (wheel rate/sprung mass carried by wheel). A characteristic of

suspension stiffness that may be used to compare cars of different weights. Race cars

typically have higher front frequencies than rear to reduce dive and improve traction;

street cars are the opposite for "flat ride" over bumps. Street car frequencies are 1.0 to 1.5

Hz. Race car frequencies are usually between 2.0 and 3.0 Hz. High downforce (esp.

ground effects) cars often have wheel frequencies well above 5.0 Hz (nearly a go-kart).

Wheel Rate (Kwh): Suspension stiffness in vertical travel, usually expressed in lbs/in. Equal to

(spring rate) * (motion ratio squared).

Wheelbase: Dimension (in side view) from front to rear wheel CL. Longer wheelbase improves

stability, shorter wheelbase improves maneuverability

Yaw: Angle between vehicle centerline and path of vehicle around a turn--related to

oversteer/understeer.